Plankton development in Lake Jonsvatn, Norway, after introduction of Mysis relicta: a long-term study

Quantitative sampling of phyto- and zooplankton has been carried out for 25 yr in Lake Jonsvatn, Norway, in order to assess changes after the introduction of Mysis relicta. In Lille Jonsvatn, the outlet basin, the cladoceran populations collapsed 7 yr after the mysid introduction. The mean cladoceran biomass was reduced by 95 to 99 % for a decade before Bosmina longispina, Holopedium gibberum and Daphnia galeata developed new populations. B, longispina and H. gibberum almost disappeared again after a few years, while D. galeata was replaced by D. longispina, which unexpectedly maintained a relatively dense population in the presence of high mysid abundance. Rotifers declined significantly in Lille Jonsvatn after the mysid establishment, whereas copepod biomass did not change. Phytoplankton biomass increased during the first years after the cladoceran collapse, but declined later to levels lower than in pre-mysid years. The changes were probably a combined effect of reduced nutrient loads and grazing pressure. In the main basin, Store Jonsvatn, there were no detectable effects on the zooplankton community during the first 10 to 15 yr after the introduction, in spite of development to common densities of mysids within 8 yr. Over time, cladocerans decreased significantly, however, and mean biomass for the last 5 yr showed a 60 % reduction compared with the first 10 yr. No significant long-term changes were detected in copepods, rotifers or phytoplankton biomass in Store Jonsvatn. Differences in temperature, stratification, light transmission and depth may partly explain the different plankton development in the 2 basins.