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 Schmidt’s 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.