Investigating the Temporal Variability of Sea Surface Temperature over the Enclosed Water Bodies of Coral Reef Lagoon at Lakshadweep Islands, India

Abstract

Lakshadweep islands is one of the unique fringing coral reef-lagoon system that are affected by various environmental forces. The coral reef in Lakshadweep islands encloses the lagoon from surrounding ocean and restricts the flow and the heat exchange with the ambient seawater. The depth observed over lagoon varies in few meter, which increase drastically from channel and outer edge of the reef. Reef bounded lagoon creates a water pool that leads to differential heating & cooling of shallow lagoon compared to the surrounding deep ocean. In this study, an observational investigation of SST in selected coral reef-lagoon system of Lakshadweep islands is conducted using LANDSAT-8 TM images of 2016-2019 timeframe. Seasonal sea surface temperature (SST) variation is analysed for Bitra, Peremul Par (PP), Bangaram-Tinnakara, Agatti and Kadmat islands. Maximum SST in the lagoon observed is 30.15°C (Kadmat Lagoon) and minimum 22°C (Pereumal Par Lagoon) whereas for reef the maximum observed SST is 32.11°C (Bangaram Reef) and minimum is 23.59°C (Kadmat Atoll). Due to differential heating, the observed SST shows warming of lagoon water with maximum SST gradient of 0.26°C for Agatti Island during the summer. During winter season, islands and lagoon shows a cooler water compared to ambient ocean, where lagoon of Agatti Island showed a gradient of -0.2 °C. The SST over the coral reef shows similar trend with an overall reduced temperature with respect to the ambient ocean waters. During northeast monsoon, lagoon water heated up more than ambient water. The maximum observed SST gradient is for Agatti island lagoon (0.99°c) and lowest for Bitra (0.12 °C) island. The island Perumal Par, Bangaram-Tinnakara and Kadmat shows 0.25 °C, 0.2°C and 0.3°C. Compared to other season, the coral reef shows entirely opposite trend with average decrease (-0.8°C) in temperature. Among the islands considered, Agatti, Perumal Par and Bitra Islands are inhabited which may be the reason for maximum observed SST variations. Generally, in coral reef-lagoon system the thermal siphon phenome is observed, arising from interaction of linearly sloping bathymetry and time-dependent heating and cooling. The persistence of warm water within the lagoon and the coral reefs during the SW and NE monsoon shall have an adverse effect of the coral reefs and the associated bio-diversity.