Kumar¹, Sanjeev, R. Ramesh¹ and M.S. Sheshashayee²

¹Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, Tel: +91 79 6302129, Fax: ext. 4060/4265, Fax: +91 79 6301502, E-mail: sanjeev@prl.ernet.in and ² Dept. of Crop Physiology, University of Agricultural Sciences, Bangalore 560 065

¹⁵N- based productivity and f ratio for the Bay of Bengal during a poor monsoon season-2002

We report ¹⁵N based productivity and the f- ratios for Bay of Bengal (BOB) during September-October 2002, measured onboard ORV Sagar Kanya. JGOFS protocol was followed and samples were analyzed for nitrogen isotopes using a Finnigan delta plus isotope ratio mass spectrometer. The total production varied from 3 to 12 mmolNm^-2day^-1. Euphotic zone integrated nitrate-induced productivity ranged from 0.09 to 2.8 mmolNm^-2day^-1, while ammonium regeneration rates lie in the range 0.8 to 2.37 mmolNm^-2day^-1. Urea regeneration rates were the highest, from 1.4 to 8.7 mmolNm^-2day^-1 .The values reported here are a factor of 3 to 4 higher than reported for BOB during July-August of 2001 (Prasanna Kumar et al., GRL, 29, 2002). The reason for lower values in 2001 could be because of the strong stratification of surface layers due to copious rainfall and river discharge. But in 2002, monsoon failed, resulting into reduced stratification and higher wind-driven vertical mixing leading to increase in productivity. In BOB urea appears to be the most preferred substrate unlike Arabian Sea where ammonia was found to be the most preferred substrate (Watts and Owens, DSR, 1999). This contrasting behavior could be either due to the overall lower productivity in the BOB compared to the Arabian Sea, or it might reflect conditions special to a poor-monsoon (e.g., reduced discharge of nutrients from the land to the sea). Further data is required to resolve this problem. In the BOB, preference for urea lowered the integrated f-ratio significantly, which had a mean of 0.13 ± 0.08 (one standard deviation).