Impact of the tropical cyclone Ockhi on ecological and geomorphological structures of the small low-lying Islands in the Central Indian Ocean

Abstract

Ockhi, was an intense tropical cyclone that devastated parts of Sri Lanka and India in 2017 and was the most intense tropical cyclone in the Arabian Sea, Central Indian Ocean, since Cyclone Megh in 2015. The ninth depression, and the third and strongest named storm during 2017, North Indian Ocean cyclone season, Ockhi, originated from an area of low pressure that formed in the eastern Andaman Sea on November 21, 2017. Here we describe the impacts of Ockhi on benthic coral reef assemblages of an atoll island directly in the path of the cyclone. We assessed the effect of the cyclone in the reefs around Kavaratti Island, by comparing the data from recent pre-Ockhi documentation with its present status. The study provided a unique opportunity to identify and describe the impacts of an intense tropical cyclone on reefs of a small low lying island in the Central Indian Ocean, which is often assumed to be vulnerable to physical disturbance due to their poorly lithified framework. For the present study, the entire Kavaratti atoll was demarcated into three zones (lagoon, western outer reef, and eastern outer reef), and data was gathered from 29 pre-fixed stations. Observed impacts of Ockhi were site-specific and spatially highly heterogeneous, but appeared to have been strongly influenced by the level of exposure, the site received, and the biological make-up of individual reefs sites. Live coral cover on western reefs had a considerably lower impact due to the shelter provided by the land and the trends in the substrate composition before and after Ockhi is almost similar. However, drastic devastation was observed in the eastern reef system, after the Ockhi event. The combination of repeated, intense cyclonic activities and mass bleaching events will act together to significantly hamper the rapid recovery of these reefs and also adversely affect the livelihood of the community.