Environmental controls on phytoplankton communities across the estuarine-nearshore continuum of a recurrent upwelling system along the southwest coast of India.

This study investigates the monthly variability of phytoplankton communities in the Cochin Estuary (CE) and adjacent nearshore waters (NSW) along the southwest coast of India, focusing on chlorophyll a (chl a) biomass, size structure, species composition, and abundance. High-frequency time-series observations during the summer monsoon (SM; June-August) revealed that the NSW experienced cooler, nutrient-enriched conditions driven by coastal upwelling and riverine inputs, leading to elevated chl a biomass dominated by microphytoplankton (20-200 μm) and strongly associated with the increase in NO3-N and PO4-P concentrations. In contrast, non-monsoonal periods were characterized by stratified, nutrient-depleted conditions with reduced biomass, primarily contributed by nanophytoplankton (2-20 μm). The CE functioned as a freshwater-influenced, nutrient-rich system during the SM, sustaining higher chl a concentrations than the NSW and maintaining relatively elevated year-round biomass dominated by micro- and nano-sized phytoplankton, mainly diatoms. Despite pronounced upwelling signatures in the NSW during July as evidenced in the time series mooring buoy data, chl a biomass increased more substantially during August, suggesting a temporal lag between nutrient enrichment and phytoplankton biomass accumulation under prevailing hydrographic conditions. Community composition also shifted seasonally, from diatom dominance during the monsoon to increased contributions of dinoflagellates and cyanobacteria during the winter monsoon (WM) and spring intermonsoon (SIM) under more oligotrophic conditions. Overall, diatoms constituted the major chl a biomass in both systems, while their size structure and contribution varied with hydrographic conditions and nutrient availability. These findings demonstrate the contrasting phytoplankton dynamics of the CE and adjacent NSW and highlight the dominant role of monsoon-driven physical and biogeochemical processes in structuring phytoplankton communities across tropical coastal ecosystems.