Factors influencing coral recruitment patterns in the Sulu Sea marine corridors

Transcription

1 Proceedings of the 11 th International Coral Reef Symposium, Ft. Lauderdale, Florida, 7-11 July 2008 Session number 10 Factors influencing coral recruitment patterns in the Sulu Sea marine corridors J.R. Garcia 1 and P.M. Aliño 2 1) Institute of Environmental and Marine Sciences, Silliman University, Bantayan, Dumaguete City 6200, Philippines 2) Marine Science Institute, University of the Philippines, Diliman, Quezon City 1101, Philippines Abstract. Coral recruitment studies provide crucial information on a reef s capacity to recover from disturbances such as storms, coral bleaching, or destructive fishing. The Sulu Sea is at the apex of a coral triangle, yet little is known on coral recruitment or reef resilience in this area. This study aimed to determine patterns in the abundance and composition of early coral recruits in three major marine corridors of the Sulu Sea. Five hundred nineteen terracotta tiles were deployed for six months along 50-m transects in 33 sites along the Cagayan Ridge, Balabac Islands, and Verde Passage. Results show significantly higher mean recruit density in the Balabac Islands, where destructive fishing methods continue to be used, than in the Cagayan Ridge, which is a protected area. At a smaller scale, density patterns of early recruits show no direct correlations with the adjacent benthic community. High early recruit density was found in sites with derived high entrainment by currents, suggesting that physical rather than biological factors play a major role in determining early recruitment at the scale of both corridors and sites. Key words: Coral recruitment; Sulu Sea Introduction The Indo-Malay-Philippines coral triangle possesses the highest marine biodiversity in the world (Hoeksema 2007), and the Sulu Sea in central Philippines has among the highest concentrations of species per unit area in this region (Carpenter and Springer 2005). Reefs in the Sulu Sea are important not only for their high biodiversity and unique geology, but also as potential sources and sinks of larvae for other reefs, thus maintaining marine biodiversity in other Philippine inland seas (Alcala 1993). Understanding reef community dynamics, particularly recruitment processes, is crucial in determining reef recovery rates from perturbations due to human and natural disturbances (Hughes et al. 2005; Coles and Brown 2007). Larval supply is determined by various factors, such as spawning season of adult coral species in the community and proportion of fecund adults (Hughes et al. 2000; Adjeroud et al. 2007). The amount and composition of larvae produced, however, can be very different from a reef s actual juvenile community structure. Studying larval settlement patterns may instead provide more insight into early recruitment dynamics. Coral recruitment patterns depend on a reef s physical and biological attributes, such as size (Glassom et al. 2006), morphology and existing community (Mumby et al. 2007; Norström et al. 2007; Perkol- Finkel and Benayahu 2007), as well as speed and direction of ocean currents. These influence how and where larvae are distributed, and eventually settle (Banks and Harriott 1996; Chen 1999; Taylor and Pearce 1999). The abundance and composition of recruits also change with increasing distance from the reef, a function of both current velocities and differing reproductive strategies among coral species (Sammarco and Andrews 1989). This study sought to identify patterns in coral recruit abundance and composition in three Sulu Sea marine corridors at two spatial scales, i.e. within and among corridors. Possible physical and biological factors that might explain these patterns were investigated to gain insights on what processes influence the relative resilience and recovery of reefs from disturbances. Material and Methods Study site and data collection The study was conducted in the major marine corridors of the Sulu Sea: Verde Passage in the north, Cagayancillo Islands and Tubbataha Reefs, comprising the Cagayan Ridge, in the central area, and Balabac Islands in the southwest (Fig. 1). Approximately 16 terracotta tiles, each measuring 10 cm x 10 cm x 1.2 cm, were deployed along a 50-m transect in each of 33 sites. Sets of four tiles were nailed individually to the substrate at roughly 12-m intervals along each transect (Mundy 2000; Quibilan and Aliño 2004). The tiles were deployed at the onset of the northeast (NE) monsoon season in October 2006, and retrieved in April and May 2007, before the onset of the southwest (SW) monsoon season. Data gathered, therefore, represents recruitment for the NE 291

2 monsoon season. This season also covered the peak coral spawning months in the Philippines (Bermas et al. 1992; Atrigenio and Aliño 1994). Upon retrieval, the tiles were air-dried, then examined under a stereoscope. The number of coral recruits on the top, undersurface, and sides of the tiles were counted and identified to family level according to the guide provided by Babcock et al (2003). Due to natural disturbances, not all deployed tiles were recovered. Data analyses Differences in mean recruit abundance at both site and corridor level were compared using the Analysis of Variance (ANOVA). Abundance data were fourth root-transformed prior to statistical analysis to conform to the Cochran C, Hartley, and Bartlett tests for homogeneity of variances. Tukey s Unequal N Honestly Significant Difference was used as post-hoc test. To determine differences in recruit composition among sites and corridors, abundance data for each coral family were simultaneously compared among sites using the Analysis of Similarities (ANOSIM) based on the Bray-Curtis Similarity index (Clarke and Warwick 2001). Recruit density and composition data in each site were tested for correlations with geographical location and benthic composition of the surrounding reef using Pearson product-moment correlation, and Similarity of Percentages (SIMPER) based on Spearman rank correlations. Data collected by Deocadez et al. (this symposium s proceedings) on live benthic cover of the adult reef community using video transects was used for this analysis. Figure 1: Location map of sampling sites with mean recruit density and composition per corridor. Each corridor spanned an area ranging from km2, with Verde Passage being the smallest and Balabac Islands being the largest, due to its offshore islands. Adjacent corridors ranged from being km apart. Each site covered an area of 100 m2. Adjacent sites within corridors ranged from being 50 m-45 km apart. There were 6 sites in Verde, 8 in Tubbataha, 9 in Cagayancillo, and 10 in Balabac. Results Corridor Level Mean recruit density was significantly higher in Balabac Islands than in the central corridors of Tubbataha Reefs and the Cagayancillo Islands (Fig. 2). Verde Passage did not differ significantly from either Balabac or Cagayancillo. Balabac also had the largest range in number of recruits per tile, while Tubbataha had the smallest (Fig. 2). Pocilloporids dominated in Balabac and Verde, while acroporids dominated in the central Sulu Sea (Fig. 1). Poritids and other families each comprised less than 10% of recruits in all corridors. Sixty-nine percent of the recruits were found on the sides of the tiles, 19% on the undersurface, and 12% on the top. This trend was consistent in each corridor. Figure 2: Mean recruit density with range of extreme values per corridor. Bars with the same letters are not statistically different. 292

3 Figure 3: Mean recruit density and composition per site. Bars with up arrows indicate sites not statistically different from Melville. Bars with down arrows indicate sites not statistically different from Balabag In. Sites with no arrows are not statistically different from both Melville and Balabag In. Figure 4: Map of mean recruit density per site, within corridors. a) Cagayancillo Islands [inset: Cawili Island, approx. 50 km SW] b) Tubbataha Reefs c) Balabac Islands [inset: Mangsee Islands, approx. 45 km SE] d) Verde Passage. 293

4 Site Level Melville in Balabac had the highest mean recruit density among all sites (Fig. 3). Balabag In, found in Cagayancillo, had the lowest. Sites with high recruit density (i.e., not statistically different from Melville but statistically different from Balabag In) tended to be in areas between islands or in island wakes, with the possible presence of eddies (pers. observations from derived current circulation; see Vilanoy et al., this symposium s proceedings). In Balabac, sites less than 500 m from the nearest shoreline (Cabuang, Poblacion, Bancalaan 1) had significantly lower mean recruit density than sites farther offshore. In the central Sulu Sea, outer reefs generally had lower recruit density than those closer to islands or other geological barriers. (Fig. 4) Melville and Malinsono 1 in Balabac, and Talaga In and Out in Cagayancillo had significantly higher acroporid recruitment than all the other sites. Pocilloporid recruits generally dominated in sites on the western side of the islands, as opposed to the eastern side, in both Balabac and Verde (ANOSIM R=0.417, p=0.006). No apparent patterns in recruit composition were found in the central Sulu Sea sites in relation to their location on the reef. Percent cover of adult Acropora along the transect, and the number of acroporid recruits on the tiles were not found to be correlated (Pearson product-moment correlation, r= ). Neither were recruit density and composition found to be associated with any other benthic category or with over-all benthic composition in each site (Spearman rank correlation, ρ s =0.086). Discussion Differences in patterns of recruit abundance among the marine corridors suggest that individual physical factors have different degrees of influence on settlement rates. Distinct features of each corridor may magnify or reduce these effects. Higher mean recruit density occurs in the Balabac Islands despite continuing destructive fishing activities. Contrary to this, Tubbataha Reefs and Cagayancillo Islands have relatively lower early recruit abundance despite being protected for years. Two factors may be most crucial: larval supply and entrainment potential. Balabac is at the junction of two possible sources of coral larvae, the South China Sea and the Sulu Sea, which potentially increases its larval supply. Monsoonal wind-driven currents interacting with the relatively more complex configuration of islands in Balabac may result in the aggregation or entrainment of coral larvae, increasing their chances of settlement. As previously noted, sites with high recruit density, regardless of which corridor they were found in, tended to be between islands or in island wakes, which have high larval entrainment potential (Magno and Villanoy 2006). The greater number of islands in Balabac may also create more heterogeneous microhabitats with varying suitability for larval settlement, as suggested by the large range in recruit densities within this corridor compared to other corridors. This may explain why the Tubbataha Reefs and Cagayancillo corridors have the lowest mean recruitment, being reef slope sites located at least 200 km from other reef agglomerations, such as those in the Visayan seas and Palawan mainland. Recruitment patterns differ between coastal waters and open seas because of very different hydrodynamics created by factors such as bathymetry and stratification nearer to shore (Pineda et al. 2007). Sammarco and Andrews (1988,1989) showed that highest recruitment occurs in the area of a reef with the lowest flushing rate. On the other hand, slower currents in inner reefs or the lee of islands tend to have higher sedimentation rates, which may inhibit recruitment (Murray et al. 1977; Connell et al. 1997). Ideal conditions for larval settlement may therefore be a complex topography that produces eddies which increase the larvae s residence time, in combination with strong currents that constantly flush out sediment. These findings highlight how major consideration must be given to current patterns in prioritizing sites for protection. Apparent patterns in recruit composition at the corridor level (N-S) and site level (E-W) suggest that monsoonal wind-driven currents may determine the sources and sinks of larvae, depending on the site s geographical location in relation to the prevailing wind. Such monsoonal variation in recruit composition was observed in Palawan shelf reefs in the central Sulu Sea, where acroporids dominated during the SW monsoon, and pocilloporids during the NE monsoon (Quibilan and Aliño 2004). These patterns may also be due to seasonal currents interacting with species-specific peaks in coral spawning, as well as latitudinal differences (Hughes et al. 2002). The lack of concordance between recruit and adult hard coral composition is not unusual, as seen in other studies (e.g. Banks and Harriott 1996; Hughes et al. 1999). This affirms the importance of post-settlement events in determining adult community structure following both anthropogenic and natural disturbances, such as crown-of-thorns outbreaks, storms and thermal anomalies resulting in bleaching (Arceo et al. 2001; Quibilan and Aliño 2004; Victor 2008). Acknowledgements Many thanks to the Community Ecology Laboratory of UP-MSI for technical and field support, AH Baird and RC Babcock for advice in recruit identification, and most importantly, Conservation International-Philippines for funding this study as part of the Completing the Connectivity Cycle for Adaptive Management: Coral reef ecosystem-based MPA network management chain project, and providing travel support to the first author for this symposium. 294

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