The Rhine river and many other river systems are easily contaminated by organophosphorous insecticides. Whereas overall concentrations of organophosphorous compounds, their dispersal, photolysis, and degradation in rivers have been modelled various times, only few studies have been done on the complex effects on community structures in rivers so far. Using acetylcholinesterase inhibtion as a parameter of sublethal toxicity, we found that organophosphorous insecticide tolerance in various Gammarus species differs widely and may help to explain recent changes in species composition, especially of the introduced G. tigrinus vs. the autochthonous species G. pulex and G. fossarum. The higher tolerance of G. tigrinus could be one of several factors explaining the success of this species in the Rhine river invertebrate fauna.

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