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 μmol m−2 d−1 with lower rates in the ACE (91 ± 18 μmol m−2 d−1) than CE (162 ± 28 μmol m−2 d−1) and no eddy regions (NE; 138 ± 27 μmol 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.

中文翻译：

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孟加拉湾固氮的严重磷酸盐限制

摘要 几个反气旋 (ACE) 和气旋 (CE) 涡流构成了孟加拉湾 (BoB) 的环流，并分别与导致贫营养和富营养条件的下流和上流过程有关。在这项研究中，氮 (N2) 固定率和控制因素通过 BoB 中的甲板孵化实验使用富 N2 气体溶解方法进行估算。我们在 CE 中观察到可测量的溶解无机氮 (DIN) 和磷酸盐浓度，并且接近混合层中 ACE 的检测限。光区综合 N2 固定率介于 53.3 和 194.1 μmol m-2 d-1 之间，ACE (91 ± 18 μmol m-2 d-1) 的速率低于 CE (162 ± 28 μmol m-2 d-1)并且没有涡流区域（NE；138 ± 27 μmol m-2 d-1）。透光区综合 N2 固定率与透光区综合叶绿素 a 和透光区平均磷酸盐浓度呈线性相关，表明磷酸盐控制着 BoB 中的 N2 固定。观察到的高 N:P 比 (25 ± 3) 也表明 BoB 中的磷酸盐严重受限。这进一步证实了 N2 固定率增加了 1.2 到 8 倍，这是由于人为增加了磷酸盐，而在原位磷酸盐浓度下。这项研究表明，虽然条件有利于 BoB 中的 N2 固定，但在向 BoB 开放的河口中去除溶解的磷酸盐，由于分层而提供来自地下的较弱输入，并且来自大气灰尘的输入较少可能会限制 BoB 中的 N2 固定.