Due to the widespread pollution of coastal groundwaters with fertilizers, submarine groundwater discharge (SGD) is often thought to be a large dissolved inorganic nitrogen (DIN) source to the ocean. Whether this N is autochthonous or allochthonous to the subterranean estuary (STE), the availability of large quantities of DIN can nevertheless interact with the cycling of other elements, such as carbon (C). In previous studies, we documented the discharge of large quantities of freshwater and NO3- from the mouth of an STE into the Ria Formosa lagoon (SW Iberian Peninsula). For the period covered in this study (2009-2011), the same STE site was dominated by recirculating seawater due to a prolonged fall in piezometric head in the coupled coastal aquifers. Total SGD rates remained similarly high, peaking at 144 cm day-1 at the lower intertidal during fall. We observed a progressive increase of NO3- availability within the STE associated with the recovery of piezometric head inland. Interestingly, during this period, the highest SGD-derived dissolved organic C and DIN fluxes (112±53 and 10±3 mmol m-2 d-1 respectively) originated in the lower intertidal. NO3- enrichment in the STE influences the benthic reactivity of fluorescent dissolved organic matter (FDOM): when seawater recirculation drives STE dynamics, only small changes in the benthic distribution of recalcitrant humic-like FDOM are observed (from -2.57±1.14 to 1.24±0.19 10-3 R.U. “bulk” sediment h-1) in the absence of DIN. However, when DIN is available, these recalcitrant fractions of FDOM are actively generated (from 1.32±0.15 to 11.56±3.39 10-3 R.U. “bulk” sediment h-1), accompanied by the production of labile protein-like FDOM. The results agree with previous studies conducted with flow-through reactor experiments at the same site and suggest that DIN enrichment in the STE enhances the metabolic turnover of sedimentary organic matter up to the point of discharge to surface waters. DIN pollution of coastal aquifers may therefore promote a contraction of the residence time of particulate organic C within the STE, driving carbon from continental storage into the sea.

中文翻译：

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硝酸盐富集会加速地下河口中的有机质更新吗？

由于化肥对沿海地下水的广泛污染，海底地下水排放（SGD）通常被认为是海洋中大量的溶解性无机氮（DIN）源。无论该N是地下河口（STE）的外源土体，还是其他外源性氮，大量DIN的可用性仍然可以与其他元素的循环（如碳（C））相互作用。在以前的研究中，我们记录了从STE口向Ria Formosa泻湖（西南伊比利亚半岛）排放的大量淡水和NO3-。在本研究涵盖的期间（2009年至2011年），由于耦合沿海含水层的测压头长时间下降，同一STE站点被海水再循环所占据。SGD总额仍然保持较高水平，在秋季潮间期较低的潮间带，在第一天的144厘米处达到峰值。我们观察到与测压头内陆的恢复相关的STE中NO3-的利用率逐渐增加。有趣的是，在此期间，源自SGD的最高溶解有机C和DIN通量（分别为112±53和10±3 mmol m-2 d-1）起源于潮间带下部。STE中的NO3-富集会影响荧光溶解有机物（FDOM）的底栖反应性：当海水再循环驱动STE动态时，仅观察到顽固性腐殖质样FDOM底栖分布的微小变化（从-2.57±1.14到1.24±在没有DIN的情况下为0.19 10-3 RU的“散装”沉积物h-1）。但是，当使用DIN时，会积极生成这些FDOM顽固级分（从1.32±0.15到11.56±3.39 10-3 RU“大量”沉积物h-1），伴随着不稳定蛋白质样FDOM的产生。结果与先前在同一地点进行的流式反应器实验所进行的研究一致，并表明在STE中进行DIN富集可提高沉积有机物的代谢转化率，直至排放到地表水为止。因此，DIN对沿海含水层的污染可能会促进STE中颗粒状有机碳的停留时间缩短，从而将碳从大陆储存带入海洋。