Decreased zooplankton density in a mountain lake resulting from predation by recently introduced Mysis relicta

Summary

Zooplankton and Mysis relicta were investigated in a mountain lake, Lake Stugusjaen, between 1976 and 1985. M. relicta, from Lake Blasjön, Sweden, was introduced into the lake in 1973. The lake has a total area of 6.75 km², a maximum depth of about 57 m, and is situated 610 meters a.s.l. The results showed a great decline in cladoceran populations since 1978 e.g. Daphnia galeata, Holopedium gibberum and Bythotrephes longimanus. The copepods Heterocope appendiculata and Diaptomus laticeps also decreased considerably, while Cyclops scutifer exhibited the same population density. Analysis of Mysis stomach contents indicated a change from dominance by cladocerans in 1976, to rotifers in more recent years. The decline of the zooplankton populations coincided with an exponential increase in the Mysis population. Examination of relevant literature supports the hypothesis that M. relicta were largely responsible for the decimation of the zooplankton populations.