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

Abstract

Abstract Nitrate (NO 3 − ) concentrations and the isotopic composition (δ 15 N and δ 18 O) 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/[NO 3 − ], both of which suggest mixing of eutrophic freshwater and oligotrophic seawater. However, δ 15 N and δ 18 O values did not closely follow the expected mixing lines. At low salinities (0–3.0), some samples exhibited high δ 15 N and low δ 18 O values compared with those of the riverine end‐member, indicating that sewage is a significant source of nitrate (up to 19.0% calculated from δ 15 N). At salinities of >3.0, Δδ 15 N and Δδ 18 O values are linearly correlated with lnƒ (the fraction of NO 3 − remaining in the system), which is attributable to isotopic Rayleigh fractionation during phytoplankton uptake of nitrate. However, the linear relationship between Δδ 15 N and Δδ 18 O 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.