Non-conservative behavior of dissolved organic carbon in the Changjiang (Yangtze River) Estuary and the adjacent East China Sea

Six cruises were conducted in the Changjiang (Yangtze River) Estuary and the adjacent East China Sea (ECS) in February, March, and July in the years 2013–2016. Bulk DOC (dissolved organic carbon) concentrations were measured at the surface, middle, and bottom aquatic layers. Correlations between salinity and DOC concentrations suggested that DOC had greater non-conservative distribution patterns in July than in February or March. In July, measurements of surface waters generally showed higher DOC concentrations than those in lower water levels having similar salinity values. Comparisons of DOC concentrations between same-salinity patches and along salinity contours were conducted at the surface of water transported from the Changjiang Estuary seaward. These comparisons showed that surface seawater was rich in DOC in July at the northeastern transport direction of the study area. The non-conservative behaviors of bulk DOC in the study area were attributed to variations of carbohydrate compounds and Chromophoric Dissolved Organic Matter (CDOM). The exception occurred in summer when processes in seawater mostly led to a sink for CDOM but a source for bulk DOC, which may be ascribed to the incomplete degradation of CDOM. This study may help improve our understanding of non-conservative DOC dynamics in the Changjiang Estuary and the adjacent ECS shelf.