Key nitrogen biogeochemical processes in the South Yellow Sea revealed by dual stable isotopes of nitrate

Abstract Coastal eutrophication has become a serious ecological problem in China and around the world. Examining biogeochemical nitrogen cycles may help to provide insights into the mechanism of the problem. Here, we mapped the spatial and temporal characteristics of the dual stable isotopes of nitrate (δN 18 O 3 − and δ 15 NO 3 − ), nitrogen isotopes within particulates, and the hydrological parameters and nutrients in the South Yellow Sea during spring, summer and autumn. We further considered the nitrogen biogeochemical processes that are revealed by these nutrient and isotope signals in the area of the Yellow Sea cold water mass (YSCWM). The results indicated that nitrate assimilation was the main biogeochemical process in the upper layer of the YSCWM area. Nitrification was the main process that occurred at the interior of the YSCWM, which resulted in increasing concentrations of nitrate and decreasing values of δ 15 NO 3 − in the bottom layer from spring to autumn. The values of δN 18 O 3 − and the calculation of the apparent oxygen utilization (AOU)-nitrate stoichiometry relationship indicated that, in addition to nitrification, nitrate losses also occurred in the interior of the YSCWM, particularly from summer to autumn. Through field observation and theoretical calculations, we observed that the stable nitrogen isotope values from samples of surface particulate nitrogen in spring (δ 15 N p ) mirrored the bottom δ 15 NO 3 − signatures that regenerated in autumn and winter, taking into consideration the shift due to uptake fractionation (e = 4.92‰). This relationship supports the hypothesis that several key nitrogen processes in the YSCWM have important ecological effects. Specifically, as the YSCWM became weak in winter and the vertical mixture takes place thoroughly, nitrate generated from nitrification in the interior of the YSCWM is mixed upwards to the surface and becomes an important source, providing nutrients for primary production in the upper layers, especially in the following spring. This research provided a new perspective for primary production and its nutrient sources, especially in a coastal area where a typical water mass was located.