The basin-wide phytoplankton succession and community behaviour in response to varying nutrient patterns during various upwelling phases are detailed, for the first time, in the eastern Arabian Sea (EAS, ∼6 to 22N) during the summer monsoon (SM) of 2018. Three consecutive observations were carried out during early SM (June-July), peak SM (August), and late SM (September-October), representing different phases of upwelling. During the early phase of upwelling, high phytoplankton biomass was observed in the south (column-integrated chlorophyll : 74.09 ± 60.05 mg m) and moderate levels in the central (25.75 ± 6.51 mg m) and north (30.31 ± 12.32 mg m) EAS coastal waters. Diatoms were the dominant group (60–90 %) in the coastal stations throughout the upwelling period. Offshore regions characterised by deeper nutriclines (>50 m) had pico-phytoplankton dominance, including cyanobacteria (14–30 %), chlorophytes (19–24 %) and prochlorophytes (12–15 %); however, due to low nitrogen to phosphorous ratio (N/P: 2.6 ± 1.31) during this period, the contribution of diatoms decreased to less than 20 % in the offshore waters compared to the coastal EAS. During peak SM, upwelling induced shoaling of nutriclines and high N/P conditions (8.4 ± 5.25) in the mixed layers of south EAS coastal waters substantially enhanced phytoplankton biomass (chlorophyll : 129.06 ± 96.24 mg m). Additionally, the shallow nutriclines supported diatoms dominance in offshore waters, particularly in the central EAS (up to 65 %), relative to the south and north EAS (22 to 33 %), where the upwelling intensity was weaker. The withdrawal of upwelling led to a deepening of nutricline and low N/P conditions (3.33 ± 2.77 in coastal and 3.35 ± 2.26 in offshore waters) during late SM. This supported the occurrence of cyanobacteria and dinoflagellates, as the contribution of diatoms to the total phytoplankton community sharply decreased to 50 %. In other words, upwelling in the EAS brings nitrogen-deficient (denitrified) waters; the available nitrogen is immediately consumed by the diatom community, resulting in low N/P conditions that favour the dominance of the cyanobacterial population towards late SM. Overall, substantial intra-seasonal variability was observed in nutrient stoichiometry, strongly modulated by the intensity of physical processes affecting the phytoplankton populations. Continuous monitoring is required to understand the phytoplankton populations, their impact on higher trophic levels, and the overall health of aquatic food web structure in the EAS.

中文翻译：

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营养化学计量在阿拉伯海东部上升流过程中驱动浮游植物种群


首次详细描述了 2018 年夏季季风 (SM) 期间阿拉伯海东部（北纬 6 至 22 度）的全流域浮游植物演替和群落行为，以响应不同上升流阶段的不同营养模式。在SM早期（6-7月）、SM高峰（8月）和SM晚期（9-10月）连续进行了3次观测，代表了上升流的不同阶段。在上升流的早期阶段，在 EAS 南部观察到浮游植物生物量较高（柱综合叶绿素：74.09 ± 60.05 mg m），中部（25.75 ± 6.51 mg m）和北部（30.31 ± 12.32 mg m）处于中等水平。沿海水域。在整个上升流期间，硅藻是沿海站的主要群体（60-90%）。以较深营养线 (>50 m) 为特征的近海区域，超微型浮游植物占主导地位，包括蓝藻 (14–30%)、叶绿藻 (19–24%) 和原叶绿藻 (12–15%)；然而，由于该时期氮磷比较低（N/P：2.6±1.31），与沿海EAS相比，近岸海域硅藻的贡献下降至不足20%。在 SM 峰值期间，上升流引起东亚地区南部沿海水域混合层中营养素的浅滩和高 N/P 条件 (8.4 ± 5.25)，显着增强了浮游植物生物量（叶绿素：129.06 ± 96.24 mg m）。此外，浅层营养线支持硅藻在近海水域中的主导地位，特别是在东亚地区中部（高达65％），而东亚地区南部和北部（22％至33％）的上升流强度较弱。 SM 晚期，上升流的撤退导致营养线加深和低 N/P 条件（沿海为 3.33 ± 2.77，近海水域为 3.35 ± 2.26）。 这支持了蓝藻和甲藻的出现，因为硅藻对整个浮游植物群落的贡献急剧下降至 50%。换句话说，东亚地区的上升流带来了缺氮（反硝化）的水域；可用氮立即被硅藻群落消耗，导致低 N/P 条件，有利于蓝藻种群在 SM 后期占据主导地位。总体而言，在营养化学计量方面观察到显着的季节内变化，并受到影响浮游植物种群的物理过程强度的强烈调节。需要持续监测，以了解浮游植物种群、它们对较高营养级的影响以及东亚地区水生食物网结构的整体健康状况。