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

墨西哥湾拥有世界上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，美国沿海水域最大的缺氧区位于其北部，与淡水氮(N)通量有关，而淡水氮(N)通量来源于密西西比河三角洲。然而，最近的研究表明，河水是局部缺氧问题的次要因素；相反，他们指出富含营养的地下水排放到沿海水域的潜在贡献。在本研究中，我的目标是(1)量化地下水衍生的N通量，(2)确定淡水N来源，(3)评估墨西哥湾北部富有机质海岸沉积物中的N转化。具体来说，我建议把重点放在海岸线上的几个较小的河口上，由于它们靠近美国大陆最大的三角洲密西西比河三角洲，地下水排放尚未得到研究。我假设，地下水衍生的N通量是造成墨西哥湾北部缺氧问题的部分原因，而富含有机物的沿海沉积物通过生物地球化学过程的组合充当了人为N的汇。为了验证我的假设，我建议结合实地和实验室工作，包括测量氡、镭、溶解的有机和无机氮浓度，以及微生物测序。本研究结果可用于评估地下水对区域总氮收支的贡献，确定地下河口在改变或/和减弱人为氮输送中的作用，并有助于更好地了解墨西哥湾水质的驱动因素和修复措施。在更广泛的范围内，该项目获得的知识将提高我们对地下水对全球海洋营养收支贡献的预测能力。

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.