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 (δ¹⁵N_NO3-, δ¹⁸O_NO3-) can complement conventional water quality monitoring programs to help differentiate natural sources of NO₃^- from anthropogenic inputs and estimate the processes involved in N cycling within an ecosystem. We measured nutrient concentrations, δ¹⁵N_NO3-, and δ¹⁸O_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 NO₃^- 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 (¹⁵ε = -2.02‰ and ¹⁸ε = -3.37‰), Δ(15,18) = -5.5‰ and δ¹⁵N_NO3- - δ¹⁵N_NO2- = 12.3‰. 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 NO₃^- 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.