Physically modulated phytoplankton production and export at submesoscales in the oligotrophic South China Sea Basin

IF 6 2区 地球科学 Q1 GEOSCIENCES, MULTIDISCIPLINARY Science China Earth Sciences Pub Date : 2024-07-11 DOI:10.1007/s11430-023-1362-1
Zhonghua Zhao, Mengdi Xu, Bangqin Huang, Wenfang Lu, Hongshuai Qi, Feng Cai, Yuwu Jiang
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Abstract

Oceanic submesoscales can significantly influence phytoplankton production and export owing to their similar timescales of days. Based on two-year Biogeochemical Argo (BGC-Argo) observations, this study investigated the development of submesoscale instabilities, particularly symmetric and mixed-layer baroclinic instabilities, and their impacts on biological production and export in the oligotrophic South China Sea basin. In the northern basin, near-surface winter blooms consistently cooccurred with seasonally deepened mixed layers. However, significantly stronger and weaker winter blooms were observed over two consecutive winters within the BGC-Argo observation period. During the first winter, symmetric-instability-induced upward nutrient entrainment played a crucial role in initiating the strong winter bloom in early December, when the mixed layer was approximately 20–30 m shallower than the nutricline. This bloom occurred approximately 20–30 days earlier than that anticipated owing to the contact between the seasonally deepened mixed layer and mesoscale-cyclone-induced uplifted nutricline. The symmetric instability also facilitated the export of fixed phytoplankton carbon from the surface to deeper layers. Conversely, during the second winter, remarkably intense mixed-layer baroclinic instability associated with an intense mesoscale anticyclone led to more significant shoaling of the mixed layer compared to the nutricline, thus increasing the vertical distance between the two layers. Under this condition, upward nutrient injection, phytoplankton bloom, and carbon export were suppressed. In contrast, the BGC-Argo float in the central basin revealed significantly inhibited seasonality of phytoplankton biomass and submesoscale instabilities compared to those in the northern basin, primarily owing to the significantly shallower winter mixed layer.

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低营养南海海盆中受物理调控的浮游植物产量和出口量
由于海洋副旋涡的时间尺度相近,可达数天,因此可对浮游植物的生产和输出产生重大影响。本研究基于两年的生物地球化学 Argo(BGC-Argo)观测数据,研究了南海低营养海盆亚中尺度不稳定性(尤其是对称层和混合层的气压不稳定性)的发展及其对生物产量和出口的影响。在北部海盆,近表层冬季水华始终与季节性加深的混合层同时出现。然而,在 BGC-Argo 观测期内,连续两个冬季都观测到了明显较强和较弱的冬季水华。在第一个冬季,对称不稳定性引起的营养物质向上夹带在 12 月初开始的强冬季藻华中发挥了关键作用,当时混合层比营养线浅约 20-30 米。由于季节性加深的混合层与中尺度气旋引起的营养线上升之间的接触,这次水华比预计的时间提前了约 20-30 天。对称不稳定性也促进了浮游植物固定碳从表层向深层的输出。相反,在第二个冬季,与强烈的中尺度反气旋相关的混合层气压不稳定性明显增强,导致混合层与营养盐层相比出现更明显的倾斜,从而增加了两层之间的垂直距离。在这种情况下,向上的营养物质注入、浮游植物繁殖和碳输出受到抑制。相反,与北部海盆相比,中部海盆的 BGC-Argo 浮漂显示浮游植物生物量的季节性和中尺度以下的不稳定性明显受到抑制,主要原因是冬季混合层明显变浅。
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来源期刊
Science China Earth Sciences
Science China Earth Sciences GEOSCIENCES, MULTIDISCIPLINARY-
CiteScore
9.60
自引率
5.30%
发文量
135
审稿时长
3-8 weeks
期刊介绍: Science China Earth Sciences, an academic journal cosponsored by the Chinese Academy of Sciences and the National Natural Science Foundation of China, and published by Science China Press, is committed to publishing high-quality, original results in both basic and applied research.
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