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 (δ15NO3), 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 δ15NO3 and supported by the mixing behavior of the nitrate and δ15NO3. From February to November, the mean nitrate content in the deep waters gradually increased (from ∼8 to ∼15 μmol l−1), whereas the mean δ15NO3 decreased (from ∼2‰ to ∼−2‰). Particularly in November, most of the δ15NO3 values in the deep waters were negative (∼−5‰ −0‰, which is much lower than the ∼3‰ riverine input from δ15NO3), 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 ∼2 to ∼20 μmol l−1) in the deep waters during November when the δ15NO3 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.