Changes to coral reef habitats

After the 1998 bleaching event, coral recruitment was high, and the reefs had largely regained coral cover levels and species composition consistent with those documented prior to 1998 by 2012. Estimates of average vertical growth rate of reefs dominated by species of Acropora corals were also high. This indicated that many of the recovered reefs, had the capacity to keep pace with sea-level rise. 

The subsequent 2015-2016 bleaching event had a major impact on reef species composition and coral growth rates. The decline in coral cover was mostly driven by mortality of tabular Acropora species, resulting in slower rates of average vertical growth of the reefs.

Overall, cover can rebound but may be fundamentally different and changes to structural complexity are likely to persist for several years. The remaining areas of reef continue to experience degradation mostly due to bio-erosion, for example by parrotfish, which tend to proliferate and feed on reef substrates following severe coral bleaching, but whose numbers tend to decline again once the corals start to regrow and recover. Many species of reef fish are negatively affected by bleaching events, either due to a direct loss of food sources, or due to a reduction in the 3-dimensional structure that protected them from predators. Some species that feed on algae and invertebrates can increase in abundance.  
 

There is high confidence that SSTs in the tropical Indian ocean will continue to rise, and that oxygen concentrations and pH will decline. 

However, there is likely to be considerable local variation in the vulnerability of reefs to these changes, and the invertebrates and fish they support, which may offer some resilience. Finer resolution models being developed will improve understanding of relative climate vulnerability of reefs across the Indian Ocean. 

The return of Acropora species-dominated communities will be crucial to restore the key geo-ecological functions of habitat complexity and carbonate production that local reefs delivered pre-bleaching. 

Early observations suggest that deeper living reefs within the BIOT MPA are highly diverse and surviving deep-water corals may provide a source to recolonise shallow reefs if the species can live in deep and shallow environments. Surviving deeper reefs may also provide a refuge for organisms that can move to their depths.

In BIOT lagoons, pH change, coupled with decreasing salinity could produce unfavourable conditions for calcifying organisms such as corals and coralline algae, which are key to reef health and structure.