Data from the International Biological Programme (IBP) and subsequent studies have been re-analysed to test the two hypotheses which previously have been suggested concerning the zooplankton in the mountain lake, Øvre Heimdalsvatn: (1) the average temperature in June, more than other summer months, is affecting the growth rate and population densities of zooplankton in the lake, (2) the invasion of the European minnow (Phoxinus phoxinus) has caused changes in the zooplankton community. The analyses have demonstrated that the June temperature strongly affects the growth rate of all the zooplankton species, but that there is no relationship with the population maxima. The species composition in the crustacean zooplankton has not changed between 1969 and 1999, and any direct impact of the minnows on the zooplankton community could not be detected. Indirectly, the minnows may have reduced the density of invertebrate predators, and thus caused an increase in juvenile survival and increased summer maximum density of Bosmina longispina. The variation in density of the copepod, Cyclops scutifer, was correlated with the density of Heterocope saliens, most likely the result of predator-prey interactions.

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