Coral atolls in the Lakshadweep are built on a framework of living coral reefs that serve as a foundation for the islands.The reefs also buffer the islands from the effects of mid-oceanic waves and storms thereby reducing shoreline erosion and flooding. In doing so, the reefs also serve as a barrier, protecting the delicate freshwater lens in the islands from contamination due to over-wash by waves. Aside from this protective function, sediments from the reef are transported to the island and play an important role in island building.

A healthy reef generates enough calcium carbonate to sustain the atoll framework and provide sediments for island building. This is maintained in a dynamic balance by processes of accretion and erosion that together determine the total amount of calcium carbonate structure added to the reef. Within the context of climate change, rising sea-levels and increasing mass bleaching events, this groups of projects ascertains the health of Lakshadweep’s reefs to secure the future habitability of the union territory.

People: Mayukh Dey, Wenzel Pinto, Radhika Nair, Rohan Arthur, Sidharth Mahesh, Siddhi Jaishankar, and Chris Perry

Timeline: 2019 - present

The ability of coral reefs to accrete more than they erode is crucial to ensure that their regulatory, protective and provisioning services are maintained. Climate change has devastated reefs worldwide, and the coral reefs in Lakshadweep are no different. After undergoing four mass bleaching events since 1998, the reefs in Lakshadweep are severely battered. However, we do not yet know how compromised the reefs in Lakshadweep are and estimating accretion rates can provide a good prognosis to the state of this critical ecosystem. This becomes critical when we contextualise the rate of vertical reef growth against the backdrop of sea-level rise. A fine balance between the accreting components of a reef (corals and coralline algae) and eroding agents (parrotfish, sponges, urchins, polychaete worms) helps ensure that the reef is able to keep-up with sea-level rise and generate sediments that are crucial for island growth. When erosion outweighs the accreting components, vertical reef growth can often slow down. Regions where vertical reef growth lags considerably behind sea-level rise are predisposed to having more frequent storm surge and flooding events in the near future.

Project objectives:

• Measure the carbonate budget of shallow water coral reefs around the atolls of Kadmat, Agatti, and Kavaratti

• Use in-water census-based technique to estimate the broad benthic composition and quantify biological agents that erode the reef

• Estimate how current levels of accretion rates compare against projected sea-level rise under different emission scenarios to identify vulnerable areas

People: Wenzel Pinto, Siddhi Jaishankar, Radhika Nair, Mayukh Dey, Farai Patel, Rohan Arthur, Teresa Alcoverro

Timeline: 2019 - present

Much of the variability in estimating reef accretion rates arises from the abundance and site-specific erosion rates of parrotfish and urchins. The abundance and community composition for parrotfish is highly varied across coral reefs while different species of sea urchins have different habits leadings to large variability in their erosion rates. Here, we seek to quantify erosion rates for these two groups of bio-eroders by estimating, i) factors that influence the occurrence of parrotfish in reefs, and ii) site-specific erosion rates for urchins.

Body size mediates the effects of wave exposure on parrotfish bioerosion

Wave exposure is a powerful environmental filter in shallow coral reefs, influencing species distributions and mediating patterns of decline and recovery. Understanding these patterns is especially important in the case of functionally important groups like parrotfish. While playing crucial roles in herbivory, corallivory and sediment generation, parrotfish are known to be the dominant force of bioerosion in several reef ecosystems.

Objectives: 

• How does wave exposure over long timescales shape parrotfish distributions and consequently bioerosion in the reef?

• How does in-water turbulence and competition with other territorial fish influence parrotfish foraging behaviour for four common parrotfish species?

Through this project, we found that parrotfish distribution was strongly influenced by wave exposure, mediated by individual size and body shape. There was a clear decoupling between density, biomass and bioerosion in relation to wave regimes. We found that size also mediated the effects of turbulence on the parrotfish foraging behaviour, with larger individuals irrespective of species, showing much lower foraging rates in turbulent conditions while small individuals showed the reverse trend. Conversely, larger individuals were unaffected by chasing by other territorial fish, while smaller parrotfish foraged significantly less the more they were chased.

Our study thus highlights the importance of body size as a critical trait in influencing community assembly and behaviour, and consequently in determining the distribution of the bioerosional function of parrotfish.

Estimating site-specific erosion rates for a species of burrowing urchins in the Lakshadweep Archipelago

Of all, the most abundant urchin is a burrowing species called Echinostrephus molaris. These urchins reside in cylindrical burrows that they carve out in the reef substrate, in turn eroding the reef. However, we do not have rates of this erosion. Also, though the sand formed out of this erosion contributes to atoll formation, an overabundant population of urchins may disproportionately boost erosion in these reefs, which along with other factors, may threaten the very existence of these islands. 

Objectives:

• Estimate the bioerosion potential of Echinostrephus molaris over a year

• Identify factors that affect the abundances and distribution of the urchins E. molaris in the Islands

• Measure recruitment rates of E. molaris 

Our work till now has found that the urchin distribution is starkly different between islands, and patterns of urchin distribution correspond with a gradient of human population (and possibly the consequent nutrient input) along the islands, hinting at a relationship between the two. Further, we calculated the potential yearly reef erosion rates by urchins, positively correlating a high erosion rate with human density and urbanisation.

We now aim to determine urchin recruitment rates across atolls as this is a major gap in our understanding of population dynamics of this critical eroding species in the Lakshadweep.