Nutrient biogeochemistry in the Chinese Seas

Anthropogenic activities have severely altered many coastal ecosystems by increasing the input of nutrients through such as rivers and groundwater, wastewater discharge, atmospheric deposition, resulting in eutrophication, modifying aquatic food webs, and provoking more severe hypoxic events in coastal marine environments. Sustainability of marine ecosystem production is unavoidably impacted by multi-stressors. A better understanding of the fate of nutrients entering the coastal ecosystem is an important issue with implications for environmental management. The Huanghe estuary and Bohai Sea ecosystem is given as an example to address the response of nutrient transport patterns to anthrogpogenic activites, such as the sediment-water regulation events, and their effects on the ecosystem of the Huanghe estuary and the Bohai.

As Chinese rivers are generally enriched with dissolved inorganic nitrogen (DIN) and depleted in dissolved inorganic phosphate (PO43-) with very high DIN: PO43- concentration ratios. The fixed nitrogen concentrations in the coastal areas of Chinese Seas have been exponentially increased. The nitrate reduction and nitrogen loss pathways within sediments of the Chinese Seas were addressed using the 15N isotope pairing technique. Considering complicated spatial and temporal variations, in particular the patchy distribution of sediments and limited shipboard sampling lead to uncertainty in the extrapolation of direct measurements of N fluxes to regional and global fluxes. Moreover, nitrogen cycling processes, such as nitrification, denitrification and nitrogen fixation, are complicated and may affect primary production, phytoplankton species composition and abundance in coastal ecosystems. The fluxes between pools of nitrogen and rates of transformation of nitrogen species are often poorly constrained in the ocean. The dual isotopes of nitrate and nitrite and other nitrogen isotopes are powerful approaches to separate nitrogen flux processes in the ocean that are difficult to obtain through more traditional measurements of solute concentration or direct rate determination, especially in complicated coastal ecosystems. Nitrogen biogeochemistry in the Yellow Sea was addressed as an example using nutrient concentrations and isotopic composition of nitrate and other nitrogen species. Nitrate N and O isotope co-variation on the continental shelf has characteristics related to active recycling of nitrogen and nitrogen assimilation with Yellow Sea Warm Current water deviating from the Kuroshio Subsurface Water. High NO3- δ18O relative to δ15N likely derives from nitrification, atmospheric deposition, and/or active recycling of nitrogen.