Globally expanding hypoxia in estuaries and coastal oceans has largely been attributed to the elevated river nutrient inputs, whereas the role of river‐delivered terrestrial organic matter (OM_terr) in hypoxia formation has been less investigated. This study uses a coupled physical‐biogeochemical model and observations to investigate how OM_terr directly (via remineralization) and indirectly (via the nutrients released from OM_terr remineralization) promotes hypoxia development in the coastal transition zone off the Pearl River Estuary. Results show that direct contribution of OM_terr remineralization to total oxygen consumption by terrestrial and marine organic matter negatively correlates with salinity, decreasing from over 60% in the upper estuary to nearly 0% in the far reaches of the river plume, and is higher in the upstream (average 30%) than the downstream region (average 18%). Nevertheless, the nutrients released from OM_terr remineralization greatly sustain an indirect contribution to oxygen depletion and hypoxia formation downstream. The increasing relative importance of indirect over the direct effect of OM_terr to hypoxia along the plume path is a combined result of the wind‐driven eastward shelf current and the OM_terr‐released nutrients being advected farther downstream than the sinking OM_terr. This highlights that without including the indirect effect of OM_terr may underestimate the role of OM_terr in hypoxia formation in aquatic systems. Examinations of the hypoxia response to varying riverine loads further suggest that reducing the nutrient and OM_terr loads is required for hypoxia mitigation in the upstream region while reducing the nutrient load alone is more effective in mitigating hypoxia in the downstream.

中文翻译：

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模拟珠江口沿岸过渡带内河流有机质在低氧形成中的作用

河口和沿海海洋缺氧的全球性扩展主要归因于河中养分投入的增加，而对河流输送的陆地有机质（OM _terr）在缺氧形成中的作用的研究较少。这项研究使用了一种物理-生物地球化学耦合模型和观测资料来研究OM _terr如何直接（通过再矿化）和间接（通过OM _terr矿化释放的养分）促进缺氧在珠江口外的沿海过渡带发展。结果表明OM _terr的直接贡献再矿化对陆地和海洋有机物的总耗氧量与盐度呈负相关，从上河口的60％以上下降到河羽远处的近0％，在上游（平均30％）高于下游地区（平均18％）。然而，从OM_矿再矿化中释放的营养物极大地维持了对下游氧气消耗和缺氧形成的间接贡献。间接作用相对于OM _ter对羽流路径中的缺氧的直接作用的相对重要性的增加是风向东陆架流和OM _TER释放的养分比下沉OM _TER向下游输送的综合结果。这突显出，如果不包括OM _terr的间接作用，可能会低估OM _terr在水生系统缺氧形成中的作用。对不同河水负荷的低氧反应的研究进一步表明，减少上游地区的低氧需要减少养分和OM _terr负荷，而仅减少养分负荷则可以更有效地缓解下游的低氧。