Impact of river discharge and suspended sediments on phytoplankton in the nearshore waters of the Indian southwest coast.

The current study presents how seasonal and spatial variations in river discharge and suspended sediments alter selected phytoplankton variables in the nearshore waters along the southwest coast of India. The study utilised satellite remote sensing and in-situ hydrography, complemented with rainfall, river discharge, and suspended sediment data sets. The nearshore waters of the Southeastern Arabian Sea (SEAS) and the adjacent Indian (Kerala) landmass were the primary focus of this research. The study region receives 2649 mm of annual rainfall, with over 70 % of that falling during the Southwest Monsoon [SWM (June-September)]. 41 short rivers from the western Ghats drain the rainfall into the SEAS. Three zones have been examined here: the Northern Region (NR) between 11 and 12.5°N with 24 rivers, the Central Region (CR) between 9 and 11°N with nine rivers, and the Southern Region (SR) between 7 and 9°N with 12 rivers. Despite high PAR levels in the study area during the Pre-Monsoon [PRM (March-May)] and the Northeast Monsoon [NEM (November-February)], nutrient concentrations were too low, resulting in lower chlorophyll a (<0.5 mg m^-3). This setting changed during the SWM when huge river discharge and coastal upwelling resulted in elevated nutrients. Upwelling was strong in the SR, while the higher river discharge and stratification weakened upwelling in the NR and CR. Overall, the NR and CR had higher suspended sediments from river discharge during the SWM, which resulted in a decrease in the euphotic column. As a result, the SR had the highest euphotic column (28 m), followed by the CR (20 m) and the NR (15 m). A much thinner chlorophyll maxima layer was found in the surface waters of the CR (av. 13 m) and NR (av. 15 m) than in SR (av. 25 m). This in turn caused higher euphotic column integrated chlorophyll in the SR (av. 77.9 ± 13.3 mg m^-2) compared to the NR (av. 42.9 ± 12.4 mg m^-2) and the CR (av. 30.63 ± 9.6 mg m^-2). FlowCAM-based analyses showed more abundance of needle-shaped microphytoplankton cells in the SR and the spherical ones in the CR and NR during the SWM. The study concluded that spatial changes in river discharge and suspended sediments in the current study area during the SWM can have an impact on the chlorophyll maximum layer and phytoplankton composition of surface waters.