Oceans and Coasts

Coping with catastrophe

Documenting patterns and processes of resilience in the Lakshadweep reefs

Local processes strongly influence patterns of reef recovery from climate change events in the Lakshadweep, and we have just begun to uncover what some of them might be

Death and revival on coral reefs

The year 1998 was disastrous for reefs across the tropics. In the Lakshadweep, within a few short weeks of the El Niño raising sea surface temperatures here, we witnessed a massive mortality of corals across the archipelago as corals bleached and died.  By the end of the year, our surveys showed that once vibrant reefs were reduced to a shadow of what they once were. We expected to see a familiar narrative begin to unfold in these reefs, where, after catastrophic mortality reefs are taken over by algae preventing the recovery of coral on these reefs.

Since 1998, we have been tracking benthic change on these reefs and what is emerging is a much more complex narrative of death and revival. Within 8 years of the initial mortality, several locations had shown a remarkable ability to recover while others showed only marginal change. 

The accidental resilience of the Lakshadweep reefs

Given the high human densities in the Lakshadweep Archipelago - among the highest rural densities in rural India - this capacity for reef recovery was, at first glance, difficult to understand.  Many tropical reefs are heavily overfished leading to a decline in herbivore numbers and making these reefs very vulnerable to being overtaken by algae.  Despite the high fisher densities in the Lakshadweep however, its reef fish communities appeared relative healthy in the wake of the coral mass mortality.  While two generations ago, fishers in this archipelago depended heavily on the reef for their sustenance, since the 1980s, fishers have shifted to targeting pelagic tuna.  This was part of a government-sponsored fisheries development programme that was radically successful and fishers quickly saw the economic benefits of this new fishery and quickly adopted it as the dominant fishery.  One happy by-product of this initiative is that it released reefs from potential overfishing, critical to the resilience we witnessed in the wake of the 1998 bleaching event.

Our current work explores the ecological mechanisms underlying these patterns of resilience. Understanding how coral recruitment interacts with the stability of structures and local hydrodynamics appears to be a critical factor driving recovery. In addition, we are beginning to unpack how the distribution of fish herbivores and their function relate to their ability to control algal growth after coral mortality events.

Identifying prime real-estate on reefs

As the primary reef builders, the growth and replenishment of scleractinian corals is a critical first process in determining the recovery trajectory of a reef post-disturbance. In the Lakshadweep, the reefs are dominated by a variety of dead structural types, the remnants of past bleaching events, on which coral settlement can occur. These structures come in a range of growth forms- from massive coral, to dead tables and branching forms, and loose unconsolidated rubble. They also differ in their inherent mechanical stabilities, and in a system like the Lakshadweep, where local hydrodynamics shape the physical habitat, the structure "chosen" by a coral larva can have significant consequences for its survival- and by extension, for the benthic recovery of that reef.

This project aims to examine how structural type and stability contributes to the early post-settlement fate of a coral larvae and final demographic survival.

Substrate, structure and final coral fate

Our initial results indicate that there some types of structural forms, such as dead massives and tables that are "chosen" by coral spat out of proportion to their availability on the reef, or on which early post-settlement survival is highest. This suggests that coral recruits do not have a uniform spatial distribution on a  reef, and that settlement choice is potentially mediated by several chemical and physical cues, such as the presence or absence of crustose coralline algae, turf or live coral on those dead structural types. For instance, "chosen" massives and tabular coral are predominantly covered in crustose coralline algae, a substrate known to attract coral larvae during settlement.

However, early coral success does not necessarily translate into final survival. Because these structural forms themselves differ so drastically in their stabilities, only the ones that are able to withstand high hydrodynamic forces will be able to support coral recruits till the adult stage. We are finding that coral that settle on dead platforms and massives are more likely to live longer than those that choose dead tables, branching forms and unconsolidated rubble. 

Can different reefs be given unique signatures of recovery, based on their structural composition? Can our documented patterns of reef recovery be explained by these mechanisms? These are the questions we hope to explore in the coming year.

The distribution and role of herbivorous fish

It has become increasingly clear that herbivores play a critical role in driving the resilience of the Lakshadweep reefs by mediating competitive interactions between coral and algae, thereby making substrate available for recruitment and preventing a phase shift to an alternate stable state dominated by algae. 

However, what is still unclear is how herbivory varies across these reefscapes and if the distribution of herbivores links directly to the function they perform on reefs. As herbivores are known to range widely, their distribution can be highly variable in space and time, and may be associated with different uses of the reef – for foraging, refuge, resting, social interactions and others. Secondly, herbivores perform several distinct functional roles on reefs, and may not contribute equally to the ability of the community to control algal biomass. This study attempts to determine the ability of herbivores to control algal biomass at three atolls in the Lakshadweep Archipelago that are recovering from a several coral mass mortalities.