Seasonal variations in the nitrogen isotopic composition of dissolved nitrate in the Changjiang River estuary, China

The Changjiang River estuary is the most eutrophic estuary in China. Although anthropogenic input from the Changjiang River (the third-largest river in the world) is considered the main source of nitrogen, the contributions of newly regenerated nitrogen from deep waters may be underestimated, in part due to the lack of information on the nitrogen biogeochemical processes that are occurring. The nitrogen stable isotope ratio is widely used as an indicator of the source of nitrogen and nitrogen transformation processes including assimilation, nitrification, nitrogen fixation and mineralization. To study the biogeochemical processes in the Changjiang River estuary, seasonal variations in the nitrogen stable isotope ratio of nitrate (delta(NO3)-N-15), salinity, temperature, dissolved oxygen concentration, chlorophyll a (Chl a) concentration, and the composition of dissolved inorganic nitrogen were investigated along two transects in the estuary in 2010. In the surface waters, assimilation between May and November was suggested by high Chl a and delta(NO3)-N-15 and supported by the mixing behavior of the nitrate and delta(NO3)-N-15. From February to November, the mean nitrate content in the deep waters gradually increased (from similar to 8 to similar to 15 mu mol l(-1)), whereas the mean delta(NO3)-N-15 decreased (from similar to 2 parts per thousand to similar to-2 parts per thousand). Particularly in November, most of the delta(NO3)-N-15 values in the deep waters were negative (similar to-5 parts per thousand -0 parts per thousand, which is much lower than the similar to 3 parts per thousand riverine input from delta(NO3)-N-15), indicating that most of the nitrates in the deep water were newly regenerated from nitrification rather than originating from riverine input Additionally, the mean ammonium abruptly increased (from similar to 2 to similar to 20 mu mol l(-1)) in the deep waters during November when the delta(NO3)-N-15 values were negative, whereas in August, high Chl a values were observed in the deep waters. These results indicate that remineralization and nitrification occurred in the deep waters of the Changjiang River estuary, implying that regenerated nitrogen may also be an important nitrogen source for uptake by phytoplankton. (C) 2015 Elsevier Ltd. All rights reserved.