Hydrochemistry of the Bay of Bengal: possible reasons for a different water-column cycling of carbon and nitrogen from the Arabian Sea

Abstract

A study on biogeochemical cycling in the western Bay of Bengal was undertaken during two seasons (pre-southwest monsoon and northeast monsoon of 1991). Relationships of the conservative tracers NO and PO with potential temperature (θ) are used to identify three end-member water masses in the Bay: the low-salinity surface water, the high-salinity intermediate water advecting from the Arabian Sea, and the deep water of circumpolar origin. A hydrochemical discontinuity at ~12°C appears to define the boundary of zones of linear mixing among these waters. The shallower occurrence of the discontinuity in the Bay as compared to the Arabian Sea appears to restrict the thickness of the oxygen minimum layer in the former region. Marked seasonal changes appear to occur in intermediate waters, presumably reflecting changes in the coastal circulation off India. A secondary nitrite maximum is not observed in the Bay, and the NO-θ relationships are indistinguishable from those observed in the oxic parts of the Arabian Sea, firmly indicating that the Bay of Bengal is not an active denitrification site. The absence of denitrification in the Bay appears to be incompatible with a slower renewal of intermediate waters and a higher sinking flux of particulate organic matter in comparison with the Arabian Sea. It is proposed that the lower respiration rates in the Bay might result from a rapid sinking of organic matter with relatively little decomposition within the water column caused by the large inputs of terrigenous material. Small N-S gradients in oxygen and carbon dioxide support this view.