ABSTRACT

The worldwide release of excessive nutrients (nitrogen [N], silicon [Si] and phosphorus [P]) into coastal waters is impacting phytoplankton growth and community structure. The coastal Bay of Bengal (BoB) is predicted to be 90% more eutrophic in the future; the impacts on phytoplankton are largely unknown. We present the first experimental results concerning the responses of coastal phytoplankton to changing nutrient stoichiometry (N:Si:P) in the southwest BoB. A gradual increase in Si, N and P supply resulted in exponential growth and significant nutrient depletion (Δ). The nutrients reaching the Bay are likely to possess a short residence time. We also show that any alteration in nutrient loading ratio may impact nutrient removal dynamics of N, Si and P differently. Growth-saturating concentrations for Si, N and P were ~55, 65 and 10 µM respectively, whereas luxury uptake was seen up to 20 µM for P and 65 µM for N and Si. Net consumption of N (ΔN) and P (ΔP) was decoupled from increasing Si concentrations. Gradual N enrichment under P- and Si-saturated conditions linearly increased ΔSi as well as ΔP, and diatoms dominated over cyanobacteria. The diatoms Thalassiosira spp. seemed to respond positively to N enrichment, probably due to their high nitrate uptake and storage capacity. Nitzschia spp, a pennate diatom, showed the opposite trend. Such modifications may have direct consequences on the food chain. The rates of N, Si and P removal under saturated concentrations were 1.03 ± 0.27, 1.35 ± 0.14 and 0.14 ± 0.02 µmol chlorophyll a^–1 d^–1, respectively and yielded a ratio of 7.5:9.8:1 (N:Si:P). The departure from the Redfield ratio (indicating eutrophication) in the Bay may potentially alter nutrient removal patterns, phytoplankton community composition and autochthonous organic matter stoichiometry.

摘要

全球向沿海水域释放过量的营养物质（氮 [N]、硅 [Si] 和磷 [P]）正在影响浮游植物的生长和群落结构。预计未来孟加拉湾 (BoB) 的富营养化程度将增加 90%；对浮游植物的影响在很大程度上是未知的。我们展示了关于沿海浮游植物对西南 BoB 变化的营养化学计量 (N:Si:P) 的反应的第一个实验结果。Si、N 和 P 供应的逐渐增加导致指数增长和显着的养分消耗 (Δ)。到达海湾的营养物质可能停留时间很短。我们还表明，养分负载比的任何改变都可能对 N、Si 和 P 的养分去除动态产生不同的影响。Si、N 和 P 的生长饱和浓度约为 55，分别为 65 和 10 µM，而对于 P 和 65 µM，对于 P 和 N 和 Si 的奢侈吸收高达 20 µM。N (ΔN) 和 P (ΔP) 的净消耗与增加的 Si 浓度脱钩。在 P 和 Si 饱和条件下逐渐富集 N 线性增加 ΔSi 和 ΔP，并且硅藻在蓝藻中占主导地位。硅藻海藻属 似乎对 N 富集反应积极，可能是由于它们的硝酸盐吸收和储存能力高。Nitzschia spp，一种羽状硅藻，显示出相反的趋势。此类修改可能会对食物链产生直接影响。饱和浓度下 N、Si 和 P 的去除率分别为 1.03 ± 0.27、1.35 ± 0.14 和 0.14 ± 0.02 µmol 叶绿素a ^–1 d ^–1，比率为 7.5:9.8:1 (N:Si:P ）。海湾中与 Redfield 比率（表明富营养化）的偏离可能会改变养分去除模式、浮游植物群落组成和本土有机物化学计量。