Reef responses to mass bleaching

Subject to repeated disturbances, including the catastrophic tsunami of 2004 and periodic bleaching events, the resilience of the Andaman and Nicobar reefs has been severely tested.  Our work attempts to understand and build support for the underlying resilience of these reefs to ensure their survival.

Climate change is considered the greatest long-term threat to tropical coral reefs. The Andaman and Nicobar archipelagos in the eastern Indian Ocean are high-diversity systems that have been recently affected by a series of catastrophic disturbances including the 2004 tsunami and repeated mass bleaching events (2008 and 2010). Managing these systems in the wake of these large unpredictable events is a complex task, made all the more urgent as global climate change increasingly makes these events part of the new normality for tropical coral reefs.

The best hope for their rational conservation lies in identifying factors that could enhance the natural resilience of these systems to a suite of disturbances that currently affect them. Reef resilience is likely highly contingent on local conditions, and driven by a combination of physical, ecological and anthropogenic factors. Identifying these factors at managerially relevant scales is critical if resilience principles have to inform rational reef conservation efforts. Additionally, while there is a felt need to incorporate resilience into management, reef managers are often unclear of how to translate these principles into practical strategies. This project proposes to identify a clear set of factors that will enable a mapping of vulnerabilities and resilience across the Andaman and Nicobar Archipelagos and to translate this into a clear set of guidelines to help reef managers prioritize and evaluate their conservation action.

The factors contributing to resilience are multidimensional and contingent on local conditions. Identifying these factors at scales relevant to managers is important if resilience principles have to be included in rational reef conservation efforts. 

In this study, we surveyed 49 reef sites across the Andaman archipelago to assess their responses to multiple disturbances of past years, including the tsunami. At each site, we measured a range of environmental and ecological parameters and estimated the putative resilience for each site based on an approach and methodology modified from reef resilience assessments developed by Maynard et al (2010), Obura and Grimsditch (2008) and from studies carried out by the Oceans and Coasts Programme in the Lakshadweep Archipelago. 

Using this score, we identify sites based on their overall resistance relative to other sites, and determine the factors that potentially contribute to this resistance. Our findings indicate that the summer bleaching event of 2010 killed almost 60-80 %of reefs within the island chain. A regression-­based approach identified that reef resistance to beaching is determined by a combination of factors i.e. the island group, current, depth and sedimentation. 

This work serves as an essential baseline for the Andaman Islands from which to track patterns of recovery and further decline and we highlight the urgent need to understand more clearly the drivers of reef resilience within this group.