Dynamics of dissolved and particulate organic matter in the Changjiang (Yangtze River) Estuary and the adjacent East China Sea shelf

Two cruises that passed over Changjiang freshwater, through the Changjiang Estuary, and finally to the adjacent East China Sea (ECS) shelf were carried out in July 2015 and March 2016. The concentrations of dissolved organic carbon (DOC) and particulate organic carbon (POC) were measured at the three layers (surface, middle, and bottom) of 99 and 89 stations during the cruises in July 2015 and in March 2016, respectively. The concentrations of DOC and POC showed overall decreasing trends from land to sea; however, their highest concentrations always occurred around the mouth of the Changjiang River, where estuarine salinity fronts and turbidity maxima appeared simultaneously. The POC/DOC ratios showed values < 1 in most samples during the two cruises, suggesting that DOC contributed to a larger fraction than POC to the total organic matter inventories. An exception occurred in the inner stations during March 2016. Variations of DOC and POC were mainly controlled by salinity and by the dynamics of suspended particulate matter concentrations, respectively, suggesting that these two organic matter pools were largely decoupled in most cases during the two cruises. One exception was found in the surface water in July 2015, when the overlying Changjiang Diluted Water (regarded as seawaters with a salinity < 31) generally supported a release of DOC with ongoing seaward transport. The high percentage values of POC (%) in suspended particulate matter, indicating the higher Chlorophyll-a abundances, were always the main reason supporting the release of DOC. And the above results provided an example of the coupling between DOC and POC that occurred on the surface in the summer. The findings from this study provide insight into the coupling and decoupling between DOC and POC in the mixing zone of a large estuary, which may largely modify the final fluxes and properties of the terrestrial organic carbon from the land to the ocean.