Nitrogen biogeochemistry of groundwater fluxes to the estuaries in the northern Gulf of Mexico

墨西哥湾拥有世界上一些最多样化的生态系统；然而，其生态多样性目前受到水质差和水资源稀缺的威胁。目前，美国沿海最大的缺氧区在其北部，它与来自密西西比河三角洲的淡水氮(N)通量相关。然而，最近的研究表明，河水是造成当地缺氧问题的一个次要因素；相反，他们指出，营养丰富的地下水排放到沿海水域的潜在贡献。在这项研究中，我的目标是(1)量化地下水来源的N通量，(2)确定淡水N的来源，以及(3)评估墨西哥湾北部发现的富含有机物质的沿海沉积物中的N转化。具体地说，我建议把重点放在沿海岸线的几个较小的河口，这些河口的地下水排放由于靠近美国大陆最大的三角洲密西西比河三角洲而尚未得到研究。i假设地下水产生的氮通量是墨西哥湾北部缺氧问题的部分原因，而富含有机物质的沿海沉积物通过一系列生物地球化学过程充当人为氮的汇。为了验证我的假设，我建议进行野外和实验室相结合的工作，包括测量氡、镭、溶解的有机和无机氮浓度，以及微生物测序。这项研究的结果可用于评估地下水对区域总氮收支的贡献，确定地下河口在转化或/和减弱人为氮输送方面的作用，并进一步有助于更好地从根本上理解墨西哥湾水质的驱动因素和修复。在更广泛的范围内，该项目所获得的知识将提高我们对地下水对全球海洋营养预算的贡献的预测能力。

The Gulf of Mexico hosts some of the most diverse ecosystems in the world; however, its ecological diversity is currently threatened by poor water quality and water resource scarcity. Currently, the largest hypoxic zone in U.S. coastal waters is found in its northern part and it is correlated with freshwater nitrogen (N) flux, which originates from the Mississippi River Delta. Nevertheless, recent studies indicate that river water is a minor contributor to the local hypoxia problem; instead, they point to potential contribution of nutrient-rich groundwater discharge into the coastal water. In this study, I aim to (1) quantify groundwater-derived N fluxes, (2) determine freshwater N sources, and (3) assess N transformation in the organic-rich coastal sediment found in the northern Gulf of Mexico. Specifically, I propose to focus on several smaller estuaries along coastlines, in which groundwater discharge has not been studied due to their proximate locations to the Mississippi River Delta, the largest delta in the continental U.S. I hypothesize that groundwater-derived N fluxes are partially responsible for the hypoxia problem in the northern Gulf of Mexico, and the organic-rich coastal sediment acts as a sink of anthropogenic N through a combination of biogeochemical processes. To test my hypotheses, I propose to do a combination of field and laboratory works, including measurements of radon, radium, dissolved organic and inorganic N concentration, and microbial sequencing. The results of this study can be used to assess the contribution of groundwater to total regional N budget, determine the role of subterranean estuary in transforming or/and attenuating anthropogenic N transport, and further contribute to better fundamental understanding of the drivers and remediation of water quality in the Gulf of Mexico. On a broader scale, the knowledge obtained by this project will improve our predictive ability of the contribution of groundwater to the global nutrient marine budget.