Productivity measurements in the Bay of Bengal using the ¹⁵N tracer: Implications to the global carbon cycle

Abstract

During September-October 2002 (Fall intermonsoon), a cruise was undertaken in the shelf and offshore Bay of Bengal onboard ORV Sagar Kanya as a part of the Bay of Bengal Process Study, when physical, chemical and biological parameters were measured in the Bay. This paper describes the first ¹⁵N based productivity measurements made in the Bay, which are comparable to the ¹⁴C based measurements made concurrently. Results show that the productivity to Chl a ratios have a mean of 2.6 ± 3.3 / hr. Offshore stations have an average productivity of ~360 mgC /m²/d, whereas shelf stations have ~280 mgC /m²/d. The highest value of 875 mgC /m²/d was found at a station where the mixed layer depth was the maximum (~55m, approximately equal to the average photic depth during the cruise). Both offshore and shelf stations have an average f-ratio (ratio of new to total production, conservative estimate) of ~0.5. The f ratios in the Bay of Bengal, higher relative to its western counterpart, the highly productive Arabian Sea (f ~ 0.3), implies that the Bay is capable of removing the anthropogenic excess CO₂ from the atmosphere at least as efficiently as the Arabian Sea, if not more, at least during the fall intermonsoon season. Nitrate from depth could be brought up during this season by frequent cyclonic activity. Present study provides the first direct confirmation of high new prodcution in the Bay, earlier suggested based on pCO₂ measurements.