The advective supply of allochthonous dissolved organic carbon (DOC) from open ocean to marginal seas through western boundary current intrusion influences the regional carbon inventory and microbial activities. However, there is limited observation about this process and its biogeochemical impacts on marginal seas. In this study, we investigated the biodegradation of allochthonous DOC carried by the intrusion of the Kuroshio Current into South China Sea (SCS). Using an isopycnal mixing model, the exchange and biodegradation processes of Kuroshio-intruded DOC were quantified. We estimated that approximately 10% of the surface DOC was remineralized due to the enhanced biodegradation in the SCS. This result was supported by the on-deck bioassay experiments that were conducted under different environmental contexts. The results of modeling and on-deck incubations indicate that DOC biodegradation was enhanced by the sharp gradient of environment factors, including nutrients supply, microbial species, and bio-lability of DOC in the frontal zone during the surface water mass mixing. The amount of carbon released from the enhanced DOC degradation by Kuroshio intrusion was estimated to be approximately equal to 8.6 Tg C yr⁻¹. Concomitantly, the amount of nitrogen released could contribute 0.19–0.70 mmol N m⁻² d⁻¹ to the surface of SCS which is comparable to the total supply from deeper water and nitrogen fixation in surface waters. This study suggests that the enhanced biodegradation of DOC during the western boundary currents intrusion could serve as an important sink of oceanic DOC, and thus provide an additional nutrient source to marginal seas.

中文翻译：

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入侵南海边缘时西边界黑潮中溶解有机碳的生物降解增强

外来溶解有机碳 (DOC) 通过西部边界洋流侵入从公海到边缘海的平流供应影响区域碳库存和微生物活动。然而，关于这一过程及其对边缘海的生物地球化学影响的观察有限。在这项研究中，我们研究了黑潮侵入南海 (SCS) 所携带的外来 DOC 的生物降解。使用等密度混合模型，量化了黑潮侵入 DOC 的交换和生物降解过程。我们估计，由于 SCS 中生物降解的增强，大约 10% 的表面 DOC 被再矿化。这一结果得到了在不同环境背景下进行的甲板生物测定实验的支持。建模和甲板上孵化的结果表明，在地表水团混合过程中，环境因素的急剧梯度增强了 DOC 的生物降解，包括营养供应、微生物种类和锋区 DOC 的生物不稳定性。由黑潮入侵引起的增强的 DOC 降解释放的碳量估计约为 8.6 Tg C yr^-1。同时，释放的氮量可以为 SCS 表面贡献 0.19-0.70 mmol N m ^-2  d ^-1，这与来自深水和地表水中固氮的总供应量相当。该研究表明，在西部边界洋流侵入过程中，DOC 的生物降解增强可以作为海洋 DOC 的重要汇，从而为边缘海提供额外的营养源。