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. 

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Post-bleaching, reefs of the Lakshadweep suffered catastrophic coral mortalities

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.

Monitoring 20the 20reef

By 2008 many reefs in the Lakshadweep showed almost complete recovery of coral

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.

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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. 

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Publications

  • Journal Article
    2016
    "Choice" and destiny: The substrate composition and mechanical stability of settlement structures can mediate coral recruit fate in post-bleached reefs
    Coral Reefs. 35: 211-222

    Increasingly frequent and intense ocean warming events seriously test the buffer and recovery capacities of tropical coral reefs. Post-disturbance, available settlement structures on a reef (often dead coral skeletons) vary considerably in their mechanical stability and substrate composition, critically influencing coral recruit settlement choice and fate. In the wake of a coral mass mortality in the Lakshadweep archipelago, we examine (1) the relative availability of recruit settlement structures (from stable to unstable: reef platform, dead massive coral, consolidated rubble, dead corymbose coral, dead tabular coral, and unconsolidated rubble) in 12 recovering reefs across three atolls in the archipelago, (2) the substrate composition [crustose coralline algae (CCA), mixed turf, macroalgae] of these structural forms, and (3) whether the choice and fate of young coral are mediated by the substrate and stability of different structural forms. For this, we measured the abundance and distribution of recruit (<1cm), juvenile (1–5 cm), and young adult (5–10) corals of 24 common coral genera. Four years after the mass mortality, reefs differed considerably in composition of settlement structures. The structures themselves varied significantly in substrate cover with dead tables largely covered in CCA [60 ± 6.05 % (SE)] and dead corymbose coral dominated by mixed turf (61.83 ± 3.8 %). The youngest visible recruits (<1 cm) clearly preferred CCA-rich structures such as dead massives and tables. However, older size classes were rarely found on unstable structures (strongly ‘‘avoiding’’ tables, Ivlev’s electivity index, E = -0.5). Our results indicate that while substrate cover might mediate coral choice, the mechanical stability of settlement structures is critical in determining post-settlement coral survival. The composition and availability of settlement structures on a reef may serve as a characteristic signature of its recovery potential, aiding in assessments of reef resilience.

  • Popular Article
    2016
    Living with change: local responses to global impacts
    Rohan Arthur, Naveen Namboothri, Vardhan Patankar
    Current Conservation, issue 10.2 http://www.currentconservation.org/?q=issue/10.2
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    PDF, 290 KB

  • Journal Article
    2006
    Local processes strongly influence post-bleaching benthic recovery in the Lakshadweep Islands
    Rohan Arthur, Terence J. Done, Helene Marsh, Vicki Harriott
    Coral Reefs. 25: 427-440
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    PDF, 463 KB

    The atoll reefs of the Lakshadweep, in the Indian Ocean suffered a catastrophic mortality of hard coral in the wake of the El Niño event of 1998. This study tracked changes to coral and other benthic elements in three atolls in the Lakshadweep from 2000 to 2003. The recovery of coral was highly site-specific, and appeared to be driven by differences in post-settlement survival of coral recruits, that were in turn, influenced by the local hydrodynamics of the atolls. Post bleaching recovery was highest on west-facing reefs, while recovery on east-facing reefs was very limited. However, no ‘phase-shift’ to macroalgal dominated reefs was evident. High herbivore pressures were perhaps the most important control of macroalgae. Five years after the mass mortality, the genera that showed the maximum gains represented a mix of different susceptibilities to bleaching, while some genera that were not particularly susceptible to bleaching showed significant declines. These results suggest that decline or recovery of coral is likely dependent on individual life history strategies, post-recruitment survival, and contingency.

  • Journal Article
    2005
    Benthic recovery four years after an El Niño-induced coral mass mortality in the Lakshadweep atolls
    Rohan Arthur, Terence J. Done, Helene Marsh
    Current Science. 89(4): 694-699
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    PDF, 99.5 KB

    The reefs of the Lakshadweep suffered a mass mortality of coral in 1998, in the wake of an El Niño event of unprecedented severity. In 2002, we conducted a broadscale benthic survey of six atolls in this group to check if there were geographic trends in recovery patterns across the archipelago. Four years after the mass mortality, live coral cover was relatively low on most atolls, and thin algal turfs dominated the benthos. Clear benthic differences were apparent between eastern and western aspects of reefs, pointing to the importance of local hydrodynamic conditions in determining recovery rates. Where recovery was the most apparent, it was dominated by fast-growing and bleaching-resistant coral genera. Despite the apparent lack of recovery at many sites, the reef system did not show signs of having suffered a ‘phase shift’ to a macroalgal state. High herbivorous fish abundance was likely responsible in controlling macrophyte levels, and may be crucial for further benthic recovery in these reefs.

  • Journal Article
    2000
    Coral bleaching and mortality in three Indian reef regions during an El Niño southern oscillation event
    Current Science. 79(12): 1723-1729
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    PDF, 120 KB

    The 1997–1998 El Niño Southern Oscillation (ENSO) event, which elevated Sea Surface Temperatures (SSTs) of tropical oceans by more than 3°C, was one of the most extreme ENSO events in recent history. Such increases in SSTs above the seasonal average can trigger widespread bleaching in coral reefs. This study examined bleaching in three Indian coral reef regions in relation to SSTs using quantitative rapid assessment methods between April and July, 1998. The Gulf of Kutch reefs showed an average of 11% bleached coral with no apparent bleaching-related mortality. In contrast, bleached coral comprised 82% of the coral cover in lagoon reefs of Lakshadweep and 89% of the coral cover in the Gulf of Mannar reefs. Bleaching-related mortality was high – 26% in Lakshadweep and 23% in Mannar. The coral mass mortality may have profound ecological and socio-economic implications and highlights the need for sustained monitoring for coral reef conservation in India.

  • Report
    1999
    Rapid assessment of reef responses to elevated sea-water temperatures caused by El Niño Southern Oscillation (ENSO) Current System in Indian waters.
    NCF Technical Report. No. 2. Mysore
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    PDF, 487 KB

  • Report
    1999
    Monitoring Lakshadweep's coral reefs: potential and pitfalls
    NCF Technical Report. No. 3. Mysore
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    PDF, 334 KB

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