Contrasting impacts of mesoscale processes on the silicate pump in the South China Sea
Jie Xiao^{1 2}, Danna Wang^{1 2}, Meng Cui^{1 2}, Yang Liu³, Zhouling Zhang⁴, Junhui Chen^{1 2}, Kuanbo Zhou^{1 2}, Xin Liu^{1 5}, Minhan Dai^{1 2}, Zhimian Cao^{1 2 *
1} State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
² College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
³ Laboratory of Marine Ecological Environment Early Warning and Monitoring, Third Institute of Oceanography, Ministry of Natural Resources, Xiamen, China
⁴ GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
⁵ College of the Environment and Ecology, Xiamen University, Xiamen, China

Abstract:
Mesoscale physical processes can strongly modify nutrient supply, phytoplankton community structure, and particle export in marginal seas. The marine silicate pump, primarily driven by diatom production and biogenic silica (BSi) formation, plays a key role in regulating particulate organic carbon (POC) export to the ocean interior. However, the relative influences of different mesoscale processes on the silicate pump remain insufficiently quantified, particularly in marginal seas where episodic nutrient inputs may disproportionately stimulate diatom production. Here, we synthesize field observations from five cruises to the South China Sea to examine the responses of BSi to various mesoscale processes, including eddies, coastal upwelling, and river plumes, in comparison with POC dynamics.
Cyclonic eddies enhanced euphotic-zone BSi inventories by approximately 2–5 times relative to background conditions, indicating stimulation of opal production and accumulation. In contrast, BSi accumulation was weaker in the anticyclonic eddy core than in its edge, where inventories were about 30% higher, suggesting the influence of frontal processes and lateral transport. Coastal upwelling off Vietnam, Qiongdong, and eastern Guangdong generally increased Si(OH)₄ supply, fucoxanthin concentrations, and BSi/POC ratios, indicating an enhanced contributions of diatom to POC. River plumes showed more variable effects: distal low-salinity plume waters promoted BSi accumulation over the shelf, whereas strong plume centers did not always show elevated BSi/POC ratios, implying co-regulation by stratification, nutrient stoichiometry, and biological response. The strongest enhancement of the silicate pump efficiency occurred in plume–upwelling coupled systems along the eastern Guangdong coast, where riverine and upwelled nutrient supplies jointly favored diatom-dominated BSi production and export.
Our findings demonstrate that mesoscale processes exert a process-specific control on the silicate pump in the South China Sea, thereby reshaping silicon cycling and carbon export in marginal seas.
Keywords: silicate pump; mesoscale processes; river plume; coastal upwelling; eddy; South China Sea

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