Coral reef monitoring at Reunion island (Western Indian Ocean) using the GCRMN method

Transcription

1 Proceedings 9 th International Coral Reef Symposium, Bali, Indonesia October 2000, Vol. 2 Coral reef monitoring at Reunion island (Western Indian Ocean) using the GCRMN method P. Chabanet 1, L. Bigot 2, O. Naim 1, R. Garnier 2, E. Tessier 3 and M. Moyne-Picard 4 ABSTRACT Since 1998, a standardised methodology consistent with GCRMN strategy has been used to assess some of the coral reefs in Reunion. Reef assessment started with two sites (St-Gilles/La Saline reef complex) and two others were added in 1999 (St-Leu reef). On each site, two stations were assessed (reef flat and outer slope around 12 m depth). In 2000, results of reef assessment show a high spatial variability between stations, with the highest live coral coverage recorded on St-Leu reef flat (47.8%). The differences between sites can be attributed to physico-chemical water conditions (oligotrophic vs dystrophic waters) and/or differences in the developmental stage of the reef communities (juvenile vs mature community structure). Between 1998 and 2000, coral reefs in Reunion displayed a relative stability that may be correlated with the absence of important cyclone and bleaching events in the last ten years. Keywords Monitoring, GRCMN, Coral and fish communities, Western Indian Ocean Introduction Since 1998, a standardised methodology has been applied to assess some of the coral reefs in Reunion. The same methodology has also been applied in four other island states of the South West Indian Ocean: Madagascar, Comoros, Seychelles and Mauritius. This monitoring formed part of the Regional Environment Program of the Indian Ocean Commission (IOC Program) and the Global Coral Reef Monitoring Network (GCRMN) (Wilkinson et al. 1997). In recent years, all the countries in the region have experienced heavy human growth pressures that are inducing reef deterioration and losses in reef resources. Natural (cyclones) and anthropogenic disturbances affect the reefs in Reunion Island. The latter are often caused by urban development of catchment areas, which has led to increase in nutrient levels in coral reef areas via submarine groundwater discharge (Cuet et al. 1988). As a result of the impact from increased nutrient levels, structure of benthic communities was modified (Cuet et al. 1988, Naim and Cuet 2000). The overall objectives of the IOC/GCRMN program are to improve the management and sustainable uses of coral reefs, and provide individuals, organisations and governments with the capacity to assess their coral reef and collaborate within a global network. In Reunion, the national network is operating at the research level, involving high resolution assessment and small scale monitoring. Since 1998, scientists and institutes currently participate in reef monitoring, and the generated data have been processed in a local database (Villedieu et al. 2000). In this paper, we will present the state of the reefs in Reunion in 2000 and its main evolution within sites between 1998 and Methods La Reunion is a high volcanic island in the western Indian Ocean (21 07 S, E). It has a tropical climate with two seasons: hot and humid (November to May), and cool and dry (June to October). Coral reefs lie along the driest western coast of the island. They are exposed to strong hydrodynamic conditions, mainly due to swells generated by the southeast tradewinds (dry season) and by tropical cyclones (humid season). Tides are semidiurnal and range from 0.1 m to 0.9 m. The narrow (maximal width: 520 m) fringing coral reefs form a discontinuous belt along a 25 km section of the island s 210 km circumference. Montaggioni and Faure (1980) have described the reef geomorphology. From open ocean towards land, the reef can be divided into three parts: the outer slope, the reef flat, and the back reef zone. This study was carried out on two sectors, the St-Gilles/La Saline reef complex (SGLS) which is the most developed reef of the island (9 km long, 520 m wide), and the St-Leu reef (SL) (3 km long, 200 m wide). On each sector (or reef), two monitoring sites were chosen (Fig.1). On each site, two stations were surveyed, one on the inner reef flat and the other on the outer slope (spurs and groove zone, average depth 12 m). The benthic communities of these sites have been studied since On SGLS, the two sites were Toboggan (TB), a healthysite and Planch'Alizés (PA), considered as disturbed site because the reef there has higher levels of nutrients than that of TB (Cuet et al. 1988, Cuet et al. 1998). On SL, the two sites were: Varangue (VA) (Naim et al. 2001), and Corne Nord (CN) (Turquet et al. 1998). On SL, hypersedimentation generated by hurricane Firinga (1989) affected mainly VA reef flat where 99% of coral mortality was recorded (Naim et al. 1997). The present surveys were carried out using a 1 Laboratoire d Ecologie marine, Université de la Réunion, Saint Denis messag Cedex 9, (Réunion, France) chabanet@univ-reunion.fr 2 Agence pour la Recherche et la Valorisation marine, 14 rue du stade de l Est, Ste Clotilde (Réunion, France) 3 Comité Régional des Pêches de La Réunion, 28 rue du Maréchal Galiéni, Le Port (Réunion, France) 4 Association Parc Marin de la Réunion, 3 rue de la Compagnie des Indes, St Leu (Réunion, France) 1

2 Standardised methodology (Conand et al. 1998). First, three replicate 20 m line intercept transects were used to recover the position and extent of different benthic organisms (hard and soft corals, algae, sponges and other benthos) and abiotic substrata. Scleractinian corals were identified to species when possible, and algae were classified into macroalgae, calcareous, turf and algal assemblage. Sponges and other benthos were grouped as others. Second, three 50 m long by 5 m wide belt transects were positioned around the benthic line transects to census fish communities. A series of transverses were conducted in order to count the individual species with respect to their behaviour (Chabanet et al. 1997). For fish community, the following parameters were determined: species richness, total abundance, and abundance of the following trophic groups: herbivores, omnivores, sessile invertebrate browsers, planktivores, carnivores and piscivores (Hiatt and Strasburg 1960, Harmelin-Vivien 1979). Through the IOC Program, sampling was conducted once a year (between December and March) and started in January 1998 for SGLS reef and in 1999 for SL reef. Clustering analysis (Bray-Curtis) followed by a nested ANOVA were conducted on benthos and fish data in order to analyse similarities between stations. Fig. 1 Location of the study sites and stations, A: Gilles / La Saline reef (SGLS), B: St-Leu reef (SL). Results Spatial scale - Results in 2000 (Table 1) St-Gilles / La Saline reef (SGLS) At TB site, the reef flat was characterised by a high percentage of abiotic substrate (59.2%), mainly composed of fine and coarse calcareous sediments. The benthic communities were represented by 23.5% of live corals, including 14.2% branching and digitate Acropora (A. muricata, A. humilis) and 11.2% of algae (essentially turf in damselfish territories). The recently dead corals covered by algae represented 6% of the total coverage. On outer reef slope, 39.4% of the substrate was covered by living corals (including 20.2% of Acropora) and 48.6% by algae. The corals were mainly represented by submassive corals (Acropora danae, A. robusta, Pocillopora sp) and the algae by mixed algal assemblage (36.3%). For fish communities, 57 species were recorded on the reef flat station at a high density (388 ind./100 m 2 ). The main species observed were pomacentrids: Stegastes nigricans (137 ind./100 m 2 ), Dascyllus aruanus (113 ind./100 m 2 ) and Chromis viridis (60 ind./100 m 2 ). These 3 species represented more than 80% of the total number of individuals recorded. Regarding fish diet, there was a high dominance of omnivores and planktivores (43.8% and 38% respectively), as a result of the dominance of pomacentrids in the communities. On the outer slope, the planktivores (mainly represented by Chromis nigrura) and carnivorous fish (mainly represented by gregarious species, Gnathodentex aurolineatus and Mulloides flavolineatus) were dominant (34.2% and 34.5% respectively). Piscivorous fish were nearly absent on this site. 2

3 Table 1 Benthos cover (%) and fish characteristics (including diet categories in % of individuals number) (± standard deviation) on each station (RF: reef flat, OR: outer reef slope) in Sector St-Gilles / La Saline (SGLS) St-Leu (SL) Sites Toboggan (TB) Planch Alizès (PA) Varangue (VA) Corne Nord (CN) Stations RF OR RF OR RF OR RF OR BENTHOS Acropora 14.2 ± ± ± ± ± ± ± 55 Other corals 9.2 ± ± ± ± ± ± ± ± 2.5 Algae 11.2 ± ± ± ± ± ± ± ± 5 Abiotic 59.2 ± ± ± ± ± ± ± ± 0.4 OTHERS FISH Sprichness Abundance 969 ± ± ± ± ± ± ± ± 54 Herbivores 52 ± ± ± ± ± ± ± ± 1.3 Omnivores 43.8 ± ± ± ± ± ± ± ± 1.5 Corallivores 4.4 ± ± ± ± ± ± ± ± 2.8 Planctonoph. 38 ± ± ± ± ± ± ± 4.7 Carnivores 8.5 ± ± ± ± ± ± ± ± 9.1 Piscivores 0.1 ± ± ± ± At PA reef flat, live coral cover was 41.3% including 39.2% of non-acropora (Montipora, Pocillopora and Porites). Algal cover was high (43.7%), mainly represented by macroalgae, turf and calcareous algae on dead corals. The outer slope had a high live coral cover (48.6%) including 28.3% of Acropora (A. digitifera, A. valida), 20.3% of non Acropora (Porites, Favites, Pocillopora) and 8.5% of soft corals. Algal coverage was 38.5%, mainly represented by calcareous algae. Fish communities were characterised by a low density on the reef flat (127 ind./100 m 2 among which 28.5 ind. of Stegastes sp) and on the outer slope (95 ind./100 m 2 among which 35.5 ind. of Chromis nigrura). Fish communities were mainly represented by planktivores (26.4% on the reef flat and 48.9% on the outer slope). Top predators were nearly absent on this site. St-Leu reef (SL) On CN site, both reef flat and outer slope had exceptionally high live coral cover for Reunion. On the reef flat, living coral cover was 53.4%, including 47.8% for Acropora. Algal coverage was low (12.3%), but a high percentage of recently dead coral was recorded (25.2%). The outer slope had even higher coral cover (73.5%), mainly submassive corals (24.6%). Other corals were represented by massive and submassive genera (Porites, Platygyra, Pocillopora, Favia). Algal cover was moderate (18.5%). For fish communities, notwithstanding the high coral coverage of CN, a relatively low density was observed both on reef flat and on outer slope (104 ind. and 107 ind./100 m 2 respectively). Omnivores were dominant (45.8% on reef flat and 28.8% on outer slope). On VA reef flat, live coral cover was 46.5% and the same benthic patterns as described in CN site were observed. Algal coverage was 15.5% and abiotic coverage 28.5%. A relatively high cover of recently dead coral (7.8%) was observed, with mortality attributed to exposure during the very low tides that occurred in On the outer slope, live coral cover was fairly high (45.8%), including 16.6% of Acropora. Other corals were represented by massive corals (Porites, Favites, Platygyra). Fish abundance was high (348 ind./100 m 2 ) on the reef flat, mainly Stegastes nigricans (64% of total individuals recorded). Due to the high density of S. nigricans, the majority of fish recorded were omnivores (69.5%). On the outer slope, a low fish density (69 ind./100 m 2 ) was observed and carnivores were dominant (36.7%, mainly represented by Mulloides vanicolensis). A low density of top predators was observed. Clustering analysis shows similarities between outer reef stations, characterised by algae, non- Acropora corals, others benthic organisms, planktivores and carnivores fish. On the reef flat, communities varied greatly among stations (significant differences between TB, PA-CN and VA, ANOVA p < 0.05). TB is characterised by Acropora and omnivorous fish, PA and CN by abiotic substrate and corallivorous fish and VA by piscivorous fish. Temporal scale - Changes between 1998 and 2000 (Table 2). Only major shifts observed between 1998 and 2000 will be described, considering the sector and not the site (data of the two sites have been pooled on each sector). St-Gilles/ La Saline reef ( ) Benthic communities were relatively stable through time on reef flat and on outer slope, for live coral (mean of the 3 years = 31% and 44% respectively), for algae (19% and 42% respectively) and for abiotic substrate (46% and 6% respectively). For fish communities, density and relative proportion of the diet categories were relatively stable through time, with a predominance of omnivores on the reef flat (mean of the 2 years = 35%) and planktivores plus carnivores on the outer slope (44 % for planktivores and 24% for carnivores). However, on the reef flat, there was a slight (but no significant) increase of corallivorous fish. 3

4 Table 2 Change in benthos cover (%) and fish diet categories (in % of individual number) (± standard deviation) on the reef flat (RF) and the outer reef slope (OR) between 1998 and For each sector, data represent the mean of the two stations (TB and PA for St-Gilles sector and CN and VA for St-Leu sector). (-): no data. Sector St-Gilles / La Saline (SGLS) St-Leu (SL) Year Station RF OR RF OR RF OR RF OR RF OR BENTHOS Live corals (all) 25.2 ± ± ± ± ± ± ± ± ± ± 421 Algae 16.1 ± ± ± ± ± ± ± ± ± ± 32 Abiotic substate 51.3 ± ± ± ± ± ± ± ± ± ± 0.9 Recent dead 7.4 ± ± ± ± ± ± ± ± ± ± 1.5 Others ± ± ± ± ± ± ± 1.1 FISH Herbivores ± ± ± ± ± ± ± 1.8 Omnivores ± ± ± ± ± ± ± ± 5.1 Corallivores ± ± ± ± ± ± ± ± 2.4 Planctonophages ± ± ± ± ± ± ± ± 3.1 Camnivores ± ± ± ± ± ± ± ± 8.6 Piscivores ± ± ± ± ± ± ± ± 0.3 St-Leu reef ( ) The live coral cover at SL was high (52%) for a typical Reunion reef flat. This result showed an apparent increase compared with 10 years ago when live coral was nearly 0% after the passage of Firinga (Naim et al. 1997, 2001). However, a rise of recently dead corals was also observed between 1999 and The same patterns were observed on the outer slope, i.e. a high live coral coverage (60%) and a slight rise in recently dead corals. Fish communities of the reef flat displayed a relative stability in terms of density and distribution of diet categories with the dominance of omnivores (55%). On the other hand, on the outer slope, while total fish density appeared stable, there was a decrease in herbivores that might be due to an increase of carnivores. The relative proportion of the different diet categories observed in 2000 in SL was more typical for Reunion outer slope (Chabanet et al. 1997). Top predators remained nearly absent in the area. Clustering analysis confirmed that there is no effect of time on benthic and fish communities structure during the sampling period (similarity higher than 80% for each station through time). Discussion Spatial scale Our results indicated an important spatial variability on benthic communities between SL and SGLS sectors. In 2000, a high live coral cover was observed on SL reef flat (CN: 53%, VA: 46%) while in SGLS reef, live coral cover varied between 23% (TB) and 39% (PA). The high coral coverage observed in SL may be explained by the young recovery state of SL reef after the impact of Firinga that allowed a high growth rate of living coral due to lowered competition between different species (Naim et al. 2001). Nevertheless, coral communities in SL have not yet reached their mature stage as in SGLS, and succession is likely to continue. On SGLS, there was an apparent algal development on PA reef flat and TB outer slope while the algal cover was low on TB reef flat and PA outer slope. The spatial variability observed within this sector could be due to hydrogeological factors as a result of the discharge of groundwater enriched in nutrients before or after the reef front (Join et al. 1988). Trophic characteristics of fish communities were relatively homogeneous within each sector for outer slopes, with the dominance of planktivores on SGLS and carnivores on SL. This is a common feature observed in the outer slope, where planktivores are often found in the speed of currents that transport plankton onto the reef (Thresher 1983). For carnivores, their high proportion reflect a better structuring of fish communities on the outer slope, disturbed environments are often characterised by a food chain reduction which favour opportunistic fish (Caswell and Cohen 1991). However, the very low density of top predators (Serranidae, Lutjanidae) may indicate an intensive fishing pressure on Reunion island outer slope, as density of predators has been commonly attributed to human pressure (Mac Manus et al. 1981). The homogeneity of the distribution of diet categories in fish communities is observed in the reef flat with the dominance of omnivores, mainly represented by Pomacentridae, although their proportions are different between sites. As the majority of pomacentrids are small, territorial, or live close to the substratum, they appear to be influenced the most by the morphological characteristics of the substratum (Roberts and Ormond 1987), and are often associated with branching corals (Chabanet et al. 1997). The absence of branching corals on PA could explain the lower omnivores percentage observed in this station. On the other hand, on VA, the high percentage of omnivores (69.5%) could be attributed to the large number of the territorial Stegastes nigricans that live on turf algae. The recent very low tides, which killed the tops of many coral colonies, might have favoured algal colonisation and the subsequent settlement of this damselfish. A relatively low fish density was observed on CN reef flat despite the high branching coral coverage. This result may be due to 4

5 the high hydrodynamic conditions found in this station which may limit the settlement of pomacentrids (mainly S. nigricans, D. aruanus), usually highly represented in Reunion's reefs. Temporal scale Between 1990 and 2000, reefs in Reunion showed no real changes over time in benthic community structure, except in SL sector (Naim et al. 1997). In the SGLS sector, all the stations show a relative stability of the living coral coverage. This pattern suggests a state of equilibrium characteristics of mature reef. In SL, the reef has been in a state of regeneration after the impact of Firinga, with the consequence of a high coral cover from recovery. The overall coral reef stability observed in our results appears to be correlated with the absence of important cyclone and bleaching events these last 10 years. Furthermore, the severe widespread coral bleaching event of summer generated by El Nino in the Indian Ocean caused only minor coral bleaching in Reunion, with moderate and patchy occurrence in localised areas (Quod 1999). In 2000, the bleaching observed in Reunion appeared to be related to exposure during extreme low tides that killed the tops of many colonies which were subsequently invaded by turf algae. Fish communities were relatively stable between 1998 and 2000, except on CN reef flat where a noticeable increase (but not significant) in Chaetodontidae was observed. In this family, species are mainly corallivores (Harmelin- Vivien 1979, Harmelin-Vivien and Bouchon-Navaro 1983). Therefore, the high vitality of this station would favour the settlement of these species often correlated with high live coral cover (Bell and Galzin 1984, Bouchon-Navaro and Bouchon 1989, Roberts et al. 1992, Chabanet et al. 1997). On a broader scale, for Reunion reefs, even with the relatively healthy conditions observed in coral communities, top fish predators are almost absent inside fish communities. This result points out a real problem of overexploitation which warrants special attention. Benthic and fish communities on SGLS and SL reefs display a relative stability that is mainly correlated with the absence of important cyclone and bleaching events in these last 10 years. In Reunion, the IOC program aims to develop a real National Reef Network including institutional, technical and scientific actors. Results of reef assessment and monitoring constitute an evolutionary and comparative database that can be especially valuable in long term. Acknowledgements Thank to C. Villedieu and K. Mété who helped entering data in COREMOI. This research was supported by Region Reunion and by Marine Park of Reunion. Great thanks are also expressed to C. Cheung and L. 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