Ecological effects of pesticides in freshwater model ecosystems

In this thesis I have investigated the effects of pesticide exposure on the ecosystem level using various types of experimental ecosystems, i.e. microcosms. The direct effect of exposure to cyperemthrin, a pyretroid insecticide, was a rapid decrease of crustancean zooplankton in enclosures in a lake. In the exposed enclosures the biomass of algae, heterotrophic nanoflagellates, ciliates and bacteria increased, likely due to indirect effects of the exposure to cypermethrin. The effects of a sulfonylurea herbicide, metsulfuron-methyl, were investigated in two studies. The experiments showed that the macrophytes were negatively affected following exposure to concentrations of metsulfuron-methyl at which no negative effects were observed on the algal communities. Instead the algae proliferated in the exposed microcosms. The alterations of the primary producing community propagated to the zooplankton, and the species composition of the zooplankton community was altered, which thus was an indirect effect of exposure to metsulfuron-methyl. Hence, these studies show that both insecticides (i.e. cypermethrin) and herbicides (i.e. metsulfuron-methyl), through different mechanisms, can induce a competitive advantage for the algal communities in relation to the macrophytes. This suggests that exposure to pesticides may shift an aquatic ecosystem over to an algal dominated turbid state, i.e. cause alterations similar to changes induced by eutrophication. Furthermore, a study of the effects of a pesticide mixture on aquatic model ecosystems of different trophic status, indicated that the structure and trophic status of aquatic ecosystems is important for the final outcome of pesticide exposure. The effects of the pesticide treatment on the macrophytes and their associated algal community was different in the mesotrophic compared to the eutrophic model ecosystems and the effects were observed at lower concentrations in the mesothrophic microcosms.