Source, composition, and reactivity of particulate organic matter along the Changjiang Estuary salinity gradient and adjacent sea

The sources, distribution, and fate of particulate organic matter (POM) in estuaries are dynamic and complex, influenced by highly intensive human activities and high productivity. In this study, water samples were collected along the Changjiang Estuary salinity gradient and adjacent sea (CEAS) in February and May 2017. Particulate organic carbon (POC), particulate nitrogen (PN), the δ13C isotope values and major biochemical constituent (total particulate amino acids, TPAA) were measured. The concentrations of POC, PN, and TPAA showed an overall decreasing trend from the river end-member to the open sea; however, their maximum always occurred around the turbidity maximum zone (TMZ). Concentrations of POC and TPAA showed a negative correlation with salinity and a positive correlation with chlorophyll a, indicating that the variation in POM concentrations and composition was mainly controlled by both terrigenous input and in situ phytoplankton production. The δ13C values gradually increased from the river mouth to the open sea in both winter and spring, in contrast to the molar C/N, reflecting the transition from terrestrial POC to phytoplankton-derived fresh POC with increasing salinity. Major biochemical indicators of TPAA/POC (%) and the degradation index (DI), showed a gradual shift towards more bioactive POM with increasing salinity in spring, although low TPAA/POC (%) values appeared within the TMZ. In spring, POC reactivity was higher than in winter. The proportions of glycine (Gly) and serine (Ser) were higher in winter, indicating that POM had suffered extensive degradation. Based on a two end-member mixing model, the contribution of marine POC in spring (53 ± 14%) was significantly greater than in winter (39 ± 19%), indicating that phytoplankton-derived POM was dominant in spring, associated with the increase in phytoplankton biomass from winter to spring. Based on mass balance, a box model showed evidence of a net POC sink over the Changjiang estuary and its adjacent East China Sea shelf in both winter and spring, with a net POC budget of 20.49 ± 7.01 and 15.87 ± 6.57 kmol s−1, respectively. Results illustrate that the spatio-temporal distribution of POM varies distinctively and will further affect the variability in its composition and reactivity in the CEAS.