Recent collapse of the copepods in the northern East China Sea: Effects of Three Gorges Dam?

Abstract

We investigated long-term trends in zooplankton in a transect off the south of Jeju Island in the northern East China Sea over the past 30 years. Rising seawater temperature was evident with the 18oC isopleth steadily moving northward. Total zooplankton wet weight increased along with the abundances of copepod, chaetognaths, amphipods, and euphausiids. Since early 2006, however, the trends have reversed that all of the zooplankton groups sharply declined in abundance despite a faster increase in total wet weight. Copepods were affected the most with decline in abundance over an order of magnitude, which apparently led to a sharp decrease in chaetognath abundance, a major predator of copepods. Concurrent declines in the abundance of amphipods and euphausids suggested that these systemic effects were driven by bottom-up processes. Chla concentration estimated from satellite image analysis showed an increasing trend since 2003, with a faster rise in spring concentration than in other seasons. Coincidentally, the Changjiang River discharge has decreased since 2003 immediately as impoundment in the Three Gorges Dam at the upper Changjiang River has started. Reduced river flow together with increasing SST could have contributed to increased phytoplankton biomass and proliferation in gelatinous zooplankton, the latter of which may have had over all adverse effects on all other zooplankton gro