The long‐term freshening and nutrient increases in summer surface water in the northern East China Sea in relation to Changjiang discharge variation

Abstract

Using historical in situ data sets, we investigated long‐term variations in summer surface salinity and dissolved inorganic nitrogen concentration (DIN) in the northern East China Sea (ECS). From 1950 to 2002, there were interannual and decadal variations in sea surface salinity, as well as a long‐term decrease. Along with the Changjiang discharge, wind fields and surface circulations appeared to play roles in determining the interannual salinity variation. Although we attributed the long‐term decrease in surface salinity to a long‐term increase in the Changjiang discharge, the high salinity between 1978 and 1988 appeared to be associated with an increase in the Kuroshio Current transport. The increase in DIN from 1971 to 2001 appeared to be caused by a combination of changes in Changjiang discharge and an increase in nitrogen fertilizer use in China. The long‐term increase in DIN modified the nutrient composition, implying that biological production in the northern ECS changed from a DIN‐limiting to a phosphorous‐limiting. We should pay attention to changes in the northern ECS ecosystem in future decades because the volume of the Changjiang discharge, as well as its physical and chemical properties, may change dramatically because of the continuous anthropogenic perturbations and climate change.