GCRMN_COI_2017-Western Indian Ocean Reef Status

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Coral reef status report for the Western Indian Ocean (2017) Global Coral Reef Monitoring Network 2.6.3 Status and trends Hard coral cover in Reunion has ben stable since 1998, averaging 40-45% (fig. 2.6.2). By contrast, a major and chronic increase has occurred in the algal community (fleshy algae, turf algae and algae growing on dead corals), which have increased from < 20% to 40- 50% over the same period. This has occurred across most sites around the island, with the exception of those at Saint-Pierre and Saint-Leu (station La Corne), which have shown a more stable coral community over time (Bigot 2008). a b NATIONAL CHAPTERS Figure 2.6.2. a) Trend in hard coral cover at 14 monitoring stations in Reunion from 1998-2015 (mean (dark blue line), 95% confidence limit (grey shaded area), individual monitoring stations (light blue lines)); b) Trend in mean hard coral cover (blue line, open circles) and fleshy algae (green line, closed circles) in Reunion from 1998-2015 (coloured shaded areas represents 95% confidence limits of the mean) Variation is high among sites and reef sectors, varying between 8% and 80% (from 2015 surveys). The healthiest fore reef sites are in the St. Pierre sector in the south, while the most degraded are in the southern part of the Saint-Gilles/La Saline reef complex. The healthiest reef flat sites are in the lagoon of St. Pierre (Alizé Plage, Ravine blanche) with coral cover of 65%, while the most degraded reef flats are in the Saint Leu sector with coral cover of 40%. 98
Coral reef status report for the Western Indian Ocean (2017) Global Coral Reef Monitoring Network Figure 2.6.3 Comparison of mean percent cover of hard coral in Reunion at fore reef (blue line, open squares) and back reef (purple line, open circles) zones with 95% confidence limits (coloured shaded areas) Investigation of the evolution in composition of the coral communities from 1998-2008 (Bigot 2008) showed that the upsurge in algal cover is associated with certain factors: • In certain cases, the algae represent early successional stages, with opportunistic species responding to the availability of new space after environmental disturbances. This is not associated with general loss of species diversity, and was observed on the reef flat stations of Saint-Gilles, Saint-Leu, Etang-Salé and the fore reef stations of Saint-Leu and Etang-Salé. Nevertheless, this could represent an early stage in a longer term loss in biological diversity. • In other cases, the increase in algae is associated with a shift in coral community structure, for example where hard coral communities dominated by Galaxea, Porites and Astreopora are replaced by the soft corals Sinularia and Sarcophyton. This is occurring on the fore reefs at Saint-Gilles/La Saline (Planch’ Alizé, Toboggan) and Saint-Leu and is more of a concern, as it reflects a shift in community dominance and structure and a more significant loss in biodiversity and associated species. These trends were reconfirmed in 2014 in the Marine Reserve (Bigot et al. 2016) where further loss of coral diversity (from 10 to 17% on average) was documented across the entire fore reef sectors of Saint-Gilles / La Saline and Saint-Leu. Fish community structure in la Reunion is strongly determined by reef geo-morphology over short term changes such as seasonal factors or disturbances such as cyclones (Chabanet 1994, Letourneur et al. 2008). Trends in the abundance of the eight families monitored are shown in fig. 2.6.4. NATIONAL CHAPTERS 99
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