Effects of water of reduced salinity on the vertical migration of zooplankton

Abstract

SUMMARYEffects of water of reduced salinity on the vertical migration of the zooplankton of Southampton Water were investigated in the laboratory using the adults of six copepod species (Acartia tonsa, A. bifilosa, A. discaudata, A. clausi, Centropages hamatus, Temora longicornis) and the larvae of one decapod species (Porcellana longicornis). Additional observations were made on Centropages typicus adults taken off Plymouth.Salinity discontinuity layers had a marked effect on the vertical migration of zooplankton. A single discontinuity layer formed by placing less dense diluted sea water over full strength sea water acted as a barrier to animals attempting to swim toward the surface of an experimental water column and no individuals were able to enter the zone of reduced salinity when an extreme dilution was used. Comparison of the behaviour of various species and of different sexes or developmental stages of a particular species suggests that vertical distribution was partly dependent on the salinity tolerances of individuals. Upward migration was also restricted by a vertical series of discontinuity layers.The swimming activity of copepods was depressed in homogeneous water columns of reduced salinity. Activity depended both on the degree of dilution and on the period of immersion. There were indications that behaviour varied according to the salinity tolerances of the different species.