Carbon and nitrogen isotopic composition of particulate organic matter in the Pearl River Estuary and the adjacent shelf

Abstract

The isotopic composition (δ13C and δ15N), organic carbon and nitrogen contents and C/N ratios of suspended particulate organic matter (POM) collected from the Pearl River Estuary and the adjacent shelf during spring and summer of 2017 were determined to understand the main controlling factors of spatiotemporal distribution and to elucidate the sources of particulate organic matter. Our study revealed considerable spatial variability of these parameters. In summer, δ13C in the Pearl River Estuary generally increased seawards, reflecting the increasing contribution of marine organic matter. Domestic sewage and industrial wastewater were responsible for relatively high δ15N in the Pearl River Estuary. In spring, δ13C on the adjacent shelf generally decreased from land to sea, with elevated δ13C occurred at the nearshore stations, suggesting that the eutrophic level, phytoplankton biomass and microbial activity played an important role in determining the sources of POM. Compared with δ13C, δ15N values displayed a complicated spatial distribution. Especially in the bottom water, enriched δ15N would be influenced by the degradation and mineralization of suspended matter. Given that isotopic values in summer can be applied in source apportionment, a Bayesian isotope mixing model was used to estimate that in summer the marine and terrestrial organic matter contributed 47% and 38% in the Pearl River Estuary, respectively. The greatest contribution from marine-derived POM (73%) was observed on the adjacent shelf.