Nutrient stoichiometry drives the phytoplankton populations during the progression of upwelling along the eastern Arabian Sea

IF 3.8 3区 地球科学 Q1 OCEANOGRAPHY
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Abstract

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 a: 74.09 ± 60.05 mg m−2) and moderate levels in the central (25.75 ± 6.51 mg m−2) and north (30.31 ± 12.32 mg m−2) 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 a: 129.06 ± 96.24 mg m−2). 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.

阿拉伯海东部上升流发展过程中营养物质化学计量对浮游植物种群的影响
首次详细研究了2018年夏季季风(SM)期间东阿拉伯海(EAS,∼6◦至22◦N)全流域浮游植物演替和群落行为对不同上升流阶段不同营养模式的响应。在夏季季风初期(6 月至 7 月)、夏季季风高峰期(8 月)和夏季季风后期(9 月至 10 月)进行了三次连续观测,代表了上升流的不同阶段。在上升流早期阶段,观测到南部浮游植物生物量较高(柱状整合叶绿素 a:74.09 ± 60.05 mg m-2),中部(25.75 ± 6.51 mg m-2)和北部(30.31 ± 12.32 mg m-2)EAS 沿岸水域浮游植物生物量中等。在整个上升流期间,硅藻是沿岸站位的主要种类(60-90%)。然而,由于这一时期氮磷比(N/P:2.6±1.31)较低,与 EAS 沿岸相比,硅藻在近海水域的比例下降到 20%以下。在 SM 高峰期,EAS 南部沿岸水域营养盐层的上升流和混合层的高 N/P 条件(8.4 ± 5.25)大大提高了浮游植物的生物量(叶绿素 a:129.06 ± 96.24 mg m-2)。此外,浅营养线支持硅藻在近海水域占优势,特别是在 EAS 中部(高达 65%),而在 EAS 南部和北部(22-33%),上升流强度较弱。上升流的减弱导致营养线加深,SM 晚期出现低 N/P 条件(沿岸水域为 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,硅藻在浮游植物群落中所占比例急剧下降到 50%。换句话说,EAS 的上升流带来了缺氮(脱氮)水域;硅藻群落立即消耗了可获得的氮,导致低氮/磷条件,有利于蓝藻种群在 SM 晚期占优势。总之,在营养物配比方面观察到了很大的季节内变化,这主要受影响浮游植物种群的物理过程强度的调节。要了解浮游植物种群、其对较高营养级的影响以及 EAS 水生食物网结构的整体健康状况,需要进行持续监测。
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来源期刊
Progress in Oceanography
Progress in Oceanography 地学-海洋学
CiteScore
7.20
自引率
4.90%
发文量
138
审稿时长
3 months
期刊介绍: Progress in Oceanography publishes the longer, more comprehensive papers that most oceanographers feel are necessary, on occasion, to do justice to their work. Contributions are generally either a review of an aspect of oceanography or a treatise on an expanding oceanographic subject. The articles cover the entire spectrum of disciplines within the science of oceanography. Occasionally volumes are devoted to collections of papers and conference proceedings of exceptional interest. Essential reading for all oceanographers.
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