Nitrogen sources and cycling revealed by dual isotopes of nitrate in a complex urbanized environment

Elevated nutrient inputs have led to increased eutrophication in coastal marine ecosystems worldwide. An understanding of the relative contribution of different nutrient sources is imperative for effective water quality management. Stable isotope values of nitrate (delta N-15(NO3-), delta O-18(NO3-)) can complement conventional water quality monitoring programs to help differentiate natural sources of NOT from anthropogenic inputs and estimate the processes involved in N cycling within an ecosystem. We measured nutrient concentrations, delta N-15(NO3-), and delta O-18(NO3-) in 76 locations along a salinity gradient from the lower end of the Pearl River Estuary, one of China's largest rivers discharging into the South China Sea, towards the open ocean. NO3 concentrations decreased with increasing salinity, indicative of conservative mixing of eutrophic freshwater and oligotrophic seawater. However, our data did not follow conservative mixing patterns. At salinities <20 psu, samples exhibited decreasing NO3 concentrations with almost unchanged NOT isotope values, indicating simple dilution. At salinities >20 psu, NO3 concentrations decreased, while dual NO3 isotopes increased, suggesting mixing and/or other transformation processes. Our analysis yielded mean estimates for isotope enrichment factors (15s = 2.02%o and (18 epsilon) = 337%0), Delta(15,18)= -5.5%o and delta N-15(NO3-) -delta O-18(NO2-)_= 12.3 parts per thousand. After consideration of potential alternative sources (sewage, atmospheric deposition and groundwater) we concluded that there are three plausible interpretations for deviations from conservative mixing behaviour (1) NO3 uptake by assimilation (2) in situ NOT production (from fixation -derived nitrogen and nitrification of sewage derived effluents) and (3) input of groundwater nitrate carrying a denitrification signal. Through this study, we propose a simple workflow that incorporates a synthesis of numerous isotope -based studies to constrain sources and behaviour of NO3 in urbanized marine environments. (C) 2018 Elsevier Ltd. All rights reserved.