Oceans and Coasts

Consequences of reef accretion in Lakshadweep

Team members: Mayukh Dey, Rohan Arthur & Wenzel Pinto

Project timeline: 2019 - Present

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.

In our study we use census-based techniques to explore how carbonate budgets varied across 3 atolls in the Lakshadweep archipelago. Our initial results indicate that net accretion rates were below optimal production rates across all atolls, with the capital Kavaratti having the lowest net accretion. Site level variation in coral species composition, as well as variations in the bio-eroder community are what seem to be driving these trends.  These results serve as a baseline for us to understand how each island is faring, and have paved the way for finer scale assessments  that we will perform across each of these islands to gauge the susceptibility of each of these islands to future climate change scenarios.

Coral reefs such as these are important builders that support life on remote oceanic islands by providing sand to build islands, freshwater to drink, and fish to eat, which are essential for any human settlement to survive.