River plume fronts and their implications for the biological production of the Bay of Bengal, Indian Ocean

Fronts are the physical interface between water masses of distinct hydrographic characteristics and are ubiquitous features of the Bay of Bengal (BoB). They form as a result of receiving high freshwater runoff from many large rivers, but their biological characteristics are mostly unexamined. We determined the distribution of fronts in the BoB during the fall intermonsoon period and examined the physicochemical couplings in the frontal regions and their concomitant effect on biological production. In situ sea surface salinity (SSS) gradients were used to delineate the fronts (threshold of 0.01 psu km−1), and 2 frontal zones with a cross-frontal SSS difference of 1–3 psu were identified. River discharge played a more significant role than direct precipitation in the frontogenesis of this ecosystem. These narrow 3-dimensional boundaries were characterized by a higher nutrient replenishment than in the contiguous non-front zones, and had a higher phytoplankton production. The highest accumulation of zooplankton biomass in water masses of intermediate salinity (31–33 psu) in the frontal regions resulted from the higher food availability and congregation by hydrodynamic convergences.