Fringing Reefs of Reunion Island and Eutrophication Effects - Part 1: Long-Term Monitoring of Two Shallow Coral Reef Communities

This study assesses changes in subtidal benthic communities on the largest reef flat in Reunion, Saint-Gilles La Saline , using several types of surveys. Temporal and spatial trends are documented over a 22 year period (1987-2009), thus spanning the 1998 and 2000s bleaching events. The most plausible explanations for the observed trends are proposed. We chose two sites that are characterized by two types of community and metabolism: (1) an oligotrophic site dominated by Acropora corals (Site-Toboggan), where sea-urchins are numerous and macroalgae rare and (2) a dystrophic site dominated by non- Acropora corals, mostly massive and submassive, where macroalgae abound and sea-urchins are almost absent (Site-Planch’Alizes). Results are presented in three parts : Part 1 : general trends of the communities, part 2 : primary producers, part 3 : living corals. Part 1 presents three surveys. Survey 1 reports status and trends across the reef flats in 1993, 1996, and 2002, with all attached benthic components reported at the level of the species where possible. Survey 2 reports composition and changes in associated sedentary organisms such as sea urchins, holothurids, and the Pomacentridae fish Stegastes . Survey 3 focuses on the period 1998 to 2009 on permanent transects established in 1987 and monitored periodically, partly as a contribution to the Global Coral Reef Monitoring Network (GCRMN). In general, community structures at both locations remained stable, with primary producer coverage around 5% at Toboggan (T) and around 60% at Planch’Alizes (P). Living coral coverage averaged around 17% at both sites, but it was dominated by Acropora on Site-T (68%) and by massive corals on Site-P (88%). Regular echinoids were very abundant at T but almost absent from P, while the opposite was true for holothurids. The territorial damselfish Stegastes was of comparable abundance at both sites (<1 individuals per m2). There were large changes in coral cover in some zones, especially due to growth in Acropora from 1993 to 2002. On the two GCRMN permanent transects, 1999 and 2004 were turning points, so results are reported in three blocks: 1987-98, 1999- 2003 and 2004-09. From 1987 to 1998 primary producers were inconspicuous on T transect but  increased after that, suggesting abiotic conditions had become more favorable to algae. On the other transect (P-one), algae began to decline from 1999 and massive corals increased. Possible causes for these changes are discussed.