Nitrogen fixation in the monsoon impacted Mekong River plume

南中国海（SCS）是世界上最大的边缘海，周围是人口稠密的沿海国家，如中国，印度尼西亚，菲律宾或越南。湄公河流域的气候变化和强烈的人为影响（计划修建18座大型水坝用于水力发电，水产养殖正在增加）将改变河流负荷，并导致近海岸生态地球化学变化。湄公河流域的人为变化的速度和幅度使得它很可能是拟议的实地计划将是为数不多的机会之一，研究南海的营养物质和有机物负荷发生显着变化之前，由于人为的影响，在河流流域。将研究湄公河目前对南海生产力与上升流营养物的作用。我们以前的研究结果表明，重氮硅藻协会（DDA）在河流羽流中的强大作用，即使在存在高硝酸盐浓度，但也存在单细胞和菌落形成蓝藻，如束毛藻。营养盐浓度和营养盐比例对固氮物种的丰度和活性起着重要作用，将是本研究的重点。将在田间样本和特定培养实验中测量营养物浓度并量化氮和碳的吸收。各种N固定基团的批量固定率可以用同位素比质谱法测定，并在细胞基础上用NanoSIMS技术研究。此外，美国和越南科学家将提供有关固氮群落的物种组成和代谢活动的重要信息。该项目的结果将提供一个坚实的理解湄公河羽流和上升流过程中的作用，在沿海的南海固氮生物。因此，未来的变化，河流生态地球化学可以评估这些结果的基础上。两次前往南海的航行由施密特海洋研究所的一项经批准的建议提供资金。因此，船上的实地取样和实验工作得到了保障。此外，DFG先前的资助与芽庄海洋学研究所建立了密切合作，因此拟议的工作建立在越南-德国合作的基础上。

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.