The study addressed the impact of the El Niño 2015-2016 on the ecosystem functioning and the subsequent effects on the distribution and community structure of zooplankton in the Kavaratti reef, a prominent coral atoll in the tropical Indian Ocean. The elevated ocean temperature (SST) associated with El Niño resulted in a mass bleaching event affecting > 60% of the live corals of the Kavaratti atoll. The concomitant changes observed in the nutrient concentration, coral health, and phytoplankton of the reef environment during the course of the El Niño led to discernible variations in the zooplankton community with markedly higher abundance and heterogeneity in distribution during the peak period of El Niño compared to its waning phase. A notable shift was also evident in the community structure of Copepoda, the dominant zooplankton taxon, with a predominance of calanoids and poecilostomatoids in the peak period and by harpacticoid copepods in the waning phase of the El Niño. The harpacticoid, Macrosetella gracilis, dominated in the waning phase because of their unique adaptability in the utilization of Trichodesmium erythraeum, both as nutritional and physical substrates in the nutrient-depleted environment of the reef ecosystem.
Keywords: Coral bleaching; Ecosystem functioning; El Niño; Macrosetella gracilis; Trichodesmium erythraeum.