Spatiotemporal variability of seawater density profile and nitrate productivity in the Bay of Bengal

The Bay of Bengal (BoB) is a dynamic marine environment shaped by complex oceanographic processes and significant freshwater influxes. Despite the critical role of seawater density variations in driving nutrient dynamics, there is limited understanding of the spatiotemporal interplay between seawater density profiles and nitrate productivity in the BoB. This study aimed to understand the relationship between seawater density variations, nitrate (NO 3 ) productivity, and their spatiotemporal dynamics in the BoB. Remote sensing data from 2004 to 2023 were utilized to analyze sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), and density variations, providing insights into how these factors influence primary productivity. The analysis revealed that SST peaks in May, with temperatures reaching 30.5 °C, while high NO 3 productivity, especially in coastal regions, occurs during August and September, with the highest concentration recorded at 80.32 mg/L. In contrast, the open ocean areas in the southwestern BoB exhibit higher densities (approximately 22.0 kg/m³) and comparatively lower NO 3 productivity during the same period. EOF analysis identifies dominant spatial patterns, with the first principal component explaining 76 % of the variance, capturing seasonal fluctuations in density and nitrate productivity. Correlation analysis reveals a strong negative correlation (-90 %) between sigma-t and nitrate, while salinity and density show a positive correlation (91 %), confirming the role of density in regulating nutrient availability. Additionally, the MLD was observed to be shallowest in April (∼20 m) and deepest in July (approximately 90 m), significantly influencing availability of vertical nutrient mixing and productivity. This study provides new insights into how seawater density fluctuations influence nitrate availability and primary productivity, contributing to improved biogeochemical modeling and sustainable marine resource management in the BOB. The findings highlight the significant influence of freshwater discharge and monsoonal cycles on the BoB's ecosystem and emphasize the importance of remote sensing for monitoring and managing marine resources to support sustainable management practices in response to climate variability . • Strong correlation between sigma-t and NO₃ (r = -0.90) indicates density-driven mixing as a key driver of nutrient transport. • Seasonal stratification limits nitrate flux, with low sigma-t (∼18.0 kg/m³) in December restricting nutrient replenishment. • Density & nitrate availability peak (sigma-t ∼22.8 kg/m³) in January-February promoted by vertical mixing. • EOF analysis (PC1: 76 % variance) confirms sigma-t as the primary driver of NO₃ variability. • MLD influences NO₃ supply: deep winter MLD (∼65–90 m) boosts upwelling; shallow summer MLD (∼20–50 m) limits input.