Effects of geomorphological zones, reefs and seasons on coral reef fish communities of Réunion Island, Mascarene Archipelago, SW Indian Ocean

The present work analyses the spatial and temporal fluctuations of fish communities on Réunion coral reef flats on three different reefs, each comprising three geomorphological zones, over eight seasons within a 6-year period. These three reefs are subjected to different environmental conditions and displayed various percentages of live coral cover. Our objectives were not only to describe the spatio-temporal patterns, but also to organize the factors involved in variation hierarchically, and to quantify the degree of community structuring that could be monitored over various spatial and temporal scales. We also focus on fish guilds to link the spatio-temporal patterns not only to species but also to the roles fish are playing (mainly involving trophic activity). We found that spatial attributes strongly determined fish distribution, with intra-reefal zones (back-reef, inner reef flat and outer reef flat) playing a much more important role than the different reefs. This suggests that the percentage of live coral cover of a given reef was less significant than its morpho-structural organization to explain fish distribution. Seasons had only a weak role in fish distribution, although fish communities were significantly more homogeneous in winter than in summer, possibly due to the arrival of numerous juveniles belonging to various species during summer settlement events. We also identified a marked temporal persistence of the spatial patterns found over the course of the study. This is discussed in relation to the current trend of increased surface seawater temperatures involved in the possible future increase in number/intensity of ENSO events. We consider the average squared Euclidian distance as a candidate for monitoring tools to quantify future changes in fish community structuring.