Tidally oriented vertical migration and position maintenance of zooplankton in a temperate estuary

Abstract

In many estuaries, maxima in turbidity and abundance of several common species of zooplankton occur in the low salinity zone (LSZ) in the range of 0.5–6 practical salinity units (psu). Analysis of zooplankton abundance from monitoring in 1972–1987 revealed that historical maxima in abundance of the copepod Eurytemora affinis and the mysid Neomysis mercedis , and in turbidity as determined from Secchi disk data, were close to the estimated position of 2 psu bottom salinity. The copepod Sinocalanus doerrii had a maximum slightly landward of that of E. affinis . After 1987 these maxima decreased and shifted to a lower salinity, presumably because of the effects of grazing by the introduced clam Potamocorbula amurensis . At the same time, the copepod P seudodiaptomus forbesi , the mysid Acanthomysis sp., and amphipods became abundant with peaks at salinity around 0.2–0.5 psu.