RAPID: Extreme Flooding Mobilized Natural Organic Matter from Forested Wetlands - Implication of Mercury and Carbon Cycling in Coastal Blackwater Rivers

季节性泛滥的淡水柏树-tupelo湿地常见于美国东南部沿海地区，从德克萨斯州到北卡罗来纳州。这些湿地是天然有机物（NOM）的主要陆地来源，导致沿海黑水河（如南卡罗来纳州（SC）的Waccamaw河）的黄色茶色水。重要的是，NOM的浓度与总汞和甲基化汞的浓度高度相关，介导了汞在沿海生态系统中的生物利用度和迁移。2015年10月1日至5日，一个停滞的温带气旋（东北风暴）和飓风华金（英语：Hurricane Joaquin）之间的相互作用在南卡罗来纳州海岸造成暴雨和短时间内的大范围洪水，导致大量NOM从森林湿地输出到沿海黑水河。如此大规模的NOM出口可能会使相对稳定的汞和碳从森林湿地流入黑水河。这项高度跨学科的研究将极大地提高极端天气条件下汞和碳的地球化学的理解，并展示生态系统背景下的关系。本研究代表了来自三所大学的四名不同专业的研究人员之间的新的和独特的合作，其中包括两名助理教授，分别在少数民族服务机构和本科院校。这项研究还将提高人们对极端天气事件对美国东南部低洼沿海地区影响的认识。沿海湿地被认为是重要的碳汇和汞汇。然而，气候变化下的极端天气事件可能会改变平衡状态，影响沿海生态系统中碳和汞库的稳定性。这项快速研究的目标是研究极端洪水对沿海湿地和黑水河中碳和汞循环以及相关生物地球化学过程的影响。具体而言，该研究将整合实地调查，控制现场实验，水文计算机建模，以检查碳和汞的出口，来源，以及沿着极端洪水事件的水文过程的时间趋势。自2015年10月4日以来，研究小组一直在沿着瓦卡莫河收集水样，代表水文过程的上升分支。对过滤和未过滤样品中的一般水质、溶解有机碳、紫外吸收、营养物质、阳离子、阴离子、总汞和甲基化汞进行了分析。为了确定碳和汞的预算，将使用水文模型来估计从这次洪水事件进入瓦卡莫河的总排水量。PI将参与一项变革性研究，以评估极端洪水事件影响下森林湿地中陆地有机物的新分解过程路线。 PI认为，新的方法，使用稳定的汞同位素沿着洪水事件的过程线，再加上测量水中溶解的有机物，使用紫外/可见光和荧光光谱，将有助于了解碳和汞之间的关系在沿海黑水河。确定碳和汞的时间变化将有助于发展一个机制和生态系统层面的理解如何极端洪水事件影响陆地有机物在沿海生态系统的生物地球化学过程。

Seasonally flooded, freshwater cypress-tupelo wetlands are commonly found in coastal regions of the southeastern United States, from Texas to North Carolina. These wetlands are the main terrestrial source of natural organic matter (NOM) that causes the yellowish tea-colored water in coastal blackwater rivers such as Waccamaw River in South Carolina (SC). Importantly, the concentration of NOM is highly correlated with concentrations of total mercury and methylated mercury, mediating the bioavailability and transport of mercury in coastal ecosystems. On October 1-5, 2015, the interaction between a stalled extratropical cyclone (nor'easter) and hurricane Joaquin caused torrential rain and extensive flooding in a short period of time in the coast of SC, resulting in a large volume of NOM exported from the forested wetlands into the coastal blackwater rivers. Such large quantity of NOM exports on an unprecedented scale could potentially mobilize relatively stable and recalcitrant pools of mercury and carbon from the forested wetlands into the blackwater rivers. This highly interdisciplinary research will dramatically improve understanding of mercury and carbon biogeochemistry under extreme weather conditions and demonstrate the relationships within the context of ecosystems. This study represents a new and unique collaboration between four investigators with different specialties from three universities including two assistant professors respectively in a minority serving institution and an undergraduate institution. This study will also raise the awareness of the impacts of extreme weather events on low-lying coastal areas in the Southeastern United States. Coastal wetlands are considered as important carbon and mercury sinks. However, extreme weather events under the changing climate could alter the equilibrium status, affecting the stability of carbon and mercury pools in the coastal ecosystems. The goal of this RAPID research is to study the influences of the extreme flooding on carbon and mercury cycles and associated biogeochemical processes in coastal wetlands and blackwater rivers. Specifically, the study will integrate field investigations, controlled field experiments, and hydrological computer modeling to examine carbon and mercury exports, sources, and temporal trends along the hydrograph of the extreme flooding event. The research team has been collecting water samples along the Waccamaw River since Oct 4, 2015, representing the rising limb of the hydrograph. Samples were analyzed for general water quality, dissolved organic carbon, ultraviolet absorbance, nutrients, cations, anions, total mercury, and methylated mercury in the filtered and unfiltered samples. To determine the carbon and mercury budget, a hydrological model will be used to estimate the total water discharge from this flooding event that went into Waccamaw River. The PIs will engage in a transformative research study to assess novel decomposition process routes of terrestrial organic matter in forested wetlands under the influence of extreme flooding event. The PIs contend that the novel method of using stable mercury isotope along the hydrograph of the flooding event, coupled with measurements of dissolved organic matter in water using ultraviolet/visible and fluorescence spectroscopy, will be useful for understanding the relationships between carbon and mercury in coastal blackwater rivers. Determining the temporal variation of carbon and mercury will be useful for developing a mechanistic and landscape-level understanding of how extreme flooding events affect the biogeochemical process of terrestrial organic matter in coastal ecosystems.