Differential response of winter cooling on biological production in the northeastern Arabian Sea and northwestern Bay of Bengal

Abstract

The northern parts of the twin seas bordering the Indian subcontinent, the Arabian Sea (AS) and Bay of Bengal (BOB), were studied during the winter monsoon. Higher biological production was observed in the AS (chlorophylla 47.5 mg m - 2 , primary production 1114 mgC m - 2 d - 1 , mesozooplankton biomass 175 mmolC m - 2 , microzooplankton biomass 26 mmolC m - 2 ) compared to the BOB (chlorophyll a 10.3 mg m - 2 , primary production 117 mgC m - 2 d - 1 , mesozooplankton biomass 71 mmolC m - 2 , microzooplankton biomass 10.6 mmolC m - 2 ). In the AS, winter cooling assisted by the high surface salinity (>36) resulted in densification of surface layers, convective mixing and deepening of the mixed layer (average 60 m). Convective mixing brought nutrients to the upper euphotic column (∼ 2 μM nitrate in the top 50 m) and stimulated biological production. The northern part of the BOB, however, responded differently to atmospheric cooling. The prevailing low salinity (27) resulted in thermal inversion and stratification of the water column. Shallow mixed layer (average 2.5°C) in the BOB and (d) in the northernmost part of the BOB, upper low-saline water restricts atmospheric cooling to the surface layer and inhibits convection.