Severe phosphate limitation on nitrogen fixation in the Bay of Bengal

Abstract

Several anticyclonic (ACE) and cyclonic (CE) eddies constitute the circulation in the Bay of Bengal (BoB) and are associated with the downwelling and upwelling processes leading to oligotrophic and eutrophic conditions respectively. In this study, the nitrogen (N2) fixation rates and controlling factors are estimated through deck incubation experiments in the BoB using enriched N2 gas dissolution method. We observed measurable concentrations of dissolved inorganic nitrogen (DIN) and phosphate in the CE and close to detection limits in the ACE in the mixed layer. Photic zone integrated N2 fixation rates ranged between 53.3 and 194.1 mu mol m-2 d-1 with lower rates in the ACE (91 ± 18 mu mol m-2 d-1) than CE (162 ± 28 mu mol m-2 d-1) and no eddy regions (NE; 138 ± 27 mu mol m-2 d-1). The photic zone integrated N2 fixation rates are linearly correlated with photic zone integrated chlorophyll a and the mean phosphate concentrations in the photic zone suggesting that phosphate is controlling the N2 fixation in the BoB. The observed high N:P ratio (25 ± 3) also indicate that severe phosphate limitation in the BoB. This is further confirmed from increase in N2 fixation rates by 1.2 to 8 times due to artificial increase in phosphate from that rates at in situ phosphate concentrations. This study suggests that though the conditions are conducive for N2 fixation in the BoB, the removal of dissolved phosphate within the estuaries opened to the BoB, provide weaker inputs from subsurface due to stratification and less input from atmospheric dust may limit N2 fixation in the BoB.