Flood-associated hydrobiological changes in the nearshore off Kochi, along the southwest coast of India

Abstract

This study presents how flood water transforms the hydrobiology of the nearshore waters close to estuarine inlets along the southwest coast of India, which are susceptible to significant freshwater discharge during the Southwest Monsoon (SWM) and depression events during the rest of the year. In the current study, coastal hydrography off Kochi that was acquired just before and during the flood event in August 2018 was compared with normal SWM conditions that prevailed in August 2014 and 2015. The SWM hydrography of August 2014 and 2015 revealed upwelling waters in the subsurface and a low saline lens in the top layer with a high concentration of chlorophyll-a (>3 mg m−3). The impact of floodwater was found in August 2018, even at a distance of 7 km from the Kochi inlet, as the low saline plume that was 3 m thick before the flood, expanded to 8 m thick and occupied almost the entire water column during the flood. It was evident that the flood water decreased the total chlorophyll-a concentration and microplankton abundance in the study zone. During the typical SWM in August 2014 and 2015, the nano-size fraction of phytoplankton dominated the chlorophyll-a (>70%), but the micro-size fraction dominated (50%) during the August 2018 flood time. All these suggest that the dilution effect of the flood could noticeably decline the phytoplankton biomass and abundance even at 7 km out to sea from the Kochi inlets, although it is unknown how this affects the higher trophic levels.