Long-term variability of chlorophyll- a in the north-western Bay of Bengal: linking phytoplankton size class and diatom dynamics

This study examines the long-term (2003–2024) seasonal and spatial variability of chlorophyll-a (chl-a) concentrations, diatom distributions, and phytoplankton size classes (PSCs) in the North-Western Bay of Bengal (NWBoB), using ocean colour remote sensing. The western boundary current (WBC), also known as the East Indian Coastal Current (EICC), reverses direction with seasons, flowing poleward during February to August and equatorward during September to January, thereby altering the hydrography. When EICC flows towards the equator, the upper 10–25 m of the water column is dominated by freshwater rich in silica and poor in nitrate and phosphate, contrasting to when it flows poleward. This would change phytoplankton concentration, composition, and size structure. Seasonal PSC analysis demonstrated microphytoplankton dominance increased towards the south (of 19°N) between February and September, associating with high nutrients due to coastal upwelling. The microphytoplankton spread towards offshore from August to December, associated with the transition of coastal currents from pole to equatorward during this period. Dominance of nano- and picophytoplankton was observed during July – September, associated with the spread of low-salinity water due to the reversal of EICC towards the equator under turbid conditions. The increased dominance of diatoms during August – December coincides with nutrient availability through coastal upwelling. Based on the PSC, nano-sized diatoms may prevail during this period. Spatially, microphytoplankton dominated nearshore regions, while picophytoplankton was prevalent in offshore regions due to a decrease in nutrient concentrations. These findings highlight that the interplay of riverine discharge and circulation-driven nutrient dynamics controls phytoplankton dynamics along the western coastal BoB. Future studies integrating high-resolution in situ measurements from autonomous observatories with validated biogeochemical models are essential to complement satellite observations and achieve a more systematic understanding of environmental controls on PSC and diatom dynamics.