Nitrogen fixation in the western coastal Bay of Bengal: Controlling factors and contribution to primary production

Abstract

N2 fixation rates were measured at 18 stations in the western coastal Bay of Bengal (BoB) to evaluate its contribution to the external source of nitrogen and its controlling factors. During the study period (fall monsoon), East India Coastal Currents (EICC) flows equatorward resulting in spreads of low saline water along the coast. This leads to stratification and inhibition of nutrient supply to the mixed layer through vertical mixing, resulting in the existence of nitrogen-poor and warm waters in the upper layer. Such conditions may trigger N2 fixation in the upper ocean. To examine such possibility, the N2 fixation rates were measured and found to vary between 0.4 and 14.8�mol N m-2 d-1 in the photic layer during the fall monsoon. A linear relationship was observed between photic zone integrated N2 fixation rates and phosphate suggesting a possible severe limitation of phosphate in the BoB. Low N2 fixation rates were observed at off Godavari, Krishna and Mahanadi River estuaries in the inshore than offshore regions associated with low phosphate and high suspended matter. This suggests that phosphate may be removed through adsorption on the suspended matter resulting in severe limitation of N2 fixation in the western BoB. Based on the available seasonal data, 0.01 to 0.94 Tg/y of N2 fixation occurs and contributes to <1% of the primary production in the BoB.