Isotopic evidence for the turnover of biological reactive nitrogen in the Pearl River Estuary, south China

Abstract

Nitrate (NO3-) concentrations and the isotopic composition (δ15N and δ18O) of water samples from the Pearl River Estuary (PRE), south China, were measured to constrain N sources and processing in springtime. Nitrate concentrations decreased at higher salinities, and dual isotopic values were correlated with 1/[NO3-], both of which suggest mixing of eutrophic freshwater and oligotrophic seawater. However, δ15N and δ18O values did not closely follow the expected mixing lines. At low salinities (0-3.0), some samples exhibited high δ15N and low δ18O values compared with those of the riverine end-member, indicating that sewage is a significant source of nitrate (up to 19.0% calculated from δ15N). At salinities of >3.0, Δδ15N and Δδ18O values are linearly correlated with ln (the fraction of NO3- remaining in the system), which is attributable to isotopic Rayleigh fractionation during phytoplankton uptake of nitrate. However, the linear relationship between Δδ15N and Δδ18O is different between the west and east PRE, with a slope of 1.18 in the west and 1.59 in the east. This difference most likely resulted from varying degrees of nitrification due to different water velocities and residence times in the two areas. Our data therefore indicate that dual nitrate isotopic signatures are a valuable way to constrain the sources and behavior of nitrate in river-dominated estuaries.