Offshore detachment of the Changjiang River plume and its ecological impacts in summer

Abstract

The Changjiang Diluted Water (CDW) significantly influences the chemical, biological, and sedimentary processes in the Yellow and East China Seas. Based on in situ observations during the summers of 2006 and 2008 and associated satellite-derived data, the offshore detachment of the CDW plume and its mechanisms are investigated, and the related ecological impacts associated with the detached CDW are examined. We show that the detached low-salinity water from the CDW plume can partially reach the seabed, with its volume gradually diminishing from the surface to the seabed, and with a horizontal distribution that initially shifts eastward and then southward. The double-upwelling system, combined with the prevailing southerly wind and the anticyclonic eddy off the Changjiang Estuary, favors the detachment of the CDW plume. In particular, the anticyclonic eddy provides a habitat or venue for the formation and maintenance of the detached low-salinity water, and is responsible for the local presence of low-salinity water along the seabed. Data analysis indicates that this detachment can induce phytoplankton blooms and that enhanced chlorophyll a (Chl-a) contents were significantly associated with moderate nutrient concentrations and good light conditions in the offshore low-salinity water. This study also demonstrates that the variability in the vertical distribution of Chl-a off the Changjiang Estuary is related to the offshore detached CDW, and that the local deepening of the DCM (depth of Chl-a maximum) and the peak primary production occur within the offshore CDW. More importantly, we find that high Chl-a concentrations in the bottom water can be induced by the anticyclonic-eddy-featuring offshore CDW. Our results may facilitate a better understanding on the role of the detached CDW in local marine biogeochemical processes.