Coral population dynamics in a subtropical coral community, Solitary Islands Marine Park, Australia

Proceedings 9 th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000 Coral population dynamics in a subtropical coral community, Solitary Islands Marine Park, Australia V. J. Harriott 1 and S.D.A. Smith 2 ABSTRACT The population dynamics of coral communities have rarely been studied in subtropical coral communities, despite hypotheses that such communities may be subject to more variable mortality and recruitment rates than tropical coral reefs. In the Solitary Islands Marine Park (30ºS) in Eastern Australia, changes in cover of coral and other biota in mapped fixed quadrats were recorded at three island sites at approximately annual intervals between 1993 and 1998. At two of the sites, coral cover was relatively stable over time and ranged between 27% and 35% cover over the five years. At the third and most inshore site on the coastline at Coffs Harbour, coral cover approximately doubled during the study period, from 11% to 23% cover. For all times pooled, coral recruitment rates averaged 1.3 to 1.8 recruits/m 2 /year, while mortality rates were 0.7 2.0 corals/m 2 /year, with recruitment rate strongly correlated with mortality rate for different taxa. Pocillopora damicornis contributed approximately 50% of the population turnover, but only 23% of the coral cover at the three sites. Principle causes of coral mortality during the period were localised storm damage and overgrowth by worm tubes. Keywords Subtropical reefs, Coral population dynamics Introduction Long-term studies of coral population dynamics in repeatedly-sampled quadrats have provided invaluable information on the dynamics and community characteristics of coral reefs (for example Connell 1978, Porter et al. 1981,1982, Hughes 1985, 1988, Connell et al. 1997, Hughes and Connell 1999). The ecological significance and timing of factors limiting coral recruitment and mortality are most readily derivable from repeated examination of the same communities at regular intervals. Such data can also be used to derive models of community population dynamics and trajectories over time (Preece and Johnson 1993, Tanner et al. 1994, 1996, Kudo and Yamano 1997, Mumby 1999). For high latitude coral communities, no such data sets have been published from which similar analyses and models can be derived. While coral reefs are largely limited to tropical regions where water temperature rarely falls below 18º C (reviews in Stoddart 1969, Rosen 1988, Veron 1995), many subtidal communities with significant coral cover occur at higher latitudes. Some of the better known of these communities include those present on both the east and west coasts of Australia, Japan, the Hawaiian chain of islands, Florida and the Gulf of Mexico, Bermuda, South America, and southern Africa. Examination of the factors regulating these communities can increase our understanding of the processes which control development of coral reefs (Grigg 1998) Hypotheses on limits of coral reef development include: a reduction in reproductive viability of corals at high latitudes (Yonge 1940, Wells 1957, Veron 1974), limited recruitment success of corals at high latitudes (reviewed in Harriott and Simpson 1997, Harriott 1999a), reduced growth rates at high latitudes (Grigg 1982, Harriott 1999b), reduced capacity of corals to compete with temperate algae or fouling organisms (Johannes et al. 1983, Miller and Hay 1996, Holmes et al. 1997, Miller 1998, McCook et al. 2001), high rates of carbonate solution at high latitudes associated with greater development of epilithic and other algal communities (Barnes and Lazar 1993), and high coral mortality rates (Edmondson 1929, Veron and Done 1979, Burns 1985, Tribble and Randall 1986). Edmondson (1929), Veron and Done (1979) further suggested that coral mortality caused by episodes of cold water exposure might limit the capacity for corals to survive at high latitudes. However, more recent evidence suggests that a significant number of species of hermatypic corals can survive at temperatures lower than 14ºC (Tribble and Randall 1986, Coles and Fadlalah 1991 reviewed in Veron 1995). Thus, we might hypothesise that an examination of coral population dynamics in a high latitude coral community would find high, and temporally variable, coral mortality rates relative to tropical sites. The absence of coral reproduction and successful recruitment has been argued to limit coral distribution and capacity of subtropical communities to recover from episodes of mortality (Yonge 1940, Wells 1957, Veron 1974, Harriott 1995). Studies at subtropical Australian sites have reported that total recruitment on settlement panels is somewhat less than for tropical Australian sites, and that most recruits are from a few brooding taxa; recruits of broadcast-spawning corals are uncommon (Harriott 1992, 1995, 1999a, Banks and Harriott 1996, Harriott and Simpson 1997). Recruitment of corals into the field has not previously been quantified from a high latitude site, but if recruitment reflected larval availability as recorded by settlement panel studies, it might be hypothesised that recruitment of corals at visible size might be lower in high latitude communities than at low latitude ones. 1 CRC Reef Research Centre, PO Box 772, Townsville, 4810, Australia. vicki.harriott@jcu.edu.au 2 Zoology Dept, University of New England, Armidale, 2351, Australia

Johannes and co-workers have argued that cold temperatures and high nutrient levels give macroalgal species a competitive advantage over corals, and that algae will progressively replace corals at high latitude sites (Johannes et al. 1983, Hatcher and Rimmer 1985, Crossland 1988). Miller and Hay (1996) also reported that macroalgae inhibited growth and recruitment of a temperate coral species in North Carolina, USA, thus suppressing the potential for reef development. Miller (1998) has reviewed the mechanisms by which temperate algal dominance might control community structure in coral/algal systems. The Solitary Islands are in a similar biogeographical zone in eastern Australia to the Houtman Abrolhos in western Australia, where Johannes and colleagues developed the hypothesis on macroalgal competition. Kelp and fleshy macroalgae form a significant component of the benthic community and, at nearshore sites, patches of coral are interspersed with kelp forests and stands of other macrophytes (Veron et al. 1974, Smith and Simpson 1991, Harriott et al. 1994). An examination of community dynamics might indicate whether competition between corals and macroalgae play a significant role in structuring these communities. The Solitary Islands Marine Park off the mid-north coast of NSW in eastern Australia, supports the most southerly significant coral communities in eastern Australia. Well developed coral communities exist in many sites, and over 90 coral species have been recorded in recent surveys (Harriott et al. 1994). The community structure, recruitment and growth of corals in the Solitary Islands have been reported in a series of recent papers (Harriott et al. 1994, Harriott 1999a, b). This paper reports on the community dynamics of coral populations at three sites within the marine park over a five year period. The study sought to investigate the hypotheses that: (a) coral recruitment success is limited at high latitudes; (b) coral mortality rates are higher and more variable than those reported from tropical sites; and (c) the outcomes of competitive interactions between corals and macroalgae structure high latitude coral communities. Methods A description and map of the Solitary Islands (153º E, 30º S) is provided in Harriott et al. (1994). The study sites were Muttonbird Island, which is close to shore at the township of Coffs Harbour and is connected to the shoreline by a breakwater; Split Solitary Islands, approx. 5 km north of Coffs Harbour and approx 2 km offshore, and North West Solitary Island, about 25 km north of Split Solitary Island and 5 km offshore. In April 1993, 20 fixed, 1 x 1 m quadrats were established on the leeward (north to north-west) side at each of island site. The location of each quadrat was marked using aluminium bars hammered into the rocky substrate, and later (when bars were becoming over grown with fouling biota) by pitons at fixed points. The locations of the quadrats at each site were marked on a map to assist relocation. Quadrats were placed on flat sections of the substratum to assist quadrat mapping. At Muttonbird Island, flat areas of the benthic community are dominated by the kelp Ecklonia radiata, with corals restricted to patches several metres in breadth and generally close to more vertical areas. At Muttonbird Island, quadrats were located in the coral patches. Other than these limitations, quadrat locations were selected randomly within the available area at each location. A number of the markers were lost over the duration of the study and consequently data collection in subsequent years was from a subset of the original quadrats. In most cases quadrats were lost because markers had become detached from the substratum, presumably under the influence of wave-action which can be severe in this region. However, at two sites (North West Solitary and Split Solitary) at least some quadrat markers were temporarily lost due to their complete overgrowth by sandy tubes produced by dense aggregations of Chaetopterus worms (Smith and Harriott 1998). The timing of samples and number of quadrats which were relocated at each location for each sample time are presented in Table 1. Table 1. Sampling dates and number of quadrats relocated at each sample time. The number of quadrats per site for which 4 or 5 sample times were available is also listed. Number of quadrats located Dates Muttonbird Is Split Solitary Is NW Solitary Is April 93 20 20 20 April 94 20 10 14 June 95 14 8 14 August 96 14 12 16 May 98 18 14 16 No. missing <1sample time 14 8 14 The biotic communities within each quadrat were documented by mapping by hand onto a reduced scale map on underwater paper. Biota were identified to the lowest possible taxonomic level, which was generally species or genus for corals. Algae were classified into major taxonomic (Chlorophyta, Phaeophyta, Rhodophyta) or life-form categories, with the easily distinguished taxa such as Halimeda sp., crustose red algae, and Chlorodesmis fastigiata scored separately. The maps were subsequently digitised to estimate the percentage cover and number of colonies/individuals for each species or taxonomic category in each quadrat at each sample time. To calculate changes in coral cover over time, quadrats missing more than one sample time were excluded from the analysis. The number of quadrats remaining for analysis is indicated in Table 1. Where only one sample was missing, values for coral cover for the missing time were interpolated from data for the preceding and subsequent sample times. There were no cases of large changes in coral cover over time in the instances where the data were derived in this way.

Maps were compared between years to examine turnover of coral colonies, for all maps where data were available from consecutive years. Any colonies present in one year and absent in the next were scored as having died, and any small colonies or fragments present in one year and not in the previous year were classed as recruits. In addition, the five-year mortality rate was calculated by tracking each coral colony present in 1993 and determining whether it was still alive in 1998. Analysis of turnover in the algal and invertebrate communities will be reported separately (Smith and Harriott, in prep.). Table 2. Percentage cover of benthic organisms (mean and standard deviation) for three sites in the Solitary Islands, pooled over sample times. Muttonbird Island Split Solitary Island North West Solitary Is. Corals Mean s.d. Mean s.d. Mean s.d. Acropora hyacinthus 0.0 0.0 0.1 0.5 3.1 10.55 Acropora solitaryensis 0.0 0.0 1.5 3.9 1.9 6.46 Acropora species. 0.0 0.0 0.0 0.1 1.3 2.60 Acanthastrea spp. 0.0 0.0 0.2 0.4 1.0 2.13 Cyphastrea serailia 0.0 0.0 0.5 1.4 1.7 5.91 Favites spp. 0.0 0.0 0.0 0.0 0.6 3.19 Goniastrea australensis 1.5 4.1 7.7 8.5 9.0 10.21 Montastrea curta 0.0 0.0 0.3 2.1 3.4 8.12 Porites heronensis 0.0 0.1 0.9 0.9 0.3 0.37 Pocillopora damicornis 7.1 10.8 3.6 3.8 5.4 5.83 Psammacora superficialis 0.1 0.3 0.0 0.0 0.0 0.00 Stylophora pistillata 0.0 0.0 0.0 0.1 0.1 0.43 Turbinaria (juvenile) 0.7 1.3 0.1 0.2 0.0 0.08 Turbinaria frondens 2.1 9.6 11.6 13.3 1.4 4.96 Turbinaria radicalis 0.2 0.9 2.0 6.1 0.1 0.23 Total corals 11.7 28.5 29.4 Soft corals Sarcophyton sp. 1.7 4.5 0.7 1.4 0.0 0.26 Soft coral 1.6 3.8 4.3 4.7 3.4 2.72 Sinularia sp. 0.7 3.2 0.1 0.5 0.9 2.58 Total soft corals 3.9 5.1 4.3 Algae Amphiroa anceps 7.0 12.1 1.1 1.4 0.1 0.19 Caulerpa spp. 1.6 1.9 0.8 1.4 4.6 11.20 Chlorodesmis fastigiata 0.0 0.0 0.0 0.0 0.1 0.41 Dictyota dichotoma 0.0 0.1 0.2 0.5 0.0 0.13 Filamentous algae 0.0 0.1 4.5 9.6 9.0 14.39 Giffordia mitchellae 0.2 0.8 0.2 1.2 0.0 0.00 Halimeda cuneata 0.0 0.0 0.1 0.4 0.1 0.51 Kelp (holdfast area) 0.5 1.0 0.0 0.1 0.0 0.00 Lithothamniales 2.1 3.9 3.5 5.5 1.2 2.04 Lobophora variegata 2.0 2.7 0.2 0.5 0.0 0.15 Padina sp. 0.6 1.2 0.1 0.3 0.0 0.15 Peyssonnellia capensis 1.6 2.4 0.0 0.1 0.0 0.03 Red algae 0.0 0.0 0.2 1.8 0.4 0.11 Ulva sp. 0.0 0.0 0.0 0.1 0.1 0.28 Total algae 15.7 10.9 15.7 Invertebrates Ascidians 11.4 10.0 8.8 7.4 11.9 13.86 Chaetopterus tubes 0.0 0.0 0.3 1.3 4.3 7.35 Bryozoan 0.3 0.5 0.1 0.2 0.1 0.20 Compound ascidians 0.7 1.0 0.7 1.2 0.4 0.61 Small ascidians 3.3 4.5 0.2 0.7 0.1 0.27 Zoanthids 0.2 0.5 0.6 1.1 0.2 1.02 Sponges 9.4 8.8 2.3 2.2 1.7 2.34 Anemones 0.1 0.2 0.0 0.0 0.0 0.13 Total invertebrates 25.2 13.0 18.8 Substrate Dead coral 0.1 0.4 2.0 3.6 2.1 4.22 Sand 1.1 3.6 1.9 4.2 0.0 0.10 Total substrate 1.1 3.9 2.1

Results Benthic cover in quadrats Summaries of cover for quadrats pooled over time for each site are presented in Table 2. Hard coral cover in the quadrats averaged 11.7% at Muttonbird Island, 28.5% at Split Solitary Island, and 29.4% at North West Solitary Island. This compares with covers of 8.5%, 34.5% and 26.3% in broad-scale line transect surveys at these sites in 1993 (Harriott et al. 1994), indicating that the quadrats were broadly representative of the communities with respect to coral cover. The coral community at Muttonbird Island was dominated by P. damicornis and Turbinaria sp, at Split Solitary Island by Turbinaria frondens, Goniastrea australensis and P. damicornis, and at North West Solitary Island by G. australensis, Acropora sp. and P. damicornis. Other dominant faunal groups were ascidians (total of 9 to15% cover), sponges (2% to 9%) and soft corals (4% to 5%). Dominant algal cover varied between sites. The red alga Amphiroa anceps was dominant at Muttonbird Island but uncommon at the more offshore islands, where filamentous algae dominated. While algal cover was the same at Muttonbird Island and North West Solitary Island (15.6%), there was a relatively low cover of fleshy algae at the offshore island (6.7%) relative to Muttonbird Island (Table 2). Changes in coral cover over time The change in cover of hard and soft corals between 1993 and 1998 is illustrated in Fig. 1. At Split Solitary Island, coral cover was reduced from 35% to 31% between 1994 and 1995. Most of the decline was attributable to a 3.6% drop in cover for the plate-forming species, Turbinaria. This change was coincident with two major storms in early 1995 and the presence of a number of large overturned coral colonies in the area during the sampling. Several quadrats at Split Solitary Island were unable to be located in this and all subsequent samples, suggesting extensive physical impacts to the area. At Split Solitary Island, hard coral cover increased over the next three years reaching 35.4% in 1998 (Fig. 1). At North West Solitary Island, hard coral cover was similar to that recorded from Split Solitary, and ranged from 27% in 1993 to 32% in 1996. At Muttonbird Island, hard coral cover increased steadily from 11% in 1993 to more than double to 23% in 1998. The increase in cover was attributable to an increase in cover of P. damicornis from 6.1% to 15.6%, and a 2% increase in cover of Turbinaria species. Coral recruitment and mortality Results for the study of turnover (mortality and recruitment) of coral colonies at each location are summarised in Table 3 and 4. Where intervals between samples were longer than 12 months, calculations of annual mortality and recruitment rates will be slightly underestimated because some corals may have recruited and died between sample times but are not included in the data set. For all taxa and sample times, annual mortality rate was 1.3 colonies/m 2, and recruitment rate was 1.6 corals/m 2. Recruitment and mortality rates varied considerably between taxa, locations and sample times. The highest recruitment and mortality rates were for P. damicornis, P. heronensis, and Turbinaria sp.; rates for all other taxa were < 0.1/m 2 /year (Table 3). The mortality rate of corals at Muttonbird Island (0.7 /m 2 /yr) was about half of that at the other two sites (1.3 2.0 /m 2 /yr), while the recruitment rate varied little between sites (1.3 to 1.8 /m 2 /year) (Table 3). The mortality rate was highest in the 1994-95 sample interval (2.2 / m 2 / year compared with 1.0 to 1.2 / m 2 / year in the other three intervals), attributable to high mortality rates at Split Solitary Island and Muttonbird Island (Table 4). For recruitment, the highest rate at each site was in the 1993-94 interval, and the lowest rate at each site was in the 1995-96 period (Table 4). Over the study period, the mortality rate was about half the recruitment rate at Muttonbird Island, but was slightly higher than recruitment at the other two sites (Table 3). For the 10 coral taxa identified at specific or generic level, there was a significant positive correlation between recruitment and mortality rates per m 2 for results pooled across sites and times to increase sample sizes (r= 0.983, P(r)<0.05). Analysis of five year mortality rates indicates that on average, 53% of corals at all sites survived for the five years between 1993 and 1998. Survival rate was highest at Muttonbird Island (73%) and lowest at Split Solitary Island (45%). Fig. 1: Changes in cover of hard and soft corals over the study period at 3 sites.

Table 3. Mean annual mortality and recruitment rate/m 2 for three sites in the Solitary Islands, and for all sites combined. Muttonbird Is Split Solitary NW Solitary All sites Species (n=60) (n=34) (n=58) (n=152) Mort. Recr. Mort. Recr. Mort. Recr. Mort. Recr. P. damicornis 0.56 0.97 0.55 0.62 0.69 0.50 0.61 0.77 P. heronensis 0.00 0.02 0.71 0.71 0.21 0.30 0.22 0.31 Turbinaria spp. 0.15 0.30 0.11 0.13 0.10 0.16 0.12 0.22 Acanthastrea sp. 0.00 0.00 0.01 0.15 0.08 0.17 0.04 0.11 Acropora sp. 0.00 0.00 0.29 0.09 0.05 0.02 0.08 0.03 G. australensis 0.00 0.00 0.20 0.11 0.14 0.09 0.09 0.07 M. curta 0.00 0.00 0.03 0.00 0.04 0.01 0.02 0.01 S. pistillata 0.00 0.00 0.00 0.00 0.07 0.02 0.02 0.01 C. serailea 0.00 0.00 0.03 0.00 0.01 0.04 0.01 0.02 P. superficialis 0.02 0.02 0.03 0.00 0.00 0.00 0.02 0.01 Encrusting coral 0.02 0.02 0.00 0.00 0.07 0.00 0.02 0.00 Total 0.73 1.31 1.96 1.81 1.46 1.30 1.25 1.57 Table 4. Mean annual mortality and recruitment rates/m 2 for corals for the four sampling periods. 93-94 94-95 95-96 96-98 N Mort. Recr. N Mort. Recr. N Mort. Recr. N Mort. Recr. Muttonbird Is 18 0.22 2.39 14 1.40 1.28 14 0.85 0.37 14 0.45 1.18 Split Solitary Is 10 1.40 3.90 6 3.99 0.71 8 1.18 0.64 10 1.26 2.00 NW Solitary Is 14 1.71 2.43 14 1.28 0.85 14 1.59 0.10 16 1.25 1.82 All sites 42 1.11 2.91 34 2.22 0.95 36 1.21 0.37 40 0.99 1.67 Discussion In a five year study of population dynamics of eastern Australian high latitude corals, coral cover and population structure was relatively stable in two locations and coral cover doubled in a third inshore location. Consistent with studies of settlement panels (Harriott and Banks 1995, Harriott 1999a) the recruitment rate of corals at visible size at the Solitary Islands (< 2 corals/m 2 /year) was substantially lower than recruitment reported for most similar field studies from tropical sites. In previous studies at tropical locations, recruitment ranged from 2 to 16 coral recruits/m 2 /year on the Great Barrier Reef (Harriott 1983, Harriott and Fisk 1989, Fisk and Harriott 1989, Connell et al. 1997) and 3.5 and 4 recruits/m 2 /year at Jamaica (Hughes 1985, 1988) and approximately 5 recruits/m 2 /year in Florida (Dunstan 1977). However, Porter et al. (1981) reported recruitment rates of 0 1.8 recruits/m 2 /year in Jamaica. There are relatively few data sets on comparative mortality rates of corals. Most studies of coral mortality describe episodes of catastrophic mortality, most commonly in terms of a decline in percentage cover of corals attributable to factors such as cyclones, starfish predation or bleaching. For other studies, differences between methodologies and in the duration of the study periods make comparisons between studies difficult. In most studies, mortality rates vary significantly with colony size introducing further variability (Hughes and Jackson 1985, Harriott 1985a). Results from mortality studies and derived estimates of annual mortality rates are presented in Table 5. In each case, derived mortality rates for corals at the Solitary Islands are lower than or similar to results from tropical regions. From the data available, there is no evidence to suggest that coral mortality rates are higher in subtropical locations than in the tropics. The general pattern for coral taxa presented in this paper, with high mortality for pocilloporid corals and low mortality for faviids is consistent with other studies (Table 5). For poritid corals, massive Porites have relatively low mortality rates, but encrusting, planulating species have high mortality. Most other studies have also reported a significant positive relationship between high mortality and high recruitment rate (e.g. Hughes 1988). Approximately 50% of the turnover in the coral population in this study was attributable to P. damicornis, which had high recruitment and mortality at all sites. Where the cause of mortality could be ascertained for Solitary Islands corals, the most common cause of mortality was storm damage following periods of high wave action. Storms are frequent within the region and, for example, there were 17 separate storm events during which significant wave heights (H sig ) exceeded 3 m between 1993-1994 (Manly Hydraulics Laboratory 1994). The storm which had an impact on corals during 1995 was associated with wave heights in excess of 6 m over a two day period, and maximum wave heights of 10 m during the peak of the storm (Smith unpublished). Evidence of storm-induced mortality was wide-spread, both at Split Solitary Island and at other sites (Smith unpublished), with breakage of corals and loss of whole colonies. While stormmage has long been recognised as a major factor regulating coral reef communities, there has recently been increased emphasis on the role of

Table 5. Comparative values for coral mortality rates from field studies. Taxon Location Mortality rate Est. annual (sample period) mortality rate Reference P. damicornis Lizard Is, GBR 44% (18mo) 31% Harriott 1985 Solitary Islands 64% (5 yr) 19% This paper Stylophora pistillata Red Sea 70-95% (3yr) 55-65% Loya 1976a,b 80-85% (2.3yr) Acroporidae (small) Green Is, GBR 26% (10mo) 30% Harriott and Fisk 1989 Acropora sp. Solitary Islands 38% (5 yr) 9% This paper Favia favus Lizard Is, GBR 5% (16mo) 3% Harriott 1985 Faviidae (small) Green Is, GBR 25% (10 mo) 30% Harriott and Fisk 1989 Goniastrea sp. Magnetic Is, 12-24% Babcock 1991 Platygyra sp. Orpheus Is, GBR G. australensis Solitary Islands 21% (5 yr) 5% This paper Diploria strigosa St Croix 32% (26mo) 18% Bythell et al. 1993 M. annularis St Croix 18% (26mo) 10% Bythell et al. 1993 M. annularis Jamaica 40% (5yr) Hughes 1988 Massive Porites Lizard Is, GBR 20% (16mo) 12% Harriott 1985 P. heronensis Solitary Islands 91% (5yr) 38% This paper Poritidae (small Green Is, GBR 26% (10mo) 30% Harriott and Fisk 1989 corals) Porites astreoides St Croix 34% (2 yr) 19% Bythell et al. 1993 All corals Solitary Islands 53% (5yr) 11% This paper All corals Heron Is, GBR 0-30% Connell 1973 Small corals Green Island, 25% (10mo) 30% Harriott and Fisk 1989 GBR All corals Curacao 2-19% Bak and Luckhurst 1980 All corals Jamaica 38% (3yr) 15% Hughes and Jackson 1985 All corals Jamaica 72% (10yr) 12% Hughes 1988 storms in preventing reef accretion in exposed shore-lines, for example in Hawaii (Dollar and Tribble 1993, Grigg 1998). The coral communities of high latitude eastern Australia (Banks and Harriott 1995, Harriott et al. 1994, 1995, 1999) resemble these exposed tropical communities in that there is a high cover of coral, with little to no build up of coral framework, almost certainly due to the removal of either living or dead corals by high wave conditions. The second clear cause of coral mortality was coverage with Chaetopterus worm tubes (Smith and Harriott 1998) at both Split Solitary and North West Solitary Islands. Worm tube aggregations were up to 22 cm deep and over 2 m in diameter in the largest patches at both sites. There was evidence of considerable mobility in the positions of tube aggregations at both islands, with aggregations in some cases moving a metre or more between census periods. Several markers became visible in 1996 and 1998 after having been buried under worm tubes for the previous two samples. The extent of the impact of the worm tubes on the coral community was variable. In some cases corals were covered and smothered by the tubes, while in a few cases, the corals re-appeared in subsequent samples with minor partial mortality. There was no evidence from this study for mortality by overgrowth by macroalgae as a significant factor structuring these coral communities. At Muttonbird Island, a nearshore turbid habitat with high macroalgal cover, including significant cover of the kelp Ecklonia radiata, coral cover doubled over the study period. There was some evidence in the study for localised abrasion of corals by kelp fronds where they occurred adjacent to each other, but no evidence that this caused more than partial mortality of the colonies affected. A change from a coral-dominated to a macroalgal-dominated community along a gradient of adverse environmental conditions for corals has been suggested in several studies for both highlatitude and low-latitude communities (Johannes et al. 1983, Crossland 1988, Coles 1988, Sheppard 1988). At the Solitary Islands during the study period, close proximity to abundant, and sometimes overtopping macroalgae, did not cause significant coral mortality, as has also been reported by Coles (1988) for the Arabian Gulf. The role of competition between corals and algae in regulating reef environments has been reviewed by McCook et al. (2001). Many studies have suggested that, for both high latitude reefs (Veron and Done 1979, Tribble and Randall 1986) and for tropical reefs subject to unusual physical events such as cold water runoff from enclosed shallow bays (Roberts et al. 1982, Burns 1985), periodic high mortality rates can regulate coral community structure and alter species composition (Davis 1982, Porter et al. 1982). Water temperature at the Solitary Islands was measured for approximately two years during the study period (Harriott 1999a). Water temperature only occasionally fell below 18º C, considered by many to be the lower temperature limit for reef corals. The Solitary Islands are commonly bathed in the southward stream of the East Australian Current, which carries warm water, and potentially tropical larvae, south from the Great

Barrier Reef. The movement of this current inshore and offshore can commonly result in temperature variations of 6º C within a few days. If periodic water temperature variations significantly affect coral mortality at the Solitary Islands, this did not occur within the five year time span of this study, but could be significant over a longer time scale. Interestingly, the water temperature at Muttonbird Island was generally about 0.5º C cooler than at offshore sites, but showed low mortality and increases in coral cover throughout the five year study period. In summary, recruitment of juvenile corals at the Solitary Islands was lower than for most tropical studies, but coral mortality was also lower, and for the dominant coral taxa, mortality was positively correlated with recruitment. The alternative, that low recruitment was not matched by low mortality, would indicate the potential for a gradual decline in the coral population, which was not evident in the five year study reported here. The most significant influence on coral mortality during the study period was periodic storm damage, followed by competitive overgrowth by worm tubes. Mortality as a result of macroalgal overgrowth was insignificant during this study. There was also no evidence for periodic death as a result of exposure to cold water below the coral s tolerance limits, but such a factor could be significant on a longer time scale. We suggest, consistent with recent theories for exposed tropical shores, that removal of living and dead coral skeletal material by episodes of severe wave conditions is the factor most likely to inhibit reef accretion at this high latitude location. Acknowledgements The authors thank the field assistants over the years for their help, in particular, Simon Banks and Lisa Roberts. Chris Connelly of Dive Quest, Mullaways Beach, also provided invaluable assistance. This research has been supported by funding from Southern Cross University and University of New England, and by the CRC Reef Research Centre. References Babcock RC (1991) Comparative demography of three species of scleractinian corals using age- and sizedependent classification. Ecol Monogr 61: 225-244 Banks SA, Harriott VJ (1995) Coral communities of the Gneering Shoals and Mudjimba Island, south-eastern Queensland. Mar Freshwat Res 46: 1137-1144 Banks SA, Harriott VJ (1996) Patterns of hard coral recruitment at the Gneering Shoals, south-east Queensland. Coral Reefs 15: 225-230 Bak RPM, Luckhurst BE (1980) Constancy and change in coral reef habitats along depth gradients at Curacao. Oecologia 47: 145-155 Barnes DJ, Lazar B (1993) Metabolic preformance of a shallow reef patch near Eilat on the Red Sea. J Exp Mar Biol Ecol 174: 1-13 Burns T (1985) Hard-coral distribution and cold-water disturbances in South Florida: variation with depth and location. Coral Reefs 4: 117-124 Bythell JC, Gladfelter EH, Bythell M (1993) Chronic and catastrophic natural mortality of three common Caribbean reef corals. Coral Reefs 12: 143-152 Coles SL (1988) Limitations on reef coral development in the Arabian Gulf: temperature or algal competition? Proc 6 th Int Coral Reef Symp 3: 211-216 Coles SL, Fadlallah YH (1991) Reef coral survival and mortality at low temperatures in the Arabian Gulf: new species-specific lower temperature limits. Coral Reefs 9: 231-237 Connell JH (1973) Population ecology of reef-building corals. In: Jones OA, Endean R (eds) Biology and Geology of Coral Reefs V2, Academic Press, New York: 205-245 Connell JH (1978) Diversity in tropical rain forests and coral reefs. Science 199: 1302-1310 Connell JH, Hughes TP, Wallace CC (1997) A 30-year study of coral abundance, recruitment, and disturbance at several scales in space and time. Ecol Monogr 67: 461-488 Crossland CJ (1988) Latitudinal comparisons of coral reef structure and function. Proc 6 th Int Coral Reef Symp 1: 221-226 Davis GE (1982) A century of natural change in coral distribution at the Dry Tortugas: a comparison of reef maps from 1881 and 1976. Bull Mar Sci 32: 608-623 Dollar S, Tribble GW (1993) Recurrent storm disturbance and recovery: a long-term study of coral communities in Hawaii. Coral Reefs 12: 223-233 Dunstan P (1977) Vitality of reef coral populations off Key Largo, Florida: recruitment and mortality. Environm Geol 2: 51-58 Edmondson CH (1929) Growth of Hawaiian corals. Bernice P Bishop Mus Bull 58, Honolulu Hawaii Fisk DA, Harriott VJ (1989) The effects of increased sedimentation on the recruitment and population dynamics of juvenile corals at Cape Tribulation, North Queensland. GBRMPA Tech Mem 20: 31 pp Grigg RW (1982) Darwin Point: a threshold for atoll formation. Coral Reefs 1: 29-34 Grigg RW (1998) Holocene coral reef accretion in Hawaii: a function of wave exposure and sea level history. Coral Reefs 17: 263-272 Harriott VJ (1983) Reproductive ecology and population dynamics in a scleractinian coral community. Ph.D. thesis, James Cook University, Townsville, Australia: 189 pp Harriott VJ (1985a) Mortality rates of scleractinian corals before and during a mass bleaching event. Mar Ecol Prog Ser 21: 81-88 Harriott VJ (1985b) Recruitment patterns of scleractinian corals at Lizard Island, Great Barrier Reef. Proc 5th Int Coral Reef Congress, Tahiti 4: 367-372 Harriott VJ (1992) Recruitment patterns of scleractinian corals in an isolated sub-tropical reef system. Coral Reefs 11: 215-219 Harriott VJ (1995) Is the crown-of-thorns starfish a threat to the reefs of Lord Howe Island? Aquatic Conserv: Mar Freshw Ecosys 5:179-190 Harriott VJ (1999a) Coral recruitment at a high latitude Pacific site: a comparison with Atlantic reefs. Bull

Mar Sci 65: 881-891 Harriott VJ (1999b) Coral growth in subtropical eastern Australia. Coral Reefs 18: 281-291 Harriott VJ, Banks SA (1995) Recruitment of scleractinian corals in the Solitary Islands Marine Reserve, a high latitude coral-dominated community in Eastern Australia. Mar Ecol Prog Ser 123: 155-161. Harriott VJ, Fisk DA (1989) The natural recruitment and recovery process of corals at Green Island. GBRMPA Tech Mem 15: 36 pp Harriott VJ, Simpson CJ (1997) Coral recruitment in Western Australia. Proc 8th Int Coral Reef Symp, Panama. 2: 1191-1196 Harriott VJ, Smith SDA, Harrison PL (1994) Patterns of coral community structure of subtropical reefs in the Solitary Islands Marine Reserve, Eastern Australia. Mar Ecol Prog Ser 109: 67-76 Harriott VJ, Harrison PL, Banks SA (1995) The coral communities of Lord Howe Island. Mar Freshwat Res 46: 457-465 Harriott VJ, Banks SA, Mau RL, Richardson D, Roberts LG (1999) Ecological and conservation significance of the subtidal rocky reef communities of northern New South Wales, Australia. Mar Freshwat Res 50: 299-306 Hatcher BG, Rimmer DW (1985) The role of grazing in controlling benthic community structure on a high latitude coral reef: Measurements of grazing intensity. Proc 5 th Int Coral Reef Congress, Tahiti 6: 229-236 Holmes NJ, Harriott VJ, Banks SA (1997) Latitudinal variation in patterns of colonisation of cryptic calcareous marine organisms. Mar Ecol Prog Ser 155: 103-113 Hughes TP (1985) Life histories and population dynamics of early successional corals. Proc 5 th Int Coral Reef Congress 4: 101-106 Hughes TP (1988) Long-term dynamics of coral populations: contrasting reproductive modes. Proc 6 th Int Coral Reef Symp 2: 721-725 Hughes TP, Connell JH (1999) Multiple stressors on coral reefs: a long-term perspective. Limnol Oceanogr 44: 932-940 Hughes TP, Jackson JBC (1985) Population dynamics and life histories of foliaceous corals. Ecol Monogr 55: 141-166 Johannes RE, Wiebe WJ, Crossland CJ, Rimmer DW, Smith SV (1983) Latitudinal limits of coral reef growth. Mar Ecol Prog Ser 11: 105-111 Kudo K, Yamano H (1997) Dynamic structure of coral reef communities: a simulation study. Proc 8 th Int Coral Reef Symp 1: 509-514 Loya Y (1976a) The Red Sea coral Stylophora pistillata is an r strategist. Nature 259: 478-480 Loya Y (1976b) Settlement, mortality and recruitment of a Red Sea scleractinian coral population. In: Mackie GO (ed) Coelenterate ecology and Behavior. Plenum Publ Corp: 89-100 Manly Hydraulics Laboratory (1994) New South Wales Wave Climate Annual Summary 1993/94. Report MHL695. Manly Hydraulics Laboratory, Sydney McCook LJ, Jompa J, Diaz-Pulido G (2001) Competition between corals and algae on coral reefs: a review of evidence and mechanisms. Coral Reefs 19: 400-417 Miller MW (1998) Coral/seaweed competition and the control of reef community structure within and between latitudes. Oceanogr Mar Biol Ann Rev 36: 65-96 Miller MW, Hay ME (1996) Coral-seaweed-grazernutrient interactions on temperate reefs. Ecol Monogr 66: 323-344 Mumby PJ (1999) Can Caribbean coral populations be modelled at metapopulation scales? Mar Ecol Prog Ser 180: 275-288 Porter JW, Battey JF, Smith GJ (1982) Perturbation and change in coral reef communities. Proc Natl Acad Sci 79: 1678-1681 Porter JW, Woodley JD, Smith GJ, Neigel JE, Battey JF, Dallmeyer DG (1981) Population trends among Jamaican reef corals. Nature 294: 249-250 Preece AL, Johnson CR (1993) Recovery of model coral communities: complex behaviours from interaction of parameters operating at different spatial scales. In: Green DG, Bossomaier T (eds) Complex systems: from biology to computation. IOS Press, Amsterdam: 69-81 Roberts HH, Rouse LJ, Walker ND, Hudson JH (1982) Cold-water stress in Florida Bay and Northern Bahamas: a product of winter cold-air outbreaks. J Sedim Petrol 52: 145-155 Rosen BR (1988) Progress, problems and patterns in the biogeography of reef corals and other tropical marine organisms. Helgol Meers 42: 269-301 Sheppard CRC (1988) Similar trends, different causes: responses of corals to stressed environments in Arabian Seas. Proc 6 th Int Coral Reef Symp 3: 297-302 Smith SDA, Simpson RD (1991) Nearshore corals of the Coffs Harbour region, mid-north coast New South Wales. Wetlands (Aust) 11: 1-9 Smith SDA, Harriott VJ (1998) Tube-building polychaete worms smother corals in the Solitary Islands Marine Park, northern NSW. Coral Reefs 17: 342 Stoddart D (1969) Ecology and morphology of recent coral reefs. Biol Rev 44: 433-498 Tanner JE, Hughes TP, Connell JH (1994) Species coexistence, keystone species and succession: a sensitivity analysis. Ecology 75: 2204-2219 Tanner JE, Hughes TP, Connell JH (1996) The role of history in community dynamics: a modelling approach. Ecology 77: 108-117 Tribble GW, Randall RH (1986) A description of the high-latitude shallow water coral communities of Miyake-Jima, Japan. Coral Reefs 4: 151-159 Veron JEN (1974) Southern geographic limits to the distribution of Great Barrier Reef corals. Proc 2 nd Int Coral Reef Symp 2: 465-473 Veron JEN (1995) Corals in space and time: the biogeography and evolution of the Scleractinia. UNSW Press, Australia: 321 pp

Veron JEN, Done TJ (1979) Corals and coral communities of Lord Howe Island. Aust J Mar Freshw Res 30: 203-236 Veron JEN, How RA, Done TJ, Zell LD, Dodkin MJ, O'Farrell AF (1974) Corals of the Solitary Islands, 193-208 central New South Wales. Aust J Mar Freshw Res 25: Wells JW (1957) Coral reefs. Mem Geol Soc America 67: 609-631 Yonge CM (1940) The biology of reef-building corals. Svi Rep GBR Exped 1: 353-391