Measuring reef growth in the Lakshadweep Archipelago

Coral atolls in the Lakshadweep are built on a framework of living coral reefs. These reefs serve as a foundation for the islands which form over them. They also play a key role in buffering the islands from the effects of mid-oceanic waves and storms reducing shoreline erosion and flooding on the islands themselves. In doing so, they also serve as a barrier, protecting the delicate freshwater lens in the islands from contamination by seawater 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 is able to generate enough calcium carbonate to sustain atoll frameworks while also providing sediments to provide for island building. This is maintained through a carbonate budget - in a dynamic balance by processes of accretion (or growth), and forces of erosion that together determine the total amount of calcium carbonate structure added to the reef. Reef accretion is determined by the abundance, species composition and growth rates of organisms that sequester calcium carbonate from the ocean; primarily coral and crustose coralline algae. Erosion rates on the other hand depend on physical processes such as waves and storms, but also the role of bioeroders such as parrotfish, sea urchins, sponges and polychaete worms which break down the calcium carbonate structure of the reef. Within the context of climate change, rising sea-levels and increasing mass bleaching events, it becomes imperative that the health of these reefs is ensured in order to secure the future habitability of the union territory.

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 in the that is 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