Nitrogen fixation in the monsoon impacted Mekong River plume

南中国海（SCS）是世界上最大的边缘海，被中国、印度尼西亚、菲律宾或越南等人口稠密的沿海国家包围。湄公河流域的气候变化和强烈的人为影响（规划18座大型水电大坝，水产养殖正在增加）将改变河流负荷，导致近海岸生物地球化学的变化。湄公河流域人为变化的速度和幅度，很可能使拟议的实地项目成为在流域的人为影响导致营养物和有机质负荷发生重大变化之前研究南海的少数机会之一。与上涌的营养物质相比，湄公河目前对南海生产力的作用将被研究。我们之前的研究结果表明，即使在硝酸盐浓度很高的情况下，重氮营养体-硅藻协会（DDA）在河流羽流中也有很强的作用，但也存在单细胞和集落形成的蓝藻，如Trichodesmium。营养浓度和营养比例对固氮物种的丰度和活性起主要作用，这将是本研究的重点。将在田间样品和特定孵化实验中测量养分浓度，并对氮和碳的吸收进行量化。各种n固定基团的体积固定率可以用同位素比质谱法确定，并在细胞基础上用NanoSIMS技术研究。此外，美国和越南的科学家将为固氮群落的物种组成和代谢活动提供重要信息。该项目的研究结果将有助于深入了解湄公河羽流和上升流过程对南海沿海固氮生物的作用。因此，可以根据这些结果来评价未来河流生物地球化学的变化。两艘前往南海的游轮是由施密特海洋研究所批准的提案资助的。因此，船上的实地采样和实验工作得到了保障。此外，以前的DFG资助与芽庄的海洋学研究所建立了密切的合作关系，以便拟议的工作建立在越南和德国的既定合作基础上。

The South China Sea (SCS) is the largest marginal sea in the world, surrounded by densley populated coastal states like China, Indonesia, Philippines or Vietnam. Climate change and strong anthropogenic impacts (18 major damming for hydropower are planned, aquaculture is increasing) in the catchment area of River Mekong will change riverine loads with the consequence of changes in near coastal biogeochemistry. The speed and magnitude of anthropogenic changes in the Mekong basin make it very likely that the proposed field program will be one of the few opportunities to study the SCS before significant changes in nutrient and organic matter loading occur as a result of anthropogenic impact in the river basin. The present role of the Mekong for the productivity of the SCS in comparison to upwelling nutrients will be studied. Previous results from us have suggested a strong role of diazotroph-diatom associations (DDA) in the river plume even in the presence of high nitrate concentrations, but also unicellular and colony forming cyanobacteria like Trichodesmium are present. Nutrient concentrations and nutrient ratios have been suggested to play a major role for the abundance and activity of the nitrogen fixing species and will be a focus of this study. Nutrient concentrations will be measured and the uptake of nitrogen and carbon will be quantified in field samples and specific incubation experiments. Bulk fixation rates of various N-fixing groups can be determined with isotope ratio mass spectrometry and on a cellular basis studied with NanoSIMS technology. Moreover, US and Vietnamese scientists will contribute important information on species composition and metabolic activity of nitrogen fixing communities. The outcome of this project will provide a solid understanding of the role of the Mekong river plume and upwelling processes on the N-fixing organisms in the coastal SCS. Future changes in river biogeochemistry can thus be evaluated on the basis of these results. Two cruises to the SCS are financed by an approved proposal of Schmidts Oceanographic Institute. Field sampling and experimental work on board is thus secured. Moreover, previous DFG funding established close collaboration with the Institute of Oceanography in Nha Trang so that the proposed work builds upon an established Vietnamese-German collaboration.