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

季节性洪水泛滥的淡水柏树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质（NOM）的主要陆地来源，导致了南卡罗来纳瓦卡莫河（SC）等沿海黑水河流的黄茶色水。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运输。2015年10月1-5日，停滞的温带气旋（东北风）与飓风华金（Joaquin）相互作用，导致SC海岸短时间内出现暴雨和大面积洪水，导致大量NOM从森林湿地输出到沿海黑水河流。如此大规模、规模空前的NOM出口，可能会将相对稳定和难以抗拒的汞和碳库从森林湿地转移到黑水河流中。这项高度跨学科的研究将极大地提高对极端天气条件下汞和碳生物地球化学的理解，并展示生态系统背景下的关系。本研究由来自三所大学的四名不同专业的研究者组成，其中包括两名来自少数民族服务机构和本科院校的助理教授。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和汞汇。然而，气候变化下的极端天气事件可能改变这种平衡状态，影响沿海生态系统碳汞库的稳定性。本RAPID研究的目的是研究极端洪水对沿海湿地和黑水河流中碳和汞循环及其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、受控实地实验和水文计算机模型，以检查极端洪水事件的水文曲线上的碳和汞出口、来源和时间趋势。自2015年10月4日以来，研究小组一直在瓦卡莫河沿岸收集水样，这条河代表了水文曲线的上升分支。分析了过滤和未过滤样品中的一般水质、溶解有机碳、紫外线吸光度、营养物质、阳离子、阴离子、总汞和甲基化汞。为了确定碳和汞收支，将使用水文模型来估计这次洪水事件进入Waccamaw河的总水量。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陆生有机质的新分解过程路线。研究人员认为，利用稳定汞同位素沿着洪水事件的水文曲线，结合使用紫外/可见光和荧光光谱测量水中溶解有机物的新方法，将有助于了解沿海黑水河流中碳和汞之间的关系。确定碳和汞的时间变化将有助于从机制和景观层面理解极端洪水事件如何影响沿海生态系统中陆生有机质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.