UNIVERSITY OF CALIFORNIA SANTA CRUZ

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1 UNIVERSITY OF CALIFORNIA SANTA CRUZ SPECIES INTERACTIONS AFFECTING CORALS AND RECRUITMENT ON A PROTECTED, HIGH-LATITUDE REEF: HERBIVORY, PREDATION, AND COMPETITION BY FISHES, URCHINS, MACRO ALGAE AND CYANOBACTERIA A dissertation submitted in partial satisfaction of the requirements for the degree of DOCTOR OF PHILOSOPHY in ECOLOGY AND EVOLUTIONARY BIOLOGY by Wendy A. Cover June 2011 The Dissertation of Wendy A. Cover is approved: Professor Donald C. Potts, Chair Professor Mark Carr Professor James Estes Tyrus Miller Vice Provost and Dean of Graduate Studies Professor Michael Graham

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3 Copyright by Wendy A. Cover 2011

4 TABLE OF CONTENTS LIST OF FIGURES AND TABLES ABSTRACT iv vi INTRODUCTION 1 CHAPTER 1. Coral recruitment on Midway Atoll: settlement patterns, cyanobacterial blooms, and grazing effects on a high-latitude reef 6 Abstract 6 Introduction 7 Methods 13 Results 20 Discussion 26 CHAPTER 2. Differentiating impacts of fish and urchin grazing on algal growth and coral recruitment 57 Abstract 57 Introduction 58 Methods 61 Results 65 Discussion 71 CHAPTER 3. Direct, species-specific impacts of sea urchins on live corals 87 Abstract 87 Introduction 88 Methods 92 Results 98 Discussion 102 LITERATURE CITED 118 in

5 LIST OF FIGURES AND TABLES CHAPTER 1 Table 1. Total counts and mean numbers of coral recruits per tile 36 Table 2. One-way ANOVA for numbers of recruits per tile among six sites, and a matrix of pairwise comparison probabilities 37 Table 3. Numbers of recruits on forward and rear tiles and two-way ANOVA 38 Table 4. Mean numbers of coral recruits per m 2 on the three tile surfaces and their subcategories (zones) 39 Table 5. Numbers and percent of coral recruits settling on different substrates 40 Table 6. Condition of recruits based on amount of overgrowth by other organisms 41 Table 7. Frequency of recruit overgrowth by various organisms 42 Figure 1. Study sites on Midway Atoll, Northwestern Hawaiian Islands 43 Figure 2. Tile surfaces and zones used when scoring data 44 Figure 3. Examples of each taxon recorded 45 Figure 4. Mean coral recruitment for three treatments and by site 46 Figure 5. Mean numbers of recruits per tile pair for each coral taxon at each site 47 Figure 6. Proportion of total benthic cover comprised of adult coral colonies at four sites 48 Figure 7. Frequency distributions of coral recruits by a) size, and b) number of corallites 49 Figure 8. Relationship between colony size and number of corallites in colony 50 Figure 9. Total numbers of recruits in grooved vs. non-grooved (exposed) zones of outer tile surfaces at each site 51 Figure 10. Numbers of recruits in three coral taxa that settled on the four most common substrates 52 Figure 11. Numbers of recruits that were Overgrown and Not Overgrown at each site 53 Figure 12. Examples of various organisms overgrowing coral recruits 54 Figure 13. Numbers of recruits overgrown by CCA or by Other Algae 55 Appendix. Additional organisms noted on settlement tiles 56 CHAPTER 2 Table 1. 2-factor ANOVA comparing algal wet weights in factorial treatments exposed to fishes and urchins 75 IV

6 Table 2. Effect of urchins and fishes on species composition of algae in each treatment 76 Table 3. 2-factor ANOVA comparing sediment wet weights in factorial treatments exposed to fishes and urchins 77 Table 4. 2-factor ANOVA comparing the number of coral recruits in factorial treatments exposed to fishes and urchins 78 Figure 1. Diagram of the 2 x 2 factorial design 79 Figure 2. Effects of fishes and urchins on the mean biomass of algae on experimental tiles 80 Figure 3. Effect of urchin density on algal biomass 81 Figure 4. Biomass of turf algae in the four treatments 82 Figure 5. Mass of sediments on tiles in the four treatments 83 Figure 6. Numbers of coral recruits per tile (± 1SE) in the four treatments 84 Figure 7. Number of coral recruits as a function of algal biomass 85 Figure 8. Quantitative spectra of light under three treatments (open, full cage, and half cage) and a natural rock overhang 86 CHAPTER Table 1. Mean daily rates (cm urchin" day") of live coral tissue loss from corals exposed to E. mathaei 108 Table 2. 2-factor ANOVA results for rate of coral tissue removal by site and coral genus 109 Table 3. 2-factor ANOVA results for day of first damage by site and coral genus 110 Figure 1. Study sites on Midway Atoll 111 Figure 2. Numbers of undamaged Pocillopora ligulata nubbins per plot 112 Figure 3. Area (mean ± 1SE) of Pocillopora ligulata tissue removed by E. mathaei on days 1,3, and Figure 4. Two Pocillopora ligulata nubbins, one exposed to E. mathaei and the other in a control plot without E. mathaei 114 Figure 5. Numbers of damaged and undamaged coral nubbins by treatment (E. mathaei removed or present) for two coral taxa after 116 days 115 Figure 6. Examples of coral nubbins (with and without E. mathaei) on three dates for a) Montipora flabellata, and b) Porites lobata 116 Figure 7. Number of days (mean ± 1SD) for nubbins to be damaged by E. mathaei 117 v

7 ABSTRACT SPECIES INTERACTIONS AFFECTING CORALS AND RECRUITMENT ON A PROTECTED, HIGH-LATITUDE REEF: HERBIVORY, PREDATION, AND COMPETITION BY FISHES, URCHINS, MACRO ALGAE AND CYANOBACTERIA Wendy A. Cover Interactions between species can affect major processes that shape community structure in many systems. Both fishes and urchins are important herbivores on corals reefs, maintaining resilience through their grazing activities and preventing coral-macroalgal phase shifts. Despite their importance, little is known about the relative importance of these different herbivore guilds and of the scale and magnitude of their positive and negative effects on corals. First, I investigated coral recruitment at six backreef sites on Midway Atoll, including two anthropogenically-impacted sites with metal debris that have periodic blooms of the benthic cyanobacterium Hormothamnion enteromorphoides. Contrary to expectations, coral recruitment was significantly higher at the two cyanobacterial bloom sites than at the four control sites. The proportion of recruits on exposed surfaces was higher at bloom sites than at controls, indicating that cyanobacteria indirectly enhanced recruitment by inhibiting fish grazers that usually remove small corals from exposed surfaces. Next, I conducted a factorial field experiment to quantify the relative effects of herbivorous

8 fishes and urchins (Echinometra mathaei) on macroalgal growth and coral recruitment. Fish grazing effectively limited algal biomass, which was >50 times higher in treatments without fishes. Coral recruitment was >2X higher in treatments exposed to fish grazing, indicating that algae inhibit coral recruitment more than do fishes. Algal biomass was negatively correlated with coral recruitment, suggesting that management to increase herbivorous fishes and grazing levels is likely to benefit coral recruitment. Finally, I investigated direct, species-specific effects of urchins on corals. Most fragments of all three coral species exposed to E. mathaei were damaged within days and the damage increased over several weeks to months, often ending with complete removal of all coral tissue and skeleton. Fragments in plots without E. mathaei, and all fragments exposed to H. mammillatus plots were unaffected. These studies demonstrate a number of positive and negative, direct and indirect effects of herbivores on corals. Negative effects of urchins are lessened by their spatial restrictions, and negative effects of fishes are outweighed by the positive effects of algae removal, clearing space for coral recruits and enhancing the resilience of coral reef communities.

9 INTRODUCTION Interactions between species can sometimes be so profound that they shape the entire structure of the community (Terborgh & Estes 2010). Massive changes in communities are often referred to as "phase shifts" or "regime shifts", where the foundational species in the community change dramatically in abundance, usually due to changes in top-down forcing (Scheffer et al. 2001, Folke et al. 2004). Some of the most striking examples in both terrestrial and aquatic systems come from trophic cascades, where a top predator has cascading effects through multiple trophic levels, indirectly controlling the abundance of lower levels, and often affecting the foundational species (Terborgh & Estes 2010). Freshwater lakes are a well-known example of such control: in the absence of a piscivore, small fish consumers dominate and consume zooplankton, which allows phytoplankton to proliferate, coloring the lake green. With the addition of a top predator, such as the largemouth bass, there are cascading effects through each trophic level, reducing small fish and increasing zooplankton abundance, which then controls phytoplankton and results in a clear, blue lake (Mittelbach et al. 1995). There are also important species interactions on coral reefs, which provide the basis of what I studied. Fishes and urchins are the primary herbivores, consuming algae, or seaweeds. Algae compete with corals for space on the reef, preventing corals from occupying new spaces, and sometimes causing coral death (McCook et al. 2001a, Jompa & McCook 2003, Smith et al. 2006). This means that herbivores 1

10 indirectly benefit corals by removing their algal competitors. On a healthy reef with abundant herbivore populations, algal densities are kept low, allowing corals to proliferate and form the framework of the reef ecosystem (Mumby & Harborne 2010). But many reefs worldwide are overfished, resulting in reduced herbivore densities and lower grazing rates (Jackson et al. 2001). When grazers are effectively removed from the system, and a disturbance, such as a hurricane or a bleaching event, kills corals, macroalgae can take over the reef, limiting space for new corals to recruit and reducing coral growth, ultimately limiting the cover of corals on the reef (Knowlton 2004). The importance of these interactions was starkly illustrated in Jamaica, where rising human populations contributed to overfishing of reef fishes, notably herbivores, and a disease killed 99% of the primary herbivorous urchins, Diadema antillarum, throughout the Caribbean (Lessios et al. 1984). Following hurricane Allen, which destroyed much of the coral cover in the region, and with the last functional herbivores effectively removed, there was a phase-shift from a coral-dominated to a macroalgal-dominated state (Hughes 1994). Coral cover declined from over 50% to around 5% cover, and macroalgae increased to over 90% cover, preventing recovery of coral populations and leaving the reefs with lower diversity and reduced habitat for fish and other organisms. Hurricanes are a natural disturbance from which reefs normally recover, but instead of recovery, Jamaican reefs experienced a phase shift due to an underlying loss of resilience in the system. Resilience is the ability of a system to recover from 2

11 disturbance and return to its original state (Gunderson 2000). As Jamaica lost its herbivores, it lost the capacity of the reef to prevent macroalgal overgrowth and recover from disturbance (Scheffer et al. 2001). One process that is particularly important to the resilience, or recovery potential, of a system is coral recruitment. Corals have a bipartite life cycle, with adults living on the benthos and larvae suspended in the water column, transported by currents until they settle onto the reef and begin to accrete a skeleton, becoming a new recruit into the coral population of that reef. Recruitment is an important process that not only replenishes coral populations on healthy reefs, but also fuels recovery on reefs that have suffered a disturbance and loss of adult corals (Williams et al. 2008, Done etal. 2010). Coral reefs are facing increasing threats to their persistence worldwide, with overfishing, pollution, and climate change among the key issues contributing to reef loss (Pandolfi et al. 2003). Overfishing not only reduces food supplies, it also lowers the resilience of the reef by limiting the grazing function of herbivorous fishes (Hughes et al. 2007a). Pollution can cause many problems, including macroalgal proliferation via nutrient inputs, but healthy herbivore populations can limit macroalgal growth despite increased nutrient loads (Smith et al. 2010). Climate change contributes to warming oceans and coral bleaching events, which are largely caused by global factors beyond the scope of local reef managers (Baker et al. 2008). However, resilient reefs with many herbivores are more likely to recover from a bleaching event (Arthur et al. 2006), causing many to advocate maintaining the 3

12 resilience of reefs through protection of herbivorous fishes (Hughes et al. 2003). Fishing regulations and no-take areas (marine protected areas, or MP As) are important tools for boosting fish populations and maintaining the resilience of reefs to many perturbations (Hughes et al. 2007a). Several conservation organizations, such as The Nature Conservancy, have major initiatives aimed at promoting coral reef resilience. Despite the importance placed on protecting herbivores and keeping algae at bay, we know very little about the specific roles of the great diversity of herbivores on the reef, nor of the strength of their interactions. In addition to their more wellknown positive effects, herbivores are also capable of negatively affecting corals through direct removal while grazing (Sammarco 1980, Mumby 2009a), but few studies have investigated the relative importance of positive and negative effects. My work investigated some of the details of these interactions, as well as trying to assess the relative importance of dominant reef herbivores. I conducted my research on Midway Atoll in the Northwestern Hawaiian Islands (NWHI), an archipelago that is fully protected from fishing and other extractive activities under the Papahanaumokuakea Marine National Monument, one of the largest marine protected areas in the world. As a result of its isolation and fishing restrictions, the NWHI have a relatively intact trophic structure, with high densities of top predators and large, abundant herbivores (Friedlander & DeMartini 2002). Since most other reefs in the world have reduced herbivore densities due to 4

13 exploitation, Midway is an ideal location to study herbivore interactions on an intact reef. 5

14 CHAPTER 1 Coral recruitment on Midway Atoll: settlement patterns, cyanobacterial blooms, and grazing effects on a high-latitude reef Wendy A. Cover Abstract: Larval recruitment fuels the resilience and replenishment of coral reef ecosystems. I investigated coral recruitment at six backreef sites on Midway Atoll (Northwestern Hawaiian Islands), including two anthropogenically-impacted sites with metal debris that have periodic blooms of the benthic cyanobacterium Hormothamnion enteromorphoides. These blooms can cover 85% of the substrate, deter herbivorous fishes, and grow over live corals. To test the hypothesis that cyanobacterial blooms inhibit coral recruitment, I measured recruitment on terra cotta tiles at two degraded (cyanobacterial) sites, two nearby unimpacted control sites, and two distant control sites. I also examined spatial patterns of coral settlement (rugosity, orientation, and exposure effects), substrate associations, organisms overgrowing recruits, and the relationship between adult cover and recruitment. Of the 743 coral recruits, 63% were Pocillopora. There was no association between recruitment and adult coral cover, largely because the most abundant coral genus, Montipora, had only a single 6

15 recruit. The majority of corals (53%) settled on crustose coralline algae (CCA), but many settled directly on the tile (16%) or on partial tile-partial CCA (24%). Many recruits (44%) had evidence of overgrowth by CCA and non-cca algae in roughly equal frequencies. Contrary to expectations, coral recruitment was significantly higher at the two cyanobacterial bloom sites than at the four control sites, indicating that Hormothamnion enhanced coral settlement or survival. Recruit densities were higher in grooves than on exposed surfaces overall, but the proportion of recruits on exposed surfaces was higher at bloom sites than at controls. I propose that the cyanobacteria indirectly enhanced recruitment by inhibiting fish grazers that usually remove small corals from exposed surfaces. Introduction: Coral recruitment is one measure of the capacity of reef-building corals to replenish their populations, and is critical in their recovery from degradation and disturbance, a fundamental aspect of reef resilience (Hughes & Tanner 2000). There are many natural and anthropogenic perturbations affecting coral reefs, but we have limited understanding of how they may influence the spatial patterns or rates of larval recruitment. Benthic cyanobacterial blooms are an anthropogenically-influenced phenomenon that may affect coral recruitment. Cyanobacteria contain numerous toxic compounds known to negatively impact humans and marine organisms 7

16 (Osborne et al. 2001, Codd et al. 2005, Pittman & Pittman 2005). Blooms of the benthic cyanobacterium Lyngbya are now common off the coast of Florida, where they are associated with elevated iron and nutrient levels (N and P) and grow over gorgonian corals (Paul et al. 2005). In Moreton Bay, Australia, river outflows containing iron, phosphorus, and organic carbon stimulated blooms of Lyngbya majuscula, with associated losses of seagrasses and reduced fish densities (Albert et al. 2005, Pittman & Pittman 2005). Benthic cyanobacterial blooms, like other harmful algal blooms (HABs), are increasing in frequency and severity, and this trend is predicted to continue due to increasing temperatures from global climate change (Van Dolah 2000). On Pacific atolls, blooms of filamentous cyanobacteria have been observed around iron shipwrecks, on larger spatial scales than that reported from continental and volcanic island reefs (J. E. Maragos, pers. comm.). It is possible that iron is a particularly important limiting resource for cyanobacteria on atolls, which are naturally iron-free, carbonate systems in iron-depleted waters. Cyanobacteria require iron for the nitrogenase enzyme that fixes nitrogen. The well-known iron-addition experiments in the South Pacific triggered blooms of planktonic cyanobacteria (Rue & Bruland 1997, Hyenstrand et al. 2000) and iron has also been implicated in stimulating a benthic cyanobacterial bloom (Arthur et al. 2009). Due to the potential negative effects from toxicity and competition, iron debris and associated benthic cyanobacterial blooms are a growing management concern. For example, a benthic 8

17 cyanobacterial bloom associated with a shipwreck on Rose Atoll prompted the USFWS to remove all shipwreck debris (Green et al. 1997, Schroeder et al. 2008). Like other macroalgae, benthic cyanobacteria can compete with corals for space, but they also have the potential to cause more damage than other macroalgal species via allelochemical effects; Titlyanov (2007) found that Lyngbya bouillonii reduced coral growth, lowered densities of symbiotic dinoflagellates, induced bleaching, and ultimately resulted in coral tissue death. In Florida, the toxic Lyngbya majuscula smothered octocorals and other invertebrates (Paul et al. 2005), while Lyngbya reduced survival and recruitment of three species of coral larvae in experiments on Guam (Kuffner and Paul 2004, Kuffner and Walters 2006). Coral recruitment is often variable both within sites (1-5 m) and between sites (0.5-3 km) across broad latitudinal gradients (Hughes et al. 1999). Despite the importance of recruitment for reef recovery after large disturbances, the link between recruitment levels and adult coral cover at a site is often tenuous (Hughes et al. 1999), with a few exceptions (Penin et al. 2007). Larger-scale variation in larval recruitment can reflect small-scale habitat features. Many studies have shown strong influences of small-scale habitat features on local patterns of coral recruitment. In particular, the composition (e.g. bare substrate, live coral, dead coral, coralline algae) and physical structure (e.g. rugosity, orientation, light exposure) of the reef surface can influence larval settlement choices or post-settlement survival, ultimately affecting the density of juvenile corals (Carleton & Sammarco 1987, Maida et al. 1994, Harrington et al. 2004). One 9

18 common approach for examining the role of these features is with ceramic tiles that can be manipulated to reflect these habitat features. For example, studies of settlement locations generally find more recruits on the inner surfaces of paired settlement plates and on the undersides of single plates (Carleton & Sammarco 1987, Maida et al. 1994). These patterns are often attributed to active larval choice, in which larvae responding to environmental cues such as light, substrate composition, or orientation, tend to settle selectively on more protected surfaces (Babcock & Mundy 1996, Mundy & Babcock 1998, Raimondi & Morse 2000). Alternatively, patterns of recruit location may result from spatially variable, post-settlement mortality. Although mechanisms of early post-settlement mortality are poorly understood, likely mortality factors include such ecological interactions as competition with other organisms for space, direct predation, or indiscriminate grazing by herbivorous fishes or sea urchins. Direct predators of recruits are largely unknown but may include mollusks (e.g. Drupella), and other mobile invertebrates (e.g. flatworms, crabs). Incidental removal through the grazing of herbivorous fishes or urchins is likely to play a large role in the post-settlement mortality of coral recruits (Sammarco 1980, Baria et al. 2010). In French Polynesia, recruit mortaility and the local density of parrotfishes were positively correlated (Penin et al. 2010). Several studies have found more recruits (Gleason 1996, Adjeroud et al. 2007, O'Leary & Potts 2011) and higher survival of recruits (Nozawa 2008, Christiansen et al. 2009, Baria et al. 2010) on surfaces protected from grazers, as well as evidence of mortality and damage to recruits exposed to fish grazers (O'Leary & Potts 2011, 10

19 Penin et al. 2011). I used paired tiles with grooved outer surfaces to provide variable exposure to grazers and to test hypothesis that more recruits survive in spaces protected from grazers. Few studies have looked at competitive effects at the scale of a coral recruit, but major competitors are likely to be rapidly-growing sessile invertebrates (e.g. bryozoans, tunicates, sponges) and algae (fleshy macroalgae, turfs, or crustose coralline algae). Macroalgae may preempt space from coral recruits, and sponges may overgrow recruits (Vermeij 2006). Macroalgae can also reduce survivorship of juvenile corals through interference competition (Box & Mumby 2007), while both turfs and macroalgae can inhibit coral recruitment (McCook et al. 2001b). In contrast, other organisms have been shown to enhance rates of larval recruitment of corals. In particular, many coral larvae are known to settle preferentially on crustose coralline algae (CCA), especially particular CCA taxa, like Titanoderma and Hydrolithon (Morse et al. 1996, Raimondi & Morse 2000, Harrington et al. 2004). However, some taxa are not strongly selective in substrate, notably Pocilloporidae (Baird & Hughes 2000, Baird & Morse 2004), which is often the most common family recruiting in high latitudes (Harriott & Banks 1995, Glassom et al. 2004). I documented settlement substrate (CCA or other), as well as which organisms were overgrowing individual coral recruits, and to what extent, to gain insight into settlement preferences and the organisms and mechanisms responsible for post-settlement mortality via competition. 11

20 High-latitude (subtropical) coral reefs survive at the extremes of coral distribution where they withstand larger temperature fluctuations than their lowerlatitude counterparts. Two primary factors implicated in limiting coral growth at high-latitudes are: colder annual temperatures with larger seasonal differences, and a lower aragonite saturation state (Kleypas et al. 1999). This means that high-latitude reefs may be indicators of coral reef responses to climate changes that are predicted to increase temperature fluctuations and lower carbonate concentrations. Recruitment levels on latitudinally marginal reefs are usually much lower and have higher proportions of brooding corals than their tropical counterparts (Harriott 1999). The extreme environment of high-latitude reefs may provide insight into upcoming changes in coral recruitment patterns under the broad-scale stresses of climate change and ocean acidification. I investigated coral recruitment across subtropical Midway Atoll to better understand its recovery potential across sites; to compare its recruitment to the neighboring Hawaiian Islands and other high-latitude reefs; to gain insight into possible causes of post-settlement mortality, especially grazer impacts, and to investigate the role of anthropogenically-induced cyanobacterial blooms on coral recruitment rates. Midway is part of the Northwestern Hawaiian Islands, an isolated and protected archipelago characterized by >50% offish biomass as large carnivores (e.g. sharks, jacks, grouper) that are rare (<3%) in the main Hawaiian Islands, and a higher biomass and mean weight of herbivores such as parrotfish and surgeonfish 12

21 (Friedlander & DeMartini 2002), making this an ideal location to study the functioning of a trophically intact coral reef ecosystem. I studied two degraded, iron-impacted sites on Midway which have periodic, ephemeral blooms of the cyanobacterium Hormothamnion enteromorphoides. While the ecology of//, enteromorphoides is little-studied, it is known to contain toxins that deter fish and gastropod herbivores (Wylie & Paul 1988, Gerwick et al. 1992, Pennings & Paul 1992, Pennings et al. 1997). Given the toxicity of many cyanobacteria and prior studies showing reduced coral recruitment rates, I hypothesized that benthic cyanobacterial blooms have negative impacts on coral recruitment at the focal degraded sites. I expected fewer coral recruits would settle on tiles placed within bloom sites than on tiles in nearby control sites. Methods Study system Midway Atoll is a high-latitude (28 N), ecologically marginal environment for corals, lying at or near the proposed Darwin Point, where reef accretion matches erosion and subsidence (Grigg 1982). It lies near the center of the North Pacific Ocean (179 W), 141 km NW of Pearl and Hermes Atoll, and 87 km SE of Kure, the last atoll in the Hawaiian archipelago. Lagoon temperatures range from a late winter low of 19 C to a late summer high of 28 C. Midway is a National Wildlife Refuge (established in 1996), and is part of the Papahanaumokuakea Marine National Monument which protects the entire 13

22 Northwestern Hawaiian Islands (NWHI), a chain of 10 emergent reefs and islands extending 2400km beyond the main Hawaiian Islands. It is one of the most isolated reef systems in the world. Historically, there has been little to no fishing, leaving an intact food chain with 260% higher fish biomass than in the main Hawaiian Islands (Friedlander & DeMartini 2002). Although reef fishing has been prohibited on Midway for at least 20 years, and probably much longer under the Navy, the atoll has experienced extensive anthropogenic influence. For over 60 years, beginning as a Pan-American seaplane base in 1935 and then a U.S. Navy base with up to 5000 residents, the island and reef were extensively modified, including removal of patch reefs, dredging and dynamiting a deep channel through the southern reef margin, dredging a ship basin in the lagoon, construction of harbors and airstrips, and clearing a reef-free region in the lagoon for seaplane landing strips. Dredged material was used to add substantially to the land area and elevation of Sand Island: approximately one quarter of the current island area was filled in to create a larger runway and a protected harbor. The filled region is retained by a sheet metal sea wall stretching around half of the island's circumference. Environmental consequences of these WWII and Cold War actions are poorly documented, but almost certainly resulted in large-scale mobilization of sediments within the lagoon and major changes in lagoonal circulation, with water now exiting through the new opening in the southern reef crest. Many contaminants, such as petroleum, lead, and PCBs, were widely used on the island, and for many 14

23 years raw sewage was released into the lagoon off the West Beach, finally ending in the 1980's. Before the Navy left Midway in 1996, they cleaned up many of the land-based contaminants and dump sites but left several shallow-water scrap metal sites, including three areas known locally as: Rusty Bucket, a nearshore patch reef where debris was bulldozed off the end of the airport runway; Reef Hotel, the remains of a former building on steel pilings on the northern backreef; and Bulky Dump, a peninsula on the south side of the island created from large debris and covered with cement blocks and soil. Periodic benthic cyanobacterial blooms occur at these sites, lasting for a few weeks and recurring episodically during the year over a broad temperature range ( C), with densities ranging from isolated thalli to 85% cover on rocky surfaces. Effect of cyanobacteria on coral recruitment To determine the effect of cyanobacteria on coral recruitment rates, I compared sites that characterized three treatment levels: cyanobacterial bloom sites, nearby controls, and more distant controls. I selected two replicate sites for each treatment level, with replicates on opposite sides of the atoll to avoid pseudoreplication (Fig. 1). The two bloom sites, Reef Hotel (RH) and Rusty Bucket (RB), have extensive subtidal metal debris and experience periodic blooms of cyanobacteria (Hormothamnion enteromorphoides). Adjacent to each of these, I identified a nearby control, Reef Hotel Control (RHC) and West Beach Control 15

24 (WBC), neither of which have iron debris and do not experience cyanobacterial blooms, but have similar geomorphology, species composition, and oceanographic influences as their matched treatment sites. Two additional "far control" sites, North Reef Far (NRF) and The Hook Far (HKF), are distant from known human impacts, do not experience cyanobacterial blooms, and provide a broader view of recruitment between the northern and southern halves of the atoll. The northern sites (RH, RHC, and NRF) are backreef habitats protected by a raised crest and consisting of scattered patch reefs with relatively high coral cover (-35%) of predominantly Montiporaflabellata. The southern sites (RB, WBC, and HKF) have much lower coral cover (-3%), primarily Pocillopora. HKF is in a region with high water flow, RB and WBC are large patch reefs close to Sand Island (Fig. 1). All sites are shallow (1-2 m) reefs within the lagoon. Sites were chosen to cover a range of backreef habitats to give a broader picture of recruitment patterns across the atoll, including regions with high and low coral cover. I used terra cotta tiles as standardized settlement habitats to sample coral recruitment across the three treatment levels. Two terra cotta edging tiles (~14 x 15 x 1.4 cm) were used to form a "sandwich" spaced ~3cm apart by curved edges along one side of each tile (Fig. 2a). Paired tiles provide a wider range of microhabitats for settlement. Each tile sandwich was attached to a vertical rock surface on a raised reef structure, using a 10 cm stainless steel lag-screw inserted through a hole in the center of each tile and screwed into a plastic wall anchor in a hole drilled into the rock. Tiles were placed vertically to minimize accumulation of sediments and maximize 16

25 recruitment (Babcock & Mundy 1996). The tiles were close to, but not touching, live corals. The outer surface of each tile had grooves (1.5 across x 2.5 mm deep) providing spaces protected from grazers alternating with non-grooved surfaces exposed to grazers. The smooth inner surfaces of the tiles faced toward each other, providing further space protected from fish grazers. Within each pair, the tile touching the rock was designated the "rear" tile; the one facing open water was the "forward" tile. I deployed ten replicated tile "sandwiches" at each site in June 2006 and collected them 13 months later in July I photographed each tile in the field after removal. After removing fleshy macroalgae and loose sediments manually, the tiles were bleached for 24 hrs to remove remaining soft tissues and to expose coral skeletons, then air dried. Each tile was searched completely under a dissecting microscope by two independent observers. All coral recruits were identified to the lowest possible taxon using reference photos and descriptions from recruitment studies in Hawaii (Brown 2004, Basch & White 2008), the Great Barrier Reef (Babcock et al. 2003), and Kenya (Mangubhai et al. 2007). Both Pocilloporidae and Poritidae have only one known genus in the NWHI, so identifications were to genus level for these families. All recruits were identified, except those that were too small (i.e. a very early developmental stage with only partial accretion of the skeleton), too overgrown, or too severely damaged (rare). I was able to identify Pocillopora recruits at a much earlier developmental stage than most of the other taxa. Many recruits were 17

26 photographed for reference vouchers (Fig. 3). Data comparing density of recruits (mean number per replicate tile pair) between sites and taxa were analyzed using ANOVA with Tukey post-hoc tests and general linear models (GLM) with hypothesis tests, after checking for conformance with the assumptions of normality and of homoscedacity of variances. To compare with other studies, the overall number of recruits was converted into recruits m" yr" using the tile area of m and the number of weeks each tile was deployed. Effects of substratum features on coral recruitment For each coral, I recorded the colony size (as number of corallites, and as maximum diameter in mm); condition (5 categories from no overgrowth to fully overgrown) and identity of organisms overgrowing it; substrate association (what the coral was growing on); tile position (rear, forward); tile surface (inner, outer, edge); and zone on tile (subcategories of surface; Fig. 2a and b). Zones were delineated to distinguish specific settlement locations of recruits for comparisons with other studies that have shown more recruits on protected surfaces and edges. On the outer surfaces of tiles, where light conditions were comparable, rugosity effects and exposure to grazers were represented by "groove" and non-groove, or "exposed", surfaces (Fig. 2a). The edges of the tiles provided a means to explore orientation effects, distinguishing between the "top", "bottom", and "sides" of tiles (Fig. 2a). The inner surfaces differ in light availability based on their proximity to the open edge of the tile; I delineated four bands that were horizontal to the open edges (A, B, C, and D; 18

27 Fig. 2b). Recruits on surfaces where the tiles touched each other were included in the total inner surface count but not in the horizontal band counts. The upper and lower bands, A and D, were 3.4 cm wide to include the typical growth region of crustose coralline algae; intermediate bands B and C were 4.2 cm wide. For comparison between tile surfaces and zones with different areas, the number of recruits was converted to recruits m" using the area of each surface and zone. These data were analyzed using GLM hypothesis tests after squareroot transformation. Substrate association was usually determined by looking down through the primary calyx; this is possible up to a colony size of several corallites, before the calices develop an opaque base. This technique was used on most recruits, but when not possible, usually due to large size, the association was inferred from the surrounding substrate. The latter method may be unreliable because CCA often grows around the primary corallite after it settles. I confirmed questionable settlement substrates by removing corals to check their underlying association. Condition of each coral recruit was assigned to one of five categories based on the proportion of the recruit had been overgrown by other organisms: No Overgrowth, Slight Overgrowth (an edge of the coral had some overgrowth, 1-9%>), Partially Overgrown (10-50%) overgrown), Mostly Overgrown (51-90%) overgrown), and Fully Overgrown (completely covered or nearly so, %). For coarser distinctions, these categories were pooled into two: Not Overgrown (0-9%> overgrown) and Overgrown (10-100%) overgrown). Frequency data on coral recruit condition, substrate association, and locations were analyzed using Chisquared tests. 19

28 Relationship between coral recruitment and adult coral cover Surveys of benthic cover were conducted at four sites, Reef Hotel (encompassing both RH and RHC), North Reef Far, Hook Far, and Rusty Bucket, using six haphazardly placed, 10 m long, line intercept transects at each site. I scored all corals, algae, and other invertebrates >3 cm, and identified them to the lowest possible taxon. Data were used to estimate percent cover of adult coral taxa for comparison with recruit taxon densities. Results Corals recruited to 52 of the 55 tile pairs, with a total of 743 recruits across the 6 sites (Table 1). Recruitment ranged from 0 to 42 recruits per tile pair and averaged 13.4 ± 6.0 SD. The mean recruitment rate was 101 recruits m" 2 yr" 1, and site means ranged from 9 to 151 recruits m" yr" (Table 1). Pocillopora was the most common taxon at 63.1% of recruits, followed by Pontes (15.7%), Faviidae (7.4%), and Montipora (0.4%), with 13.3% that could not be identified (Table 1; Fig. 3). Only one of three Montipora recruits established from a settling larva, the other two Montipora colonies came from the horizontal spread of adjacent colonies of Montipora flabellata onto the tile. Recruit size was assessed in two ways: mean diameter and number of corallites (Fig. 7A). Sizes ranged from 0.7 to 6 mm diameter, with a median of 2 mm (Fig. 7A). The average size of a recruit with one corallite was 1.4mm ± 0.5 SD. 20

29 Over half the recruits (57%) had only one corallite, while the largest colony had 67 corallites (Fig. 7B). The number of corallites in a colony tended to increase with increasing colony diameter (Fig. 8). Effect of cyanobacteria on coral recruitment There were significant differences in the numbers of recruits per site (Table 2; ANOVA, po.ool). Contrary to the initial hypothesis that cyanobacteria inhibit settlement and/or survival, the two sites that experience cyanobacterial blooms, Rusty Bucket (RB) and Reef Hotel (RH), had the highest, not the lowest, numbers of recruits (Table 1, Fig. 4). Recruitment at these cyanobacterial bloom sites was significantly higher than at nearby control sites (RHC and WBC) (GLM hypothesis test, p < 0.001) and at the far control sites (NRF and HKF) (GLM Hypothesis test, p = 0.032). The two cyanobacterial sites in different regions of the lagoon had very similar recruitment rates (p = 0.999; Table 2). Recruitment rates at the four control sites were much more variable (Fig. 4B), with the pooled far control sites intermediate between the treatment sites and the nearby controls (Fig. 4A). Distributions of coral recruit taxa by site are shown in Fig. 5 and Table 1. The two Reef Hotel sites (RH and RHC) had the highest taxon richness, with all four taxa present (Pocillopora, Pontes, Faviidae, and Montipora). Pocillopora is the only taxon that was present at all sites, and it was the sole taxon identified at two sites (RB and WBC). Relationships between taxon and all other variables (position, substrates, 21

30 condition) were tested, but only the results that were either statistically significant or notable are described below. Relationship between coral recruitment and adult coral cover There was little relationship between relative adult coral cover (Fig. 6) and recruitment of coral taxa at each site (Fig. 5, Table 1). Adult coral cover did not predict recruitment densities, especially for Montipora, which was by far the most abundant coral at the three northern sites (RH, RHC, NRF) but had almost no recruitment (and only one by a larva). Excluding Montipora, there was some concordance between the species composition at a site and the species composition of recruits at that site. The adult colonies at RB and WBC consisted solely of Pocillopora, and this was reflected in the recruit composition. Adults at the other sites were more diverse, with the addition of Pontes and Faviids, and this was reflected in the diversity of recruits at these sites. Note that although the surveys at NRF did not record Pontes or Faviids on the transects, these taxa were present as adults at this site. Three coral families are present but uncommon to rare on the Midway backreef: Agariciidae (Pavona and Leptoseris), Fungiidae, and Siderastreidae (Psammocora). None of these were identified as recruits, but a few of the small recruits that I could not identify may have been from these families. The most common species of Faviidae on Midway are in the genus Cyphastrea (as indicated in the surveys), but colonies of Leptastrea are present in some habitats. It is likely that 22

31 Cyphastrea ocellina, the most common faviid on Midway, also makes up most (if not all) of the faviid recruits, but since there are no detailed descriptions of the structure of early settlers of this genus, I was unable to confirm this. Effects of substratum features on coral recruitment Orientation effects. Recruitment rates on the three main tile surfaces, outer, inner, and edges, were compared using data expressed as density m" 2 to account for differences in area between the three surfaces (Table 4). There were more recruits on the outer surfaces than on the edges (marginally significant, GLM hypothesis test df = 1, F = 3.582, p = 0.051) and significantly more on the edges than the inner surfaces (Table 4; GLM hypothesis test df = 1, f = 6.023, p = 0.015). There were significantly fewer coral recruits on the top edges of tiles than on the sides and bottom edges (Table 4; x 2= 29.68, df=l, po.ool). Rugosity effects / exposure to grazers. On outer surfaces, there were significantly more recruits in grooves than on exposed areas (353 vs. 155; Table 4; 2x2 contingency table, x =77.17, df=l, po.ool), and this pattern was consistent for all coral size classes. This pattern also held for all sites except Rusty Bucket (RB), which had the opposite pattern of more recruits on the exposed areas (Fig. 9). Reef Hotel (RH), the other cyanobacterial site, had relatively more recruits on exposed surfaces than at the four remaining sites, but when RB and RH were removed from the analysis, the % 2 was not significant. When the sites were grouped by treatment, the cyanobacterial sites had significantly more recruits on exposed areas than did the 23

32 two control treatments (x 2 =3.97, df=l, p=0.046). For each taxon there were more recruits in grooves than in exposed zones (Pocillopora: 197 vs. 121, Pontes: 52 vs. 14, Faviidae: 45 vs. 2). Light availability. Recruitment was significantly different among the four inner surface bands of the tiles (Table 4), and this appeared to correspond directly to the presumed amount of light reaching each band, based on the vertical orientation of the sandwiched tiles with openings at the top and bottom but closed sides (band A high light, band D medium light, band B medium-low light, and band C low light). Correspondingly, band A had the most recruits, followed by band D, then band B, then band C. The four bands were significantly different, with non-overlapping confidence intervals (% 2 =66.30, df=3, po.ool). The high light bands near the edges (A and D) had significantly more recruits than the low light bands toward the inside (B and C; x 2 =45.37, df=l, po.ool). Proximity to reef. There were roughly equal numbers of recruits on rear and forward tiles (364 vs. 379), but considerable variation among sites (Table 3A,B). Reef Hotel had many more recruits on rear tiles than forward tiles, three sites (HKF, RB, RHC) had the opposite pattern, and two sites (NRF and WBC) had roughly equal distribution (Table 3A). This variation contributed to a significant interaction between site and tile position (rear or forward tile) (Table 3B; ANOVA, p=0.027). There was a significant difference between sites when Reef Hotel was included (% 2 = , df = 5, p < 0.001), but sites were independent when it was excluded (% = 3.166, df = 4, p = 0.531). There were also some taxon differences, with Pontes and 24

33 faviids more abundant on rear tiles (76 vs. 41 and 33 vs. 22, respectively), and Pocillopora significantly more abundant on forward tiles (210 vs. 259) (% 2 = , df=2,p<0.001). Use of settlement substrates Coral recruits settled on a variety of substrates on the tiles, including crustose coralline algae (CCA), partial crustose coralline algae (PCCA), other algae, tile, tube worms, vermetid snails, bryozoans, and coral (Table 5). I defined PCCA as small patches of CCA interspersed with open tile at a scale smaller than a single corallite such that the coral was on both CCA and tile. Associations with tube worms and vermetid snails were pooled due to their low frequencies, and were scored if the coral was partially touching the organism; coral recruits usually nestled in crevices of the tubes while also growing on CCA, and therefore could have been included in the CCA category. Although the relative proportion of settlement substrates varied somewhat by site, the same rankings generally applied. Pocillopora had a relatively higher proportion of recruits associated with PCCA and tile than did both Porites and faviids (Fig. 10; x 2 =18.38, df=6, p=0.005). Recruit condition and overgrowth Coral condition was assigned one of five categories based on the proportion of the recruit that was overgrown by other organisms. The majority of corals were in the 25

34 No Overgrowth category (53%), but 43% had some form of overgrowth (Table 6). 16% of coral recruits had >50% of their skeletons covered by other organisms. After grouping the condition data into two broad categories, Not Overgrown and Overgrown (Fig. 11), Rusty Bucket (RB) had significantly fewer overgrown corals (x 2 =36.46, df=5, po.ool) than all other sites (for which condition and site were statistically independent). The broad condition categories were independent of taxon, tile surface, zone, tile position, and recruit size. While a variety of organisms overgrew coral recruits, the main one was CCA (46%), followed by four groups of non-cca algae (totaling 50%), PCCA (2.3%), and a few invertebrates (1.3%) (Fig. 12; Table 7). There were no significant differences among sites in the numbers of recruits overgrown by CCA, algal filaments, and algal films (x =15.41, df=8, p=0.052). Several coral recruits seemed to be resisting CCA overgrowth by extending their epitheca upwards, with CCA growing up to the top edge of the epitheca while the remainder of the recruit remained untouched inside this barrier. When algae were grouped into CCA vs. non-cca, there were significantly more recruits overgrown by non-cca algae at sites NRF and RHC (x 2 =9.69, df=4, p=0.046) (Fig. 13). The remaining sites were independent of overgrowth category, as were coral taxa and groove vs. non-groove zones. Discussion: Settlement Densities 26

35 9 1 The coral recruitment rate of 121 recruits m" y" (13 recruits per tile pair) on Midway Atoll (28 N) was higher than reported for some high-latitude reefs in southwestern Japan (2 recruits m" 2 y~', 32 N) and the Solitary Islands, Australia (17 recruits m" 2 y"!, 30 S) (Harriott & Banks 1995, Harriott 1999, Nozawa et al. 2006). Rates from my study represent lagoonal habitats and include some sites with much higher coral diversity and cover than occurs on the more exposed forereef habitats. Therefore, these rates may be higher than the average atoll-wide recruitment. Rates I measured fall within the range of recruitment rates in the more tropical, main Hawaiian Islands (Maui: recruits m" 2 y -1 (Brown 2004); Kaneohe Bay, Oahu: 8 recruits m^y'^demers 1996)), indicating that Midway has a reasonable capacity for reef replenishment and recovery via recruitment. Most of the Hawaiian rates, however, are much lower than reported for the Great Barrier Reef (4258 recruits m" y" 1 (Hughes et al. 1999); 489 recruits m" 2 y _1 (Fisk & Harriott 1990), and elsewhere in the Indo-Pacific (Fiji: 734 recruits m" 2 y"' (Kojis & Quinn 2001); Zanzibar: 594 recruits m" 2 y"' (Franklin et al. 1998)). Over half the recruits (63%) were Pocillopora, which is consistent with other studies showing a preponderance of Pocillopora and brooding taxa over broadcast spawning species at high latitude (Harriott 1992, Harriott & Banks 1995, Harriott 1999, Glassom et al. 2004). The nearly nonexistent recruitment of Montipora despite its high cover at many sites, is also consistent with the same studies which documented reduced recruitment of Acroporidae at high latitudes. Studies in the main Hawaiian Islands have much higher proportions of Montipora recruits (approx. 27

36 75% in Maui) but these sites are characterized primarily by Montipora capitata (Brown 2004), whereas M. flabellata (cf. turgescens) is the dominant member of the genus on Midway. This species is common only in the Northwestern Hawaiian Islands, and its reproductive biology is unknown, so I cannot evaluate whether its recruitment is always low or is reduced with latitude. M. flabellata is an encrusting to foliaceous species that spreads quickly and may rely on this strategy rather than larval recruitment. Alternatively, it may prefer a different settlement substrate, experience disproportionately high post-settlement mortality or have infrequent recruitment events. Adult coral abundance was a poor predictor of recruitment, a pattern that also is consistent with other studies, and probably reflects high post-settlement mortality, episodic recruitment, and the long life of adult corals. The three sites with the highest coral cover (Fig. 6) had only the second, fourth, and fifth highest recruitment rates (Table 1). The site with the highest mean recruitment (RB) has the lowest adult coral cover (<3%) of all the sites. Species richness patterns between adults and recruits were more similar: all taxa present as recruits are present as adults at each site. The converse is not true, as several less common taxa were not seen as recruits (Pavona, Leptoseris, Psammocora, Fungiidae) and the common taxon, Montipora, was represented by only one larval recruit and two "recruitment" events via lateral growth onto the tile from a nearby colony. The lack of Montipora recruits, but dominance of adult Montipora at the northern sites was the primary driver of the dichotomy in recruit and adult abundances. At both RB and WBC the only adults are Pocillopora, 28

37 and this was the only taxon found recruiting at these sites. Concordance between richness of adults and recruits at a site may indicate some degree of local retention of recruits. Site Differences and Cyanobacteria Intra-atoll variation in recruitment was very high on Midway, with a 19-fold difference in recruit densities between the highest and lowest sites. High variation among sites is common in recruitment studies (Hughes et al. 1999, Glassom et al. 2004, Adjeroud et al. 2007), making recruitment a difficult metric to compare sites with and without anthropogenic impacts. Nevertheless, due to the high cover and toxicity of cyanobacterial blooms, I initially hypothesized that there would be lower recruitment at sites affected by iron debris and benthic cyanobacterial blooms. Contrary to my expectations, the highest recruitment was at the more impacted sites (Fig. 4A). While it seems unlikely that either cyanobacteria or metal debris would be attracting recruits, high recruitment at these sites indicates they are not substantially reducing recruitment. There are several possible explanations for the high recruitment at cyanobacterial bloom sites. The pattern may simply be a coincidence reflecting variation among sites in other characteristics. Hormothamnion may not have direct negative effects on recruits, or the ephemeral blooms may be too short to have major negative effects, or any signal of negative effects may have been overwhelmed by other, spatially covarying positive effects. Alternatively, the cyanobacteria may deter 29

38 predators that would otherwise reduce recruitment (in the absence of blooms). Predation on coral recruits is poorly documented, but many herbivores, especially parrotfishes, are believed to remove recruits incidentally while grazing (Box & Mumby 2007, Penin et al. 2010). The primary cyanobacterium forming blooms on Midway, Hormothamnion enteromorphoides, is known to deter herbivorous fishes (Gerwick et al. 1989), and this may have contributed to reduced grazing during blooms and consequently reduced mortality of coral recruits. Despite high densities of herbivorous fish at Midway, we did not observe any grazing on or around Hormothamnion blooms, which suggests it is effective at deterring herbivores which graze heavily in the absence of cyanobacteria. When the tiles were collected at Rusty Bucket, they were covered in a thick mat of turf-like algae, which has been shown to deter recruitment in other localities (Birrell et al. 2005), so the high RB recruitment was unexpected. WBC had by far the lowest recruitment (a fifth of the next lowest site), but may have been influenced by prolonged local water retention or by hidden debris, such as a large piece of lead found later near the study site. Recruit Orientation and Substrates There were substantial differences in the numbers of recruits on different surfaces of the tiles, indicating larval settlement preferences and/or post-settlement mortality were acting. On outer tile surfaces, most (69%) recruits were in grooves rather than on exposed regions (Table 4, Fig. 9), which is consistent with previous 30

39 studies (Carleton & Sammarco 1987, Nozawa 2008). Rusty Bucket had the opposite pattern; recruits were much more evenly distributed, with slightly more recruits in the exposed zone than the protected zone. Since Rusty Bucket also has very low herbivorous fish densities compared to the other sites, reduced grazing may allow recruits to survive on exposed surfaces, whereas they would be removed at highherbivory sites. At the other cyanobacterial bloom site, Reef Hotel, approximately one third of recruits were in exposed than in grooved zones, which is still a much higher proportion than other sites (Fig. 9). It is possible that the cyanobacterial blooms deterred the usually common fish herbivores, reducing incidental take of recruits and increasing the proportion seen in exposed regions. If the effect of grazing is removed by only considering recruitment into grooves, mean recruitment at Rusty Bucket would fall from first to third among sites, and Reef Hotel would move from second to fourth (Table 1). Unlike many studies in which more recruits were on the protected undersides of tiles (Fisk & Harriott 1990, Maida et al. 1994, Tioho et al. 2001), there were significantly more recruits on the exposed outer surfaces than on the protected inner surfaces (Table 4). This may be because the grooves on the outer surfaces may have attracted recruits or enhanced their survival by protecting them from incidental removal by grazers (Carleton & Sammarco 1987, Nozawa 2008). The inner surfaces were also protected from large grazers the close spacing between tiles reduced light on the innermost surfaces, which may explain the lower recruitment on these protected surfaces, especially since 82% of recruits on inner surfaces were in the 31

40 outer (higher light) zones (A and D; Table 4). Although small invertebrate predators or competition with other sessile organisms may also reduce recruitment on inner surfaces, these scenarios seem less likely than responses to light, because recruitment to the inner surface zones corresponded to the order of declining light intensity, with the highest recruitment near the brightly illuminated opening at the top of the vertical sandwich (zone A), followed by the region near the bottom (zone D), then the uppermiddle (zone B), and the lowest recruitment in the lower-middle zone C, which presumably receives the least light. Several other studies have found larval settlement to be positively correlated with light, as well as species-specific responses to light (Maida et al. 1994, Mundy & Babcock 1998). Well-lit zones also had higher cover of crustose coralline algae (CCA), some of which are known to stimulate coral larvae to settle (Harrington et al. 2004), and so may have induced higher recruitment in these zones. There were strong orientation effects, with the highest densities of recruits on the edges of tiles rather than on the larger inner and outer surfaces. Most edge recruits were on the sides and bottom edge, with very few on the top edge, which is consistent with studies finding low recruitment on upward-facing surfaces (Carleton & Sammarco 1987, Maida et al. 1994). Although certain CCA are inductive, stimulating larvae to settle, and although the majority of recruits settled on CCA (53%), a sizable number of them (16%), from all taxa represented, settled on bare tile, and even more (24%) on partial tile (PCCA), indicating that CCA is not essential for coral settlement on Midway. This agrees with 32

41 several studies in which Pocilloporidae larvae did not show a strong preference for CCA (Baird & Hughes 2000, Baird & Morse 2004). Among the coral taxa, Pocillopora had the highest settlement rates on tile and PCCA, consistent with it being less selective in substrates during settlement. Tile Community Interactions I recorded the amount of overgrowth of each recruit at the time of tile collection as an indicator of its condition and of its competitive interactions with other benthic organisms. A substantial number of recruits (36%) were at least slightly overgrown by another organism. While overgrowth probably leads to underestimates of the total number of recruits because many may have been overgrown beyond the point where they could be detected, it does provide an index of the proportion of corals involved in competitive interactions at around the time the tiles were removed. A smaller number of recruits (7%) were completely overgrown, presumably having already succumbed to post-settlement mortality due to competition. The organisms overgrowing recruits were almost entirely non-cca algae (50%) and CCA and PCCA (48%o; Table 7), although complete overgrowth by invertebrates such as bryozoans would prevent detection of the coral recruit, and so was likely underestimated. The pattern of overgrowth indicated that it was often a non-cca alga that was the first to overgrow, and then CCA followed, possibly after tissue stress or mortality in that region. 33

42 Conclusions Recruitment patterns on Midway were variable among sites, but were comparable to rates from the tropical main Hawaiian Islands. Pocillopora, a brooder, was the most common taxon and Montipora the least common, despite its high adult cover. Recruit densities did not reflect adult cover at a site, nor did recruit species richness match that of adults, although the recruit taxa were a subset of adult taxa at each site. Recruits settled on a variety of substrates and showed overgrowth indicative of competitive interactions with other benthic organisms. While blooms of the benthic cyanobacterium, Hormothamnion enteromorphoides, may have other negative effects on reef ecosystems, they do not appear to significantly reduce recruitment, since recruit densities were highest at sites with Hormothamnion blooms. Most sites had lower recruitment on exposed surfaces than in protected grooves, but sites with cyanobacterial blooms had a higher proportion of recruits on exposed surfaces, reinforcing the suggestion that the cyanobacteria deterred herbivorous fishes that removed exposed recruits, reducing recruitment at the other sites. Acknowledgements Thanks to Jamie Barlow and Don Potts for field assistance in both years; Mitsubishi volunteers Derek Jones, Vanessa Melgar, Brian Navarro and Michael Hojnacki for 34

43 tile preparation in 2006; Barbara Pimentel, Becky Ingold, Luis Alvarez, and Steve Miller for field processing of tiles in 2007; Brittany Schlotfeldt, Nick Bers, Aiko Watanabe, Eleanor Gilbert, Piara Sandhu, Gillian Parcells for scoring and measuring coral recruits; and Pete Raimondi for statistical advice. Eric Brown and Andrew Baird provided help with identifications. USFWS and Midway Atoll National Wildlife Refuge staff Barry Christenson, John Klavitter and John Miller provided access to Midway Atoll, use of its facilities, and other in-kind support. The Mitsubishi Corporation (Tokyo) provided the primary funding as part of their Global Coral Reef Conservation Project. Work was conducted under permit PMNM

44 Table 1. Total counts and mean numbers of coral recruits per tile, classified by site and taxon. Site N tile pairs Taxon: Pocillopora Pontes Faviidae Montipora unknown Total Mean recruits/tile pair, ±SD Mean recruits/ m 2 /yr RB ± RH * ± WBC ±0.8 9 RHC HKF ± NRF ± Total / Mean ± % Recruits 63.1% 15.7% 7.4% 0.4% 13.3% 100% These two Montipora were from existing colonies that spread onto the tile.

45 Table 2. One-way ANOVA for numbers of recruits per tile among six sites, and a matrix of pairwise comparison probabilities from Tukey's tests. Significant probabilities in bold font. Source Sum-of-Squares df Mean-Square F-ratio P Site Err0r < Matrix of pairwise comparison probabilities: North Reef West Beach, _, _ ^ _,.. Reef Hotel ^. Hook Far Rusty Bucket _ ^. Far Control Control North Reef Far West Beach Control Hook Far < Rusty Bucket < Reef Hotel Control < Reef Hotel <

46 Table 3. a) Numbers of recruits on forward and rear tiles at six sites, b) Two-way ANOVA for numbers of recruits per tile among six sites and two tile positions (forward, rear). A. Tile Position Rusty Bucket Reef Hotel West Beach Control Reef Hotel Control Hook Far North Reef Far Total Forward tile Rear tile B. Source Sum-of-Squares df Mean-Square F-ratio P Site O.001 Tile Position Site * Tile Position Error

47 Table 4. Mean numbers of coral recruits per m on the three tile surfaces and their subcategories (zones). N = 59 tile pairs for each category (surfaces and zones). Tile Surface Zone Total # Recruits Mean # Recruits/m 2 SE SD Outer Groove Non-groove Inner A B C D Edges Top Edge Bottom Edge Sides

48 Table 5. Numbers and percent of coral recruits settling on different substrates (ranked by abundance of recruits). CCA = crustose coralline algae; PCCA = partial crustose coralline algae. Settlement Substrate Recruit Percent Count CCA PCCA Tile Vermetid and Tube worm Algae Bryozoan Coral Can't determine Grand Total % 24.0% 15.6% 4.3% 0.3% 0.3% 0.1% 2.7% 100% 40

49 Table 6. Condition of recruits based on amount of overgrowth by other organisms. Broader grouping totals are in bold type. Recruit Condition Total Recruits Percent of Recruits No Overgrowth Slight Overgrowth Total Not Overgrown Partially Overgrown Mostly Overgrown Fully Overgrown Total Overgrown Can't be determined Grand Total % 14% 67% 13% 9% 7% 29% 3% 100% 41

50 Table 7. Frequency of recruit overgrowth by various organisms. Overgrown by Number of Recruits Relative Percent CCA other algae algal filaments algal film golden algae PCCA bryozoan tube worm sediment Not overgrown Total % 15.4% 14.5% 10.9% 9.3% 2.3% 1.0% 0.3% 0.3% 100% 42

51 I r II N NAUTICAL MILES KILOMETER" fv»- I -Tfl-V ' - >r it orthreef Midway Atoll fl IN JL'r. m GO fe;bi-j I*,' 1JJ?»,«' ' W"«l li»1 "» ^h»ji l l «# k 4 % s -.? - iwpt Hit a* N 60 IN lit-., ( W V '*^--' V^T -*->«*».4 *- 4: - ^-^^^^i^p-';-^ ww 'W *W 177 t8, W Figure 1. Study sites on Midway Atoll, Northwestern Hawaiian Islands. IKONOS satellite image; NOAA Atlas

52 A. Outer surface of tile B. Inner surface of tile Figure 2. Tile surfaces and zones used when scoring data, (a) Outer surface of paired tile "sandwich" in situ showing forward and rear tiles, central attachment (lagscrew), grooved and non-grooved zones, and three edge zones (top, bottom, and sides), (b) Inner surface of tile after bleaching, showing horizontal zones A, B, C, and D. 44

53 A Poallopora E Could not be determined i Tr '- **- - * 1 v -i ~%" v : Figure 3. Examples of each taxon recorded. Scale bars are lmm. (a) PociUopora, (b) Pontes, (c) Favudae, (d) Montipora, (e) could not be identified. 45

54 A w -*^ o (D a: "CO!= 0 Q S c 05 0) Cyano Nearby Far Sites Controls Controls Treatment B Cyano Sites Nearby Controls Far Controls U Rusty Bucket Reef Hotel West Beach Control Reef Hotel Control Hook Far North Reef Far Site Figure 4. a) Mean coral recruitment (± 1 SE) for three treatments, b) Mean recruitment per tile pair (± 1 SE) at six experimental sites. Treatment categories distinguished by shading. 46

55 Pocillopora Pontes Faviidae Montipora Rusty Bucket (9) Reef Hotel (10) West Beach Control (10) Reef Hotel Control (10) Hook Far (9) North Reef Far (7) Figure 5. Mean numbers of recruits per tile pair (± SE) for each coral taxon at each site. Number of tile pairs is in parentheses. 47

56 (/> U.4 2 o D < O fc 0.2 > o Proportion p p d -» - p^ra H Pocillopora ED Pontes E3 Favndae (Cypftasfrea) j Montipora -» H Pavona i Other * Rusty Bucket (9) Reef Hook Hotel Far Region < 9 > (13) North Reef Far (6) Figure 6. Proportion of total benthic cover comprised of adult coral colonies (± 1 SE) at four sites. Number of transects is in parentheses. Reef Hotel Region encompasses both the Reef Hotel and Reef Hotel Control sites. 48

57 30 (A "I 20h CD CD E 10-- Z3 If r.[ I V- 1 ( Colony Diameter (mm) 20 t w i_ o <o a: a> E z 5- n..llp XL O-M Number of Corallites in Colony Figure 7. Frequency distributions of coral recruits by a) size, and b) number of corallites. 49

58 >s 351 c o 0 30 O i o 2 15 o I 10 E m isni 99 #Gt> Colony Diameter (mm) H Figure 8. Relationship between colony size and number of corallites in colony. h 50

59 I^U 4 * Z3 O Groove Non-groove O * n MM- Rusty Bucket Reef Hotel West Beach Control Reef Hotel Control Hook Far North Reef Far Figure 9. Total numbers of recruits in grooved vs. non-grooved (exposed) zones of outer tile surfaces at each site. 51

60 Pocillopora Pontes Faviidae 100» _ CCA PCCA Tile Tube worm or vermetid Substrate Figure 10. Numbers of recruits in three coral taxa that settled on the four most common substrates. 52

61 150 Not Overgrown Overgrown 100 1P&& ^i^ip 50 Rusty Bucket Reef Hotel West Beach Control Reef Hotel Control Hook Far North Reef Far Figure 11. Numbers of recruits that were Overgrown and Not Overgrown at each site. 53

62 Figure 12. Examples of various organisms overgrowing coral recruits, (a) Porites colony mostly overgrown by CCA, (b) algal filaments, (c) recruit fully overgrown by algal film, (d) mostly overgrown by algal film, (e) golden algae, (f) bryozoan overgrowing Pocillopora. Scale bars are 1mm. 54

63 C/3 50 \J\J O 0 L< I. 1 j 1 1 [ 1, Organism Overgrowing _, CCA m All Other Algae " ~J fcrt * ' '*,-. 1. c -t.', ' 1._ 1 rm J F Ru sty Bucket Re sef Hote West Beach Control Reef Hotel Control Hook Far North Reef Far Figure 13. Numbers of recruits overgrown by CCA or by Other Algae. The All Other Algae category includes algal filaments, film, golden, PCCA, and other (Table 7). 55

64 Appendix. Additional organisms noted on settlement tiles. Organism Site Notes amphipods crab goby RHC RHC RHC E. mathaei urchin turban snail numerous invertebrates, esp. crabs oysters (3) cowrie (Cypraea) brittle star E. mathaei urchin thick algal mats RHC RHC WBC WBC WBC HKF HKF RB Small; in between tiles. Grazing marks visible on CCA. Mid-sized On outer surfaces of most tiles 56

65 CHAPTER 2 Differentiating Impacts of Fish and Urchin Grazing on Algal Growth and Coral Recruitment Wendy A. Cover Abstract Both fishes and urchins can be important herbivores on corals reefs, maintaining resilience through their grazing activities and preventing coral-macroalgal phase shifts. Despite their importance, little is known about the relative importance of these different herbivore guilds. I conducted a factorial field experiment to quantify the relative effects of herbivorous fishes and urchins (Echinometra mathaei) on macroalgal growth and coral recruitment in backreef habitats of Midway Atoll, a protected Northwestern Hawaiian Island reef with largely intact fish assemblages and abundant urchins. Algal growth and coral recruitment were measured on ceramic tiles that were open, partially caged, caged with urchins, or fully caged. Fish grazing effectively limited algal biomass over months (0.5 ± 1.3g dry wt. per tile). Algal biomass was >50 times higher in treatments with no grazers (69.8 ± 81.9 g) and treatments exposed only to urchins (53.5 ± 63.7g). Urchin grazing reduced algal biomass only at higher urchin densities (3 or 4 per tile). Coral recruitment was >2X 57

66 higher in treatments exposed to fish grazing than those exposed to urchin grazing or no grazers, indicating that algae inhibit coral recruitment much more than does fish grazing. Herbivorous fishes apparently have a strong positive but indirect effect on coral recruitment by reducing algal growth. Algal biomass was negatively correlated with coral recruitment, suggesting that management to increase herbivorous fishes and grazing levels is likely to benefit coral recruitment. This study underscores the importance offish herbivores in removing macroalgae and promoting coral recruitment, enhancing the resilience of reef communities. Introduction Herbivores play important, often critical roles in the structure, functions, and dynamics of benthic community dynamics in aquatic systems. On coral reefs, herbivores consume algae that compete for space with the framework-building corals, and by allowing space for corals to recover following disturbances (e.g. hurricanes or bleaching events), grazing promotes resilience in reef communities (Nystrom et al. 2000, Hughes et al. 2007b). Beneath a critical threshold of grazing intensity, reefs loose their resiliency and can shift to an alternate, coral-impoverished state characterized by much higher densities of macroalgae (Mumby et al. 2007b, Mumby 2009b). Promoting healthy herbivore populations through protection, as in no-take marine reserves, has been shown to enhance coral recovery (Mumby & Harborne 2010) and is increasingly a focus of management efforts. 58

67 Given the primary role of herbivores in maintaining resilience on reefs facing numerous natural and anthropogenic threats, management efforts should be enhanced by understanding the relative grazing contributions of different herbivore guilds, as well as any negative effects that may result from their grazing activities. Fish and urchins are the dominant reef herbivores, but species composition and abundances of herbivore guilds varies with habitat (e.g. forereef vs. backreef), region (e.g. Diadema antillarum is the dominant urchin herbivore in the Caribbean, but does not have an equivalent in the Indo-Pacific), fishing method (e.g. spearfishing may preferentially target parrotfish), and fisheries management (e.g. certain urchins common in fished areas in Kenya are uncommon in marine reserves with high fish biomass (McClanahan et al. 1994)). Because of this variability, it is important to understand the contribution of particular species or guilds present at a given site, since not all herbivores have equivalent impacts (Burkepile & Hay 2008). Most experimental comparisons of fish and urchin grazing on coral reefs have been conducted in the Caribbean, where the urchin Diadema antillarum has a greater impact than herbivorous fishes on shallow reefs (Carpenter 1986, Foster 1987, Morrison 1988). However, these studies were all in heavily fished regions with relatively few herbivorous and predatory fishes, and high urchin densities; on lessimpacted reefs with few urchins and more fish, different outcomes have been seen (Hay 1984). When abundant, Diadema is a formidable herbivore that is credited with maintaining resilience and preventing phase shifts on overfished Caribbean reefs until its populations collapsed from disease in After the collapse, many coral 59

68 populations failed to recover from hurricane damage and these reefs shifted to a macroalgal-dominated state (Hughes 1994). Temperate urchins on the Pacific coast of North America also exert strong effects on kelp communities (Estes & Palmisano 1974, Harrold & Reed 1985), but no other tropical urchin has been shown to have an impact equivalent to that of Diadema antillarum. Although most urchin species are herbivores, they vary in their feeding preferences and ability to reduce macroalgal biomass (McClanahan et al. 1994). The positive effect of grazing on corals is indirect, and mediated via competitive interactions between corals and algae. Contact with algae (perhaps mediated by microbes (Smith et al. 2006, Vermeij et al. 2009)) can cause bleaching and mortality in adult corals (Jompa & McCook 2002, Rasher & Hay 2010) and has triggered coral disease (Nugues et al. 2004). Coral recruits can also be negatively affected by turf algae and macroalgae, which may prevent access to suitable surfaces for larval settlement or initiate avoidance behaviors in larvae, as well as inhibiting growth and increasing mortality of settled recruits (Kuffner et al. 2006, Box & Mumby 2007, Arnold et al. 2010). In some studies, reduction in macroalgal cover due to recovery of fish and urchin grazers, facilitated coral recruitment (Carpenter & Edmunds 2006, Mumby et al. 2007a). While grazers may indirectly help coral recruitment by removing competitive algae, they may also harm recruits directly by consuming them during grazing activities. Several studies have found more coral recruits on surfaces protected from grazers (Gleason 1996, Adjeroud et al. 2007, Nozawa 2008)(Chapter 1), and Penin et 60

69 al. (2010) found a positive correlation between recruit mortality and the local density of parrotfishes (Scaridae). The hard dentition of parrotfishes makes them more likely than many other fishes to remove coral recruits incidentally while feeding, but even small grazing blennies can reduce survival of single-polyp recruits (Christiansen et al. 2009). Sea urchins can have strong, calcareous mouthparts and some species have been implicated in consumption of adult coral tissues in some locations (Bak & van Eys 1975, Glynn et al. 1979), and survival of coral recruits can be suppressed under high Diadema densities (Sammarco 1980). In one study starting with six-week old coral spat, survival after three months was three times higher in caged than uncaged treatments, likely due to exclusion of herbivores and corallivores (Baria et al. 2010). While herbivores can have both positive and negative effects on corals and coral recruits, it is unknown in most cases whether consumption or competition is more important in determining recruit survival, and whether fishes or urchins have the major impact. This study examines the relative roles of herbivorous fishes and urchins {Echinometra mathaei) in controlling the growth of algae and the recruitment of corals on settlement tiles by employing a caging experiment with orthogonal treatments including and excluding fishes and urchins. Methods Location The study was conducted in the shallow backreef (1-2 m) habitat of Midway Atoll, an isolated, high-latitude (28 N) atoll in the Northwestern Hawaiian Islands, 61

70 that is part of the Papahanaumokuakea Marine National Monument. Midway has been protected from reef fishing during its 23 years as a National Wildlife Refuge, and the U.S. Navy is reputed to have prohibited reef fishing during most of its tenure from due to a high incidence of ciguatera fish poisoning. Consequently, Midway has an intact trophic structure with high densities of herbivorous fishes (Friedlander & DeMartini 2002), primarily parrotfishes and surgeonfishes, making it an ideal place to study grazing impacts. The study was conducted at a site in the north backreef ( N, W) with high coral cover (36%) compared to the southern backreef (3%), and moderate urchin densities (4 m" ). The primary coral genera are Montipora, Pocillopora, and Pontes, with colonies largely restricted to patch reefs (1-3 m wide) separated by sand and hard substrate. An uplifted ancient reef crest protects the site from heavy waves, but currents can sometimes be strong. Experimental Design A balanced 2x2 factorial design was used to differentiate between the impacts of herbivorous fish and herbivorous urchins on algal growth and coral recruitment (Fig. 1). Ceramic tiles were deployed as replicate settlement substrates for algal and coral propagules, and different kinds of cages around each tile included or excluded fish and/or urchins. The four treatments were: Fish & Urchin - no cage, allowing free access by both fishes and urchins; Fish Only - a half cage, with a fence preventing urchin entry but allowing fishes access from above; Urchin Only - a full cage enclosing 4 urchins (Echinometra mathaei) while excluding fishes; and No 62

71 Herbivore - a full cage preventing access by both fishes and urchins. The half-cage design of the Fish Only treatment also served as a cage control, although a pilot experiment with a cage control that had a partial top and two sides showed no difference from the uncaged treatment, and other caging studies in Hawaii have also shown no effect of the cage on algal growth (Smith et al. 2001). There were 20 replicates of each treatment, for a total of 80 settlement tiles. The tiles were unglazed, terra-cotta flooring tiles (~14 x 15 x 1.4 cm). One surface had 1.5 x 2.5 mm grooves alternating with non-grooved surfaces, while the other surface was smooth. Tiles were deployed with the grooved surfaces facing upwards and the smooth surfaces facing the natural substrate. Similar amounts of the grooved upper surfaces consisted of groove (138 cm ) and exposed portions (157 cm 2 ). The grooves provided spaces where recruits might be protected from herbivores. Full cages (Urchin Only and No Herbivore treatments) were supported by a hard PVC plastic frame (40 cm tall and 30 cm diameter). The frame was made in one piece by cutting three panels from a section of PVC sewer pipe, leaving three upright lengths connecting an upper and lower ring. The sides and top of the frame were covered with separate pieces of green, plastic, garden fencing (2.5 cm square holes) attached with small cable ties (Fig. 1). Plastic mesh was used to prevent cyanobacterial growth that can occur on metal mesh hardware cloth (Ch. 1). Half cages (Fish Only treatment) had a partial PVC frame consisting of a 30 cm diameter ~3 cm high ring supporting a wall of plastic fencing mesh approximately 8 cm tall. 63

72 The Fish and Urchin treatment had no frame. Each tile was attached with cable ties to a plastic mesh base (25 x 25 cm with 2.5 cm holes). The full and half cages were attached to their mesh bases with cable ties, and the entire unit was secured to a hard rocky substrate using large stainless steel lag screws inserted into plastic anchors in holes drilled into the rock (Fig. 1). Each unit was placed in close proximity (0.7 ± 0.7 m) to small patch reefs with live coral and algae to ensure adequate supply of algal and coral recruits. Tiles were assigned randomly to treatments and positions within sites. Rock-boring urchins (E. mathaei) were collected from the reef near the experimental sites, and four urchins were enclosed in each Urchin Only replicate. For comparison of urchin densities between the cages and natural densities seen on the reef, the area of the base of the circular cages was calculated using a radius of 15 cm. All tiles and cages was deployed between 8 Oct and 13 Oct 2007, left to gather recruits for months, and retrieved between 18 Aug and 16 Sep Retrieval and Processing Cages and tiles were retrieved by clipping the cable ties attaching them to the lag screws, and placing the entire unit in a large plastic garbage bag for protection and to prevent loss of algae while transferring them onto the boat and back to shore. In the lab, the cages were clipped open, tiles were clipped off of the base mesh, and both sides of the tile were photographed. 64

73 All macroalgae inside the cages and on the tiles were removed, separated while fresh into genera or morphospecies, hand dried with a towel, weighed, and then dried in a domestic oven (32-60 C). Sediments and small turf algae were scraped from the tiles and wet weights taken. Turf algae were defined as any small, finely filamented alga. Tiles were then rinsed and dried in the sun. Samples and tiles were later transported to U. C. Santa Cruz where the algae were further dried in an oven at 60 C for at least 24 hrs, cooled in a vacuum dessicator, and weighed on an analytical balance. Each tile was searched completely for coral recruits under a dissecting microscope at 6x - 25x power. Records included the position of each recruit (top, bottom, or sides of tile), and, for recruits on the tops of tiles, whether it was on an open surface or in a groove. Light Measurements Because algal growth requires adequate light, spectral quality and light intensity reaching the tiles were measured at the end of the experiment using a spectroradiometer (GER 1500, Spectra Vista Corporation; 1.5 nm band widths) in an underwater housing. Data were taken from two full cages (with minimal macroalgal growth), one half cage, two open tiles, and under one natural overhang to provide comparison with naturally low light levels in which algae were present. Reference (incident irradiance) and target (reflected radiance) data were visualized using the 65

74 9 1 1 program Gerplot (NASA), and the integrated radiance values (W cm" nm~ sr" ) across the spectrum were used for comparisons. Analyses Algal wet weight data was analyzed using one- and two-factor ANOVAs with General Linear Models. A log (x+1) transformation aided with normality and homoscedacity of variances when comparing total algal biomass among all treatments. Transformation was not needed when comparing the Urchin Only with No Herbivore treatments. Wet weight data was used instead of dry weights because some dry samples were lost. Turf biomasses were square root transformed. Further analyses with GLM hypothesis tests were used to make more specific comparisons between treatments. Numbers of coral recruits were analyzed using ANOVA (GLM) with hypothesis tests, after square root transforming the data to conform to statistical assumptions. Results Within a week of deployment, all tiles in both full-cage treatments (with and without urchins) had grown a uniform layer of bright green turf algae which was absent in open and half-caged treatments; this turf layer persisted for at least another week (after which cages were left to overwinter). 66

75 Between October 2007 and August 2008, one Urchin Only treatment (n = 19) cage and two No Herbivore (n = 18) cages were lost, and all tiles and half cages remained from the Fish & Urchin (n = 20) and Fish Only (n = 20) treatments. Some urchins chewed through the plastic mesh caging material and escaped. When cages were retrieved in August 2008, ten of the cages in the Urchin Only treatment retained at least one urchin inside; only the cages with urchins still present (n = 10) were used to represent this treatment. One cage retained all four urchins, three cages held three urchins, two cages held two urchins, and four cages held one urchin. Algal Growth Algal biomass (wet weights) differed significantly among treatments (ANOVA df = 3, F = , p < 0.001). Treatments protected from fish grazing (Urchin Only & No Herbivore) had 50- to 70-fold more algae than those exposed to fish (Fish Only and Fish & Urchin) (Fig. 2; Table 1; 2-factor ANOVA, df = 1, F = , p < 0.001). The presence of urchins also significantly reduced algal biomass (2-factor ANOVA, df = 1, F = 6.160, p = 0.016), but to a lesser degree than for fishes. Urchins added a smaller, but not quite statistically significant reduction in algal biomass in Urchin Only cages compared to No Herbivore cages (GLM hypothesis test, df = 1, F = , p = 0.056), while algal biomass was very similar between the two treatments exposed to fish (Fish Only and Fish & Urchin) (Fig. 2; GLM hypothesis test, df = 1, F = 2.364, p = 0.129). 67

76 The three most abundant algal types, Gracilaria, Lobophora, and turf/ thin red branching accounted for over 97% of all algal biomass (Table 2). Most algal types had similar proportions between the No Herbivore and Urchin Only treatments, except for turf/ thin red branching algae which were significantly more abundant in No Herbivore cages than in Urchin Only cages (Fig. 4; one-way ANOVA, df = 3, F = , p < 0.001; GLM hypothesis test, df = 1, F = 9.988, p = 0.002). There was significantly more turf in Urchin Only cages than in both treatments exposed to fish (GLM hypothesis tests; Fish Only: df = 1, F = , p < 0.001; Fish & Urchin: df= 1, F = , p < 0.001). Three morphotypes of cyanobacteria (green, tan, and general) were grouped together in the "cyanobacteria" category. Cyanobacterial biomass may have been overestimated because it was difficult to distinguish what was growing outside vs. inside the cages, and some external cyanobacteria may have been included. Cages in the Urchin Only treatment contained from zero to four urchins at the end of the experiment. Algal biomass declined with increasing urchin density only after passing a threshold of two urchins (Fig. 3 or Table Z). Algal biomass was significantly lower in the cages with three or four urchins than in cages with two or one urchins (ANOVA, df = 4, F = 4.774, p = 0.012; GLM hypothesis test, df = 1, F = , p = 0.002), and lower in cages with three urchins than with two urchins (GLM hypothesis test, df = 1, F = 8.984, p = 0.010). The amount of sediment on tiles also varied significantly among treatments (Fig. 5; one-way ANOVA, df = 3, F = , p O.001), with 2-3 times as much on 68

77 the two treatments exposed to fish than on the Urchin Only and No Herbivore tiles (Table 3; 2-way ANOVA; df = 1, F = , p < 0.001). These later two treatments did not differ significantly in their sediment loads (GLM hypothesis test, df = 1, F = 1.935, p = 0.169). Cages with high algal biomass were usually characterized by dense clumps of macroalgae that had many attachments to the cage itself rather than to the settlement tile: the macroalgae were attached to the tile in only one of 32 cages with macroalgal clumps. Gracilaria was unusual because many of its attachments were directly to Lobophora. In a few cages, the macroalgae formed free-floating masses that were not attached to either the cage or the tile. The area of the base of the cages and half-cages was 707 cm (0.07 m ). At the average density of urchins at this site (4 m" ), this equates to 3/10 of an urchin per cage. The highest average density of urchins at a site, 34 m" in the southwest backreef, equates to 2.4 urchins per cage. The highest density in this experiment, 4 urchins per cage, is equivalent to 57 urchins m", which is sometimes seen in the most dense patches. Coral Recruitment A total of 159 coral recruits were recorded from 59 of 77 tiles. Overall coral recruitment was low (2.1 ± 2.0 SD recruits per tile) but the number of recruits differed significantly among treatments (Fig. 6; one-way ANOVA, df = 3, F = 2.990, p = 0.037), with more recruits in treatments exposed to fish (Fish Only and Fish & 69

78 Urchin) than in treatments protected from fish (Urchin Only & No Herbivores) (Table 4; 2-way ANOVA; df = 1, F = 8.082, p = 0.006). There were no significant differences between the two treatments exposed to fish (GLM hypothesis test, df = 1, F = 0.010, p = 0.919) or between the two treatments protected from fish (df = 1, F = 0.332, p = 0.567). On the upper surfaces of tiles, there were significantly more recruits in grooves (0.35 ± 0.89 SD recruits per tile) than on non-grooved surfaces (0.06 ± 0.29 SD recruits per tile) (one-way ANOVA, df = 1, F = 9.985, p = 0.002), despite the slightly lower area of grooved surfaces (138 cm vs. 157 cm ). Treatment did not significantly affect the number of recruits in grooves or non-grooves. There were a total of 31 recruits on upper tile surfaces, 49 on lower (smooth) surfaces, and 79 on the edges. Recruitment was particularly low in the No Herbivore (1.4 ± 1.3 SD) and Urchin Only (1.2 ± 1.8 SD) treatments. There was no obvious relationship between the number of urchins and the number of recruits in the Urchin Only cages (one-way ANOVA, df = 4, F = 0.621, p = 0.652). While most recruits on tiles were very small ( mm), 25 larger colonies (approx. 25 to 75 mm) recruited to the mesh underneath the tiles or comprising the sides of cages on 22 different replicates. Eighteen colonies were Montipora (14 of them clearly M. flabellata [cf. turgescens]); the others were growing under very low light conditions and had atypical morphologies, making positive identification difficult, but are also likely Montipora. 70

79 Relationship Between Algal Biomass and Coral Recruitment The number of coral recruits was significantly negatively correlated with the wet biomass of algae (Fig. 7; Linear regression, R 2 = 0.157, df = 1, F = , p < 0.001). Light Reaching Tiles The amount of light reaching the tiles with and without cages, in half cages, and under a natural overhang was measured and integrated across spectra ( nm). The lowest spectral readings for uncaged tiles were very similar to the highest readings for fully caged tiles (Fig. 8). Integrated values and variances were highest among the open (uncaged) readings, which ranged from 11.3 x 10 6 to 36.8 x 10 6 W cm" 2 nm _1 sr" 1. Beneath full cages, values ranged from 7.6 x 10 6 to 10.6 x 10 6 W cm" 2 nm" 1 sr" 1. While readings for half cages were lower than this (5.9 x 10 6 W cm" 2 nm" 1 sr" 1 ), they were not as low as under the natural rock overhang (2.0 x 10 6 W cm" 2 nm" 1 sr" 1 ) (Fig. 8). Cage Observations At the end of the experiment, urchins in the Urchin Only treatment were usually located on the tile or around its edges, but in a few cases, urchins were living on the upright side ribs of the cage and had cleared out urchin-sized "tunnels" (approx. 30 cm long) in the dense macroalgae filling the rest of the cage. 71

80 Numerous other organisms were inside cages, including many small crustaceans, a small sea cucumber, a juvenile wrasse, a juvenile lobster, two juvenile Echinometra mathaei urchins underneath a tile, a cone shell and eggs, and a brittle star. Two cages contained a Hawaiian whitespotted toby (Canthigaster jactator) that was much larger than the 2.5 cm mesh size, and evidently had entered the cages as small juveniles and grown to a size at which they could not escape. These fish are reported to consume primarily zoobenthos (78%) with some algae and detritus (22%) (Randall 1985). Discussion In this experiment, fishes had a > 50-fold larger impact on macroalgal biomass than did urchins {Echinometra mathaei). Tiles exposed to fish grazing had little to no macroalgal growth after a year of deployment, whereas tiles protected from fish had variable, but often substantial macroalgal growth, whether urchins were present or not (Fig. 2). Final urchin densities varied within the Urchin Only treatment, and the cages with more urchins had significantly less macroalgae and turf (Fig. 3), indicating that higher urchin densities have a greater impact on macroalgae. Urchin effects were not detectable until densities reached three or four urchins per cage, which equate to higher densities than average at sites with the most urchins, but are within the range observed on the backreef That urchins had an effect only at 72

81 higher than average densities indicates that fish have a greater effect than urchins in this system. Large-bodied spectacled parrotfish (Chlorurus perspicillatus) and big schools of surgeonfishes (various spp.) are common on the backreef and were observed grazing on exposed tiles and cages. After many years of complete protection from reef fishing, Northwestern Hawaiian Island reefs have fish stocks 260% greater than in the main Hawaiian Islands (Friedlander & DeMartini 2002) and are therefore should reflect how these systems function in their more natural, undisturbed state. Large-bodied fishes tht could not enter cages presumably have a bigger grazing effect than their smaller-bodied counterparts. While juvenile parrotfish were observed swimming through the 2.5 cm mesh of the cages and grazing on the tiles, these juveniles did not graze enough to prevent macroalgal growth in cages. Urchin impacts were more spatially limited than for fishes. In some cages, the only areas clear of macroalgae were in the area immediately surrounding an urchin. E. mathaei is a bioeroder that carves out "channels" in the rock surfaces: opportunities to carve channels did not exist within the experiment, but some urchins created a different sort of channel by clearing urchin-sized tunnels in the algae along the side of the cage. Urchin grazing did not seem to affect the taxonomic composition of macroalgae growing inside cages; both cages with and without urchins had very similar types and amounts of macroalgae (Table 2). 73

82 The full cage treatments not only had much higher macroalgal biomass, but also had significantly lower coral recruitment than treatments exposed to fish, regardless of the presence of urchins (Fig. 6). Macroalgae are known to inhibit coral larval settlement and decrease survival of coral recruits, but the grazing activity that limits macroalgae has also been implicated as a mechanism for removing recruits. In this study, grazing by fishes may have removed some coral recruits, but any negative effects offish grazing were completely overshadowed by the negative impacts of macroalgal growth when fishes were excluded. Coral recruitment was negatively related to algal biomass across all treatments and replicates (Fig. 7). The macroalgae may have deterred settling larvae by physically preventing access to the tiles, through chemical or microbial means (Smith et al. 2006, Vermeij et al. 2009) or by slowing water flow regimes and limiting the supply of larvae to the tile (cite). Alternatively, larvae may have settled then died post-settlement. However, this seems less likely since coral skeletons should persist after death, and would have been seen when tiles were examined. Coral recruitment is notoriously variable, even within a site (Hughes et al. 1999), but in this study, the variation in coral recruitment declined with increasing macroalgal cover, i.e. as macroalgae increased, the maximum number of coral recruits declined (Fig. 7). This suggests that with more macroalgae present on a reef, there will be less recruitment; therefore, any increase in grazing intensity is likely to have a positive effect on recruitment. 74

83 While tiles exposed to fish grazing had less algae, they also had higher sediment loads. Sediment can hinder settlement (Hodgson 1990, Birrell et al. 2005), but it apparently was less inhibiting than macroalgae, since recruitment was higher on these tiles. In this experiment E. mathaei did not seem to affect coral recruits, since coral numbers in Urchin Only and No Herbivore treatments were similar. However, not all urchins moved across the entire surface of the tiles (some made tunnels in macroalgae along the cage side), so their behavior may have lessened the impact of urchins on coral recruits. In some respects, the caging mesh may have been a better recruitment substrate than the terra-cotta tiles, since large colonies of Montipora grow on the mesh in all treatments, and macroalgal clumps were often attached to the mesh rather than to the tiles. The largest coral recruits (>25mm) were all Montipora colonies growing on the mesh caging material; no recruits on the tiles reached nearly the same size (all < 3mm). Since light levels influence macroalgal growth (Coutinho & Zingmark 1993), we measured the reflectance of light from tiles under open, half caged, and caged scenarios. Light levels varied more due to temporary changes in cloud cover and focused light from surface ripples than the from the cage material itself. The large difference between high and low reflectance measurements from uncaged tiles (Fig. 8), along with the very low reflectance from the half caged tile, indicate the broad range of natural light levels on a typical, partly-cloudy day in the shallow backreef. 75

84 The high values under the caging material were very similar to the lowest reading on open tiles, and the low value under a full cage was still higher than the reading for the half cage, further emphasizing the natural variability in light levels. All reflectances from tiles were higher than that recorded under a small rock overhang nearby that had macroalgae present, so light should not be limiting for macroalgal growth. In summary, in an intact Hawaiian reef system, fishes are the dominant herbivores, greatly outgrazing the abundant Echinometra mathaei urchins, and largely preventing growth of macroalgae. In addition to being highly effective grazers, these fishes also promote coral recruitment, probably by limiting competition with macroalgae. The direct negative relationship between algal density and coral recruits indicates that any reduction in algal cover will assist coral recruitment and facilitate reef recovery from natural and anthropogenic perturbations. 76

85 Table 1. 2-factor ANOVA comparing algal wet weights in factorial treatments exposed to fishes and urchins. Source Type III SS df Mean Squares F-ratio p-value Fishes I Urchin Urchin*Fishes Error

86 Table 2. Effect of urchins and fishes on species composition of algae in each treatment; mean wet weight (g ± SD), ranked by overall abundance. Algal Type Gracilaria Lobophora Turf/Thin red branching Cyanobacteria Hydroclathrus Hypnea ' Codium Dictyota Ulva Cladophora Cryptonemia Total Percent Fishes and Urchins Mean SD Fishes Mean Only SD Urchins Only Mean SD No Herbivores Mean SD Total SD Percent '

87 Table 3. 2-factor ANOVA comparing sediment wet weights in factorial treatments exposed to fishes and urchins. Source Type III SS df Mean Squares F-ratio p-value Urchin Fishes Urchin*Fishes Error

88 Table 4. 2-factor ANOVA comparing the number of coral recruits in factorial treatments exposed to fishes and urchins. Source Type III SS df Mean Squares F-ratio p-value Urchin Fishes Urchin*Fishes Error

89 Urchins No Urchins *V * LIU*.**** '» ^ ^ ^ JIT W W - - LV6*I '"'" I $ a m v w».w w - - m Figure 1. Diagram of the 2x2 factorial design with four treatments: a) Fish and Urchin, b) Urchin Only, c) Fish Only, and d) No Herbivore (No Grazers). Photographs were taken just before cage removal. 81

90 100 D) * 50 I I- Fish & Fish Urchin No Urchin Only Only Grazers Treatment Figure 4. Biomass of turf algae (wet weight ± ISE) in the four treatments. Letters indicate significant differences in GLM hypothesis tests (a:b, p <0.001; b:c, p = 0.002). 82

91 Fish & Fish Urchin No Urchin Only Only Grazers Treatment Figure 5. Mass (wet weight ± 1 SE) of sediments on tiles in the four treatments. Letters indicate significant difference in GLM hypothesis tests (p < 0.001). 83

92 4-i 3- (fi o - -i «- 1-0 Fish& Urchin Fish Only Urchin Only No Grazers Treatment Figure 6. Numbers of coral recruits per tile (± ISE) in the four treatments. Letters indicate significant differences in GLM hypothesis tests (p = 0.004). 84

93 Algal wet weight (g) Figure 7. Number of coral recruits as a function of algal biomass (wet weight). The red line is the best fit estimate, blue lines are control limits, and black lines are probability limits (R 2 = 0.157, p < 0.001). 85

94 (/> CNJ E o Open (High) Open (Low) Cage (High) Cage (Low) Half Cage Overhang 0 O c CO CO a: <& <& t^ & & <&tfp op N^V Wavelength (nm) Figure 8. Quantitative spectra of light under three treatments (open, full cage, and half cage) and a natural rock overhang. High and Low refer to the highest and lowest readings for this category. Key is listed in decreasing order of magnitude. 86

95 CHAPTER 3 Direct, species-specific impacts of sea urchins on live corals Abstract Interactions between species can shape major processes that influence coral cover and community structure on coral reefs. Grazing by sea urchins on macroalgae has strong indirect benefits on coral cover and reef resilience. While there is much research investigating urchin bioerosion of rock, direct impacts of urchins on live coral tissue are rarely studied. I conducted two experiments on the backreef of Midway Atoll (NW Hawaiian Islands) investigating direct, species-specific impacts of two abundant, bioeroding urchins (Echinometra mathaei and Heterocentrotus mammillatus) on three coral species {Pocillopora ligulata, Montipora c.fturgescens, and Porites lobatd). I epoxied nubbins of each coral species inside bioeroded channels of E. mathaei and the eroded surfaces of//, mammillatus after removing urchins from half the plots. Most fragments of all three coral species exposed to E. mathaei were damaged within days and the damage increased over several weeks to months, often ending with complete removal of all coral tissue and skeleton. Fragments in plots without E. mathaei, and all fragments exposed to H. mammillatus plots were unaffected. These results indicate that grazing by E. mathaei can have direct negative impacts on live corals, while H. mammillatus has little or no effect. 87

96 This differential grazing has important implications for recruitment success of corals, community structure and reef growth. Introduction Consumers may shape community structure directly through consumption of their prey, or indirectly, as in trophic cascades, where predators benefit primary producers by consuming their herbivores (Terborgh & Estes 2010). Some consumers may have mixed effects: causing direct harm to a foundational species while indirectly benefiting it via effects on a third species. In the ecological literature, interactions between sea urchins and corals are often treated only as indirect processes, mediated through urchin herbivory on macroalgae, a primary competitor with corals (Edmunds & Carpenter 2001, McCook et al. 2001b). By preventing macroalgae from establishing after a disturbance that causes coral mortality (e.g. bleaching, hurricane), herbivores allow coral to recover, enhancing resilience of the reef (Diaz-Pulido et al. 2009). It is widely accepted that grazing by diademnid urchins maintained the resilience of Caribbean reefs that had few remaining herbivorous fishes by the late 1970's. In 1983, a mass-mortality event decimated the urchin populations, reducing the resilience of the reefs, and the subsequent loss of coral cover from a hurricane resulted in a shift to a macroalgaldominated state (Lessios et al. 1984, Hughes 1994). 88

97 Most geological literature treats urchins as bioeroders, carving away the limestone foundation of reefs. Urchin bioerosion rates are measured by the amount of carbonate material passing through the gut (Bak 1994, Asgaard & Bromley 2008), sometimes giving estimates much higher than reef accretion rates and leading to predictions of collapse of the reef structure (Bak 1990). Bioeroding urchins consume a high proportion of CaCCb; in one study, Echinometra mathaei gut contents contained 73% CaCCh, 20% organic matter, and 7% refractory organic matter (Mills et al. 2000), and Carreiro-Silva and McClanahan (2001) concluded that urchin bioerosion in Kenya was greater than herbivory for all four urchin species studied, including E. mathaei. The rock-boring urchin, Echinometra mathaei, has a broad tropical Indo- Pacific distribution and is often very abundant. Each individual resides in a "channel" (about cm long and the width of its test and spines), created by scraping into the rock with its mouthparts (Aristotle's lantern), and variously referred to as a "boring", "groove", "burrow", "crevice", or "cavity" (Hart & Chia 1990, McClanahan & Kurtis 1991, Asgaard & Bromley 2008). Although sometimes treated solely as an herbivore (Ogden et al. 1989, Mills et al. 2000), E. mathaei also consumes floes of aggregate organic matter from the water column (making it a detritivore as well as an herbivore), "farms" algae within its boring, and traps bits of drift algae (Huettel et al. 2006, Asgaard & Bromley 2008). Another important bioeroding species is the red pencil urchin, Heterocentrotus mammillatus, which does not create channels for itself but does bioerode by scraping at the rock surface (Dart 89

98 1972). Bioerosion by urchins on dead carbonate substrates may be intense, and is well documented, but direct impacts of urchins on live coral tissue are rarely reported. However, the bioerosional capabilities of these urchins make them likely candidates to be predators of live corals. Ecologists usually treat urchins as herbivores or detritivores (Asgaard & Bromley 2008), but three studies have described predation of live tissues on adult coral colonies. Bak and van Eys (1975) described a site with high coral cover and unusually high densities of Diadema antillarum (8.5 individuals m" ) where 8% of the urchins were feeding on live coral; Sammarco (1982) described both D. antillarum and Echinometra viridis damaging larger coral colonies; and in the Galapagos, where predators are rare and Eucidaris thouarsii occurs at high population densities (10-50 individuals m" 2 ), it feeds heavily on live pocilloporid corals, unlike its mainland counterparts that eat marine plants and sponges (Glynn et al. 1979). Urchins may also directly affect coral recruits through what is generally thought to be incidental take during grazing activities, rather than by deliberate acts of predation. Coral recruitment may be optimized at intermediate densities of Diadema, with urchins at high densities consuming coral recruits, and macroalgae hindering recruitment at low urchin densities (Sammarco 1980, 1982). These observations are consistent with the larger conceptual framework of the Intermediate Disturbance Hypothesis (Connell 1978), where high disturbance by Diadema reduces recruitment directly, and low disturbance by Diadema allows competitive exclusion of recruits by macroalgae. More recent studies following recovery of Diadema populations in the 90

99 Caribbean support this idea: coral recruitment and growth of juveniles appears higher in regions where Diadema has recovered to what were previously considered "intermediate" densities than where Diadema remains rare or absent (Edmunds & Carpenter 2001, Macintyre et al. 2005, Carpenter & Edmunds 2006, Idjadi et al. 2010). Direct and indirect interactions between urchins, algae, and corals can be species-specific, but have not been studied extensively in urchins other than Diadema. McClanahan and Muthiga (1988) reported lower coral cover on Kenyan reefs with high densities of Echinometra mathaei, implying that heavy grazing reduces coral cover. Conversely, Dart (1972) suggested that E. mathaei and H. mammillatus indirectly benefit recruitment of corals in the Red Sea by removing turf algae that inhibit settlement (Arnold et al. 2010). Others have suggested the bioeroded channels created by E. mathaei may facilitate establishment of recruits (Birkeland & Randall 1981). In a specific experimental comparison of D. antillarum and E. viridis, high densities of D. antillarum suppressed coral recruitment, while high densities of E. viridis did not affect recruitment (Sammarco 1982). Speciesspecific effects have also been noted in the Florida Keys where sites with recovering D. antillarum populations were associated with lower macroalgae and higher coral cover, but sites with high densities of E. lucunter had high macroalgal cover and few corals (Furman & Heck 2009). Reports of direct impacts of urchins on live corals are mixed, and estimates of the amount of coral tissue removed over time have not been reported. Given that 91

100 corals have potential defenses against predators and competitors, e.g. nematocysts, symbiotic crustaceans (Glynn 1980, Bruno & Witman 1996), it is possible that many urchins are deterred from consuming or even moving over live corals and that qualitative accounts of urchins feeding on corals do not translate into substantial quantitative impacts. This study quantifies both the frequency of direct interactions between urchins and corals, and the rates of removal of coral tissues by urchins. I investigated two main hypotheses: 1) urchins (Echinometra mathaei and Heterocentrotus mammillatus) are capable of removing live coral tissue from three coral species (Pocillopora ligulata, Montipora c.fturgescens, and Pontes lobata); and 2) urchin impacts are species-specific (i.e. vary by species of urchin and species of coral). To test these hypotheses, I transplanted coral fragments, also called "nubbins", of three coral species into the bioeroded grooves of E. mathaei and into the grazing zones of// mammillatus at two sites on the shallow backreef of Midway Atoll. Methods Location This study was conducted on Midway Atoll, a National Wildlife Refuge within the Papahanaumokuakea Marine National Monument, which encompasses the Northwestern Hawaiian Islands. Midway is a high latitude, subtropical (28 N, 179 W) atoll near the limits of reef growth in the North Pacific Ocean (Grigg 1982). 92

101 All experiments were conducted on the shallow backreef (1-2m depth) at two sites (Fig. 1): one in the southwest ( N, W), where benthic surveys documented low coral cover (3%) and high urchin densities (35 m" 2 ), the other in the northeast ( N, W), characterized by high coral cover (36%) and much lower urchin densities (6 m~ 2 ). The southwest site is a rocky, subtidal reef flat extending a few hundred meters behind the reef crest, terminating in a vertical zone (1-2 m) with high densities of Heterocentrotus mammillatus that adjoins a large region of lagoonal sediments. Echinometra mathaei is abundant on all rocky substrates (34 m"); the overall density ofh. mammillatus is 0.3 m" at this site. The northeast site is more protected from ocean waves by uplifted portions of an ancient reef. The site has a narrower region of subtidal reef flat that gives way to coralcovered patch reefs separated by sandy areas. The interface between the reef and lagoonal sand flat is also heavily populated with Heterocentrotus mammillatus on vertical rock surfaces (0.3 m"), and Echinometra mathaei occurs throughout (6 m" ) on hard substrates. A third urchin, Echinostrephus aciculatus, is common at both sites (0.7 m" 2 in the southwest and 0.1 m" 2 in the northeast), but because it does not leave its deep, cup-shaped boring (Asgaard & Bromley 2008), it is unlikely to have any impact on corals, and it was not considered in this study. A few other urchin species are present on Midway (e.g. Echinothrix spp., Diadema spp., Tripneustes gratilla), but they are rare. Urchin damage on live coral 93

102 Impacts of the urchin Echinometra mathaei on live tissue of the coral Pocillopora ligulata were quantified at the southwest backreef site in an experiment using 12 plots (each 1 m 2 ). Urchins were removed from six plots (selected randomly) to serve as controls. An additional 0.5 m buffer zone was cleared of urchins around each control plot to ensure no encroachment from the periphery. Urchins remained undisturbed in the other six plots. Small, 5-8 cm long, fragments (hereafter called nubbins) of Pocillopora ligulata were collected from live colonies at the site, transported underwater in plastic bags to the experimental plots, and epoxied into bioeroded channels created by Echinometra mathaei. There were ten coral nubbins per plot, for a total of 120 transplanted nubbins. Either Z-Spar epoxy or Sea Goin' Poxy Putty was used to attach the coral nubbins to the substrate; neither had a detectable negative impact on the corals, as evidenced by live tissue that maintained its color even directly adjacent to the epoxy. Each nubbin was identified by numbered, plastic swine ear tags attached to the nearby substrate with cable ties. The experiment began in mid-august 2006 and all coral nubbins were monitored on 1, 3, 6, 9, and 32 days following attachment to the substrate. On each date, the number of nubbins damaged by urchins was noted, and each nubbin was photographed with a scale. Only substantial and sustained tissue loss consistent with that caused by urchins was counted as damage; temporary nicks were not included in this category. 94

103 Species-specific effects and site differences This experiment expanded the design of the first experiment to investigate species-specific impacts by including an additional urchin species and two new coral species, and to investigate site-specific effects by adding an additional site. The urchin species used were Echinometra mathaei and Heterocentrotus mammillatus, with 30 plots (1 m 2 ) at each site, for a total of 60 plots between two sites. Within a site, 20 plots were assigned to E. mathaei and 10 plots to H. mammillatus. Half the plots for each urchin were then selected randomly for removal of urchins, including a 0.25 m buffer zone around each plot. The plots were spread across the rocky backreef, with Heterocentrotus plots closer to the inner margin of the backreef and the sandy lagoon where Heterocentrotus is most abundant. Two coral species were exposed to urchin grazing: Montipora flabellata [c.f. turgescens, (James Maragos pers. comm.)] and Porites lobata, both common corals on the north backreef. A third coral, Pocillopora ligulata, was originally included in this experiment (as in the first experiment), but apparently due to handling stress (transporting corals by boat in plastic bags) most nubbins of Pocillopora bleached and died shortly after placement; therefore, only the data from Pocillopora in the first experiment are presented and discussed. Nubbins of each coral species were collected from the northeast site and transported by boat to both sites, protected by water-filled plastic bags placed in large coolers. All coral nubbins were subjected to the same transport time (approx. 25 min); immediately after arrival, the bags were placed underwater. Four coral nubbins were epoxied to the substrate in each plot: one 95

104 Porites, one Montipora, and two Pocillopora. The Porites and Montipora nubbins survived transport and transplant with no ill-effects, unlike most of the Pocillopora nubbins. Porites and Montipora nubbins were placed within areas of obvious urchin activity (i.e. a bioeroded channel in Echinometra plots and relatively bare, grazed substrate in Heterocentrotus plots) to ensure exposure to urchins. One Pocillopora nubbin was placed within a visible urchin grazing area, and the other outside the visibly grazed area, to test for differential impacts over space. Because the Pocillopora nubbins were lost, this experiment could not determine whether Echinometra mathaei ever leave their channels to graze and so might impact corals over broader areas during nocturnal feeding activities, but a separate study of urchin movements was conducted to address this question (see below). The final experimental design contained two nubbins in each plot (one Porites, one Montipora) for a total of 80 nubbins exposed to Echinometra and 40 nubbins exposed to Heterocentrotus. The experiment began in early July 2007, and nubbins were checked for damage and photographed with a scale at increasing intervals following attachment. At the northeast site, nubbins were checked on days: 1, 2, 6, 13, 34, 57, and 116. At the southwest site, nubbins were checked on days: 1, 3, 9, 25, 54, and 111. "Damage" was more broadly defined than in the first experiment to include many types of tissue loss, including that which may have been caused by fishes or other sources. Each nubbin was identified by a numbered, plastic swine ear tag attached to the nearby substrate. 96

105 Spatial effects of urchins In order to understand the spatial influence of urchins across the reef, movement patterns were observed in both species. A total of 384 Echinometra mathaei were observed during their active period at night (two hours after sunset, with no moonlight) on a nearshore patch reef at Rusty Bucket ( N, W) on September 21, Two observers swam haphazard transects and recorded the position (inside or outside its bioeroded channel) of every E. mathaei visible with a dive light. Heterocentrotus mammillatus positions were monitored overnight in October 2007 at two sites: one in the southwest backreef ( N, W) and one in the north backreef ( N, W). Ten urchins at each site were tagged with marked surgical tubing slipped over a spine and their current position marked using cable ties. Their positions were checked the next day and the distance from the last location measured. Any urchin movements observed during the day were also noted throughout the course of the experiment, as well as during other activities. Surveys The surface area of reef affected by E. mathaei grazing and bioerosion was estimated from surveys at both sites in November 2007, using 10 m long lineintercept transects, randomly placed and oriented within a 100 m region. The length 97

106 (in cm) under the line that consisted of urchin channels was recorded along ten replicate transects at each site. Analyses Sequential nubbin photographs were analyzed for area of tissue lost, using the software Coral Point Count with Excel extensions (CPCe; available as freeware). The area of tissue removed from nubbins in the first experiment was analyzed using a Repeated Measures analysis after square root-transforming the data to conform to assumptions of normality and homoscedacity of variances. Estimates of damage to Pocillopora nubbins did not require statistical analysis because no controls were damaged. For the second experiment, damage data were analyzed using Log linear models with three terms: treatment (urchins present or urchins removed), coral genus (Montipora or Porites), and nubbin status (damaged or undamaged). Rates of removal were compared by coral genus and site using a 2-way ANOVA on square root-transformed data. Time from start to first damage was square root transformed and the effects of site and coral taxon were evaluated in a 2-way ANOVA. Results Echinometra impacts on Pocillopora 98

107 Within 24 hours of exposure to E. mathaei, 90% of Pocillopora nubbins were damaged (9.0 ± 1.4 SD out of 10 nubbins per plot), but none of the nubbins in control plots sustained urchin damage (Fig. 2). These trends were consistent over time; all remaining nubbins in urchin plots were damaged by day 32, while 100% of control nubbins remained free of urchin damage. In a few cases, urchins were able to pry the nubbin off the substrate and evict it from the channel. These were considered "damaged", since they were affected by urchins; measurement of the area damaged was not applicable in this case. The area of damage increased over time as urchins gradually removed more and more coral tissue and skeleton (Fig. 3). On day one, just 0.21 ± 0.19 SD cm 2 was missing, but by day nine, that amount had increased 12-fold to 2.56 ± 2.07 cm (Fig. 4; Repeated Measures, SS = , F = , df = 2, p <0.001) Over the first nine days, the average rate of tissue loss from Pocillopora nubbins was 0.32 ± 0.23 cm 2 day" 1 (Table 1). Species-specific effects: corals Over the three month duration of the second experiment, nearly all nubbins of both Montipora (95%) and Pontes (97%) were damaged by E. mathaei in the plots with urchins present (Fig. 5). By contrast, only a small proportion, 6% of Pontes and 11% of Montipora nubbins were damaged in control plots without E. mathaei. In a Log linear model analysis of treatment, coral genus, and nubbin status, the only significant term was treatment (urchins present or urchins removed) by nubbin status 99

108 (damaged or undamaged), indicating that treatment had a highly significant effect on the number of damaged nubbins for both Montipora and Porites (x = , df = 4, p < ), with more nubbins damaged in urchin plots than in plots with urchins removed (Fig. 5). As with Pocillopora, E. mathaei urchins removed increasing amounts over time from both Montipora (Fig. 6a) and Porites (Fig. 6b). Coral removal rates were calculated after the first day of exposure for Porites and Montipora at both sites (Table 1). Removal rates were significantly higher at the southwest site than at the northeast site (Table 2; 2-way ANOVA, df = 1, F = 5.414, p = 0.030), but did not differ between coral genera (df = 1, F = 0.044, p = 0.836) or in the interaction term (df= 1, F = 0.114, p = 0.340). The time to first damage by E. mathaei differed significantly between sites, with the average time until damage taking more than twice as long at the northeast than at the southwest site (Fig. 7; Table 3; 2-way ANOVA, site effects, df = 1, F = 6.844, p = 0.011). While there was an apparent tendency for Montipora nubbins to be grazed down more quickly than Porites nubbins (Fig. 7), this was not significant (Table 3; 2- way ANOVA, taxon effects, df = 1, F = 0.350, p = 0.556). Species-specific effects: urchins In plots with Heterocentrotus urchins at both sites, there was no damage to any coral that could be definitively attributed to urchins. Despite the fact that 100

109 Heterocentrotus were often seen on top of and even completely obscuring nubbins, such nubbins always remained undamaged upon later checks. One Porites nubbin in a Heterocentrotus removal plot did lose most of its tissue between the second and fourth days after deployment, but the surrounding damage marks were linear, and so unlikely to be made by urchins, which tend to produce star-shaped marks. Two Montipora nubbins in the northeast (one in an urchin plot and one in a removal plot) were damaged by day 14, but neither seemed caused by urchins. By day 14, a few Heterocentrotus had moved into removal plots in the northeast site, and they were removed upon discovery. Because Heterocentrotus has no visible impacts, those urchins that invaded removal plots after day 14 were no longer removed, effectively ending the treatment differences at this point. Between day 4 and day 10, nearly all Montipora in both treatments in the south acquired pale spots, but no Montipora at the northeast site developed this condition. Three months after deployment, most nubbins in Heterocentrotus plots at the northeast site, regardless of species or treatment, looked healthy and undamaged, but after the same period of time most nubbins of both species at the southwest site appeared stressed, with lighter color and dead patches. Urchin movements Of 384 Echinometra mathaei observed after dark on a moonless night, all but two clearly remained in their channels. One exception was on sand underneath an overhang, where the sand may have encroached over or filled its channel, and the 101

110 other was not in a channel but immediately adjacent to an octopus which appeared to be interacting with it, and may have extracted it from its channel. Tagged Heterocentrotus mammillatus urchins, ten at each site, moved an average of 42 ± 31 cm SD overnight at the northern site and 46 ± 35 cm at the southwest site. Two urchins at the southwest site remained in the same daytime resting spot (i.e. moved 0 m), while the farthest distance traveled was 100 cm. Reef area affected by E. mathaei In ten surveys at the southwest site, E. mathaei urchin channels covered 13.2% ± 4.5 SD of the reef substrate, with the highest value reaching 18.5%. This was almost five times the area covered by channels at the northwest site 2.8% ±1.7 SD. Discussion. This study experimentally demonstrated the direct and sustained removal of live coral tissue by the sea urchin Echinometra mathaei but not by Heterocentrotus mammillatus. E. mathaei removed both live tissue and skeletal material from three coral species, supporting the hypothesis that intensive removal of coral tissue is possible and consistent across coral species tested, but varies by urchin species. E. mathaei acted quickly, with 90% of Pocillopora nubbins sustaining urchin damage within the first 24 hrs of exposure (Fig. 2). The urchins continued removing coral 102

111 tissue over subsequent days (Fig. 3), gradually reducing the coral nubbins to small remnant pieces, and often removing the entire nubbin (Figs. 4, 6). Controls (without E. mathaei) remained undamaged over one month for Pocillopora, and the small amounts of damage sustained over three months on a few Porites and Montipora nubbins were not consistent with urchin damage. The overwhelming majority of Porites and Montipora nubbins exposed to E. mathaei were damaged (Fig. 5). Indirect positive effects of tropical sea urchins on live corals are welldocumented mainly for Diadema antillarum, whose grazing activities in the Caribbean have either prevented or reversed phase shifts to low-coral, high-algal states (Hughes 1994, Edmunds & Carpenter 2001). In contrast, direct negative effects of urchins on live corals are rarely reported, but they are not necessarily less common, since some species are both abundant and strong bioeroders of carbonate substrates (Sammarco 1980, Carreiro-Silva & McClanahan 2001, Asgaard & Bromley 2008) The results of this study support the idea that, in addition to its known diet of algal films and flocculant organic matter (Asgaard & Bromley 2008), E. mathaei may also consume invertebrates, such as corals that it may encounter as recruits or adult colonies attempting to grow into its burrow. This pattern on Midway Atoll also appears to be more pervasive than in some other studies of coral predation by urchins, in which only a fraction of the population were damaged and only at exceptionally high urchin densities (Bak & van Eys 1975) or at sites with high urchin densities and under low predation pressure due to few predatory fishes (Glynn et al. 1979). Given 103

112 the evidence of herbivory on algae, detritivory on organic aggregates, and, at times, predation on corals (and probably other invertebrates), E. mathaei are perhaps best described as opportunistic omnivores, capable of feeding at multiple trophic levels on whatever enters (or grows within) their grooves. Species-specific differences in grazing effect of urchins on algae and corals are likely to be common but they are rarely investigated directly (Sammarco 1982, Furman & Heck 2009). In contrast to the behavior of E. mathaei, Heterocentrotus mammillatus did not damage any live corals in these experiments, despite multiple occasions where urchins were seen resting directly on top of one of the coral nubbins. H. mammillatus is also a bioeroder, but is more mobile than E. mathaei, grazing over a broader region and creating zones of intensive grazing characterized by reduced turf and macroalgae, rather than creating distinct bioeroded channels like E. mathaei. H. mammillatus is active at night when tagged individuals moved an average of 44 ± 32 SD cm between daytime resting places. Rates and outcomes of grazing by E. mathaei grazing did not differ significantly among three coral species (Pocillopora ligulata, Montipora flabellata, and Porites lobata), indicating that coral identity did not substantially influence the behavior of E. mathaei or its effects on coral mortality. All coral taxa tested were susceptible to urchin grazing at similar rates. Understanding the movement patterns of urchins is critical for interpreting their broader influences on the reefs. Nighttime observations were consistent with daytime observations on E. mathaei behavior, confirming that E. mathaei, although 104

113 more active at night, does not leave its channels. The common observation that small macroalgae and corals often live right up to the edges of, but not within, E. mathaei channels further indicates that urchin activity is restricted to the channels. Behavior of E. mathaei seems more variable in Kenya, where it uses crevices on the outer reef but remains on exposed surfaces on the inner reef (Khamala 1971). Similar to the behavior we observed for E. mathaei, the congeneric E. leucunter apparently does not leave its borings (Hoskin et al. 1986, Asgaard & Bromley 2008). Such restricted movement severely limits Echinometra's sphere of influence, and while its impacts on corals and algae may be intense within channels, they are highly localized and patchy in nature. The borings of E. mathaei have been described as traps for drift algae or as algal "farming" areas (Asgaard & Bromley 2008), but it is likely that they also play a large role in protection from predators. During our urchin removals, any individual taken out of its groove and left in the open was snatched up almost immediately by nearby wrasses (Labridae), notably including the Hawaiian Hogfish, (Bodianus albotaeniatus) which consumed smaller urchins whole, and readily ingested pieces of larger urchins, including the test and spines. Elsewhere, the primary predators of urchins are usually triggerfishes, e.g. on Kenyan reefs (McClanahan & Shafir 1990), but these fishes are rare on the shallow backreef of Midway Atoll where Echinometra is most abundant; triggerfishes are common only on the forereef where urchins are less dense. E. mathaei seems to have a strong channeling instinct; urchins in a 105

114 separate caging experiment created channels in the dense fleshy macroalgae growing within the cages but did not consume macroalgae beyond these channels (Chapter 2). Because Echinometra does not leave its channels, and because channels are non-overlapping, each coral nubbin was exposed to only one urchin, and site-specific impacts related to differences in urchin densities were not expected. However, there were site differences, with urchins acting more quickly and removing a greater area in the higher density southwest site than in the lower density northeast site (Fig. 7, Table 1). This suggests that there are other characteristics of the site itself that may affect urchin behavior and grazing rate. For example, strong water movement is linked to maintaining refuge in crevices (Tsuchiya & Nishihira 1984), and current velocities or temperature differences may also influence urchin behavior. Previous exposure to corals may also have a role; the northeast site had much higher coral densities and many urchin channels were immediately adjacent to a coral colony, so previous experience with corals may have made urchins initially more reluctant to attack the corals. Direct removal of coral tissue by urchins has strong implications for coral recruitment. Some studies suggest that urchin grazing is detrimental to recruitment of corals (Sammarco 1980, 1982), while others provide evidence of indirect benefits for coral recruitment (Edmunds & Carpenter 2001, Carpenter & Edmunds 2006, Idjadi et al. 2010). Channels and grazed zones are largely turf-free spaces that seem ideal for coral recruitment. While this study looked at juvenile- to small adult-sized corals, rather than recruits, the intensive removal of larger colonies by E. mathaei implies 106

115 that recruits would not survive within E. mathaei channels. It is less clear whether Heterocentrotus, which did not seem to harm the experimental coral nubbins, also avoids tiny recruits, or whether recruits would be removed incidentally during its grazing activities. It is also possible that E. mathaei inhibits recruitment within its channel, but facilitates some recruitment along the edges of its channel, where recruits could establish in a cleared area before expanding laterally away from the channel (Birkeland & Randall 1981), but I did not see evidence supporting this hypothesis. Whether by removing recruits or by limiting growth of adult colonies, the spatial influence of E. mathaei is mostly limited to the area of reef covered by its channels. On the northeast backreef, this averaged 3% of the reef surface area, but it was four times higher in southwest backreef, where an average 13% of the reef consisted of urchin channels a substantial area over which E. mathaei can directly reduce coral cover and influence community structure. 107

116 Table 1. Mean daily rates (cm 2 urchin" 1 day" 1 ) of live coral tissue loss from corals exposed to E. mathaei, calculated across the cumulative number of days exposed. Coral Pocillopora ligulata Pocillopora ligulata Pocillopora ligulata Montipora flabellata Porites lobata Days exposed Southwest mean SD Site N mean n/a n/a n/a Northeast SD n/a n/a n/a N

117 Table 2. 2-factor ANOVA results for rate of coral tissue removal by site and coral genus. Source Type III SS df Mean Squares F-ratio p-value Site Coral genus Site*Coral genus Error

118 Table 3. 2-factor ANOVA for day of first damage by site and coral genus Source Sum-of- Squares df Mean-Square F-ratio P Coral genus Site Site*Coral genus Error

119 m o WAUTICAl MILES KILOMETERS ^ 0 H Midway Atoll Northeast SMS< HI o GO ** i*+ -**"* -*, «Ik - + f -w * if ^SEbn&fe^^svinMtt' j[77024'w Figure 1. Study sites on Midway Atoll, Northwestern Hawaiian Islands. IKONOS satellite image; NOAA Atlas J. _. ~ Q _. Il«f, M«tQ, Q'VA/ Ill