Differential response of winter cooling on biological production in the northeastern Arabian Sea and northwestern Bay of Bengal
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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 (chlorophyll a 47.5 mg m sup(-2), primary production 1114 mgC m sup(-2) d sup(-1), mesozooplankton biomass 175 mmolC m sup(-2), microzooplankton biomass 26 mmolC m sup(-2)) compared to the BOM (chlorophyll a 10.3 mg m sup(-2), primary production 117 mgC m sup(-2) d sup(-1), mesozooplankton biomass 71 mmolC m sup(-2), microzooplankton biomass 10.6 mmolC m sup(-2). In the AS, winter cooling assisted by the high surface salinity (is greater than 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 (approx. 2 mu M 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 is less than 15 m) and the absence of nitrate in the top 50 m water column were the reasons for low biological production. The results indicate that (1) biological production in the northeastern AS is high compared to the northwestern BOB; (2) primary production at 21 degrees N of the AS during the peak of the winter monsoon (December-January) is comparable to the production of the upwelling regions of the central and southeastern AS during the summer monsoon; (3) winter cooling along with low salinity cause large-scale thermal inversion (greater than 2.5 degrees C) in the BOB and (4) in the northernmost part of the BOB, upper low-saline water restricts atmospheric cooling to the surface layer and inhibits convection.
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