Effects of the herbicide paraquat on the ecology of a reservoir

Summary Toxicity experiments indicated that changes in the fauna following the use of paraquat for weed control were likely to be indirect effects caused by the death and destruction of angiosperms. Generally, invertebrates living amongst the emergent vegetation in the margins of the reservoir and on the sediments were unaffected though the density of the benthic ostracod Cypridopsis vidua increased rapidly after the death of angiosperms. Of the planktonic invertebrates, those which were generally closely associated with the macrophytes were eliminated or survived at lower densities. Many of the invertebrates intimately associated with the angiosperms (e.g. Lepidoptera, Tricho‐ptera. Gastropoda) were lost completely or colonized the replacement growth of Chara globularis at reduced densities: these effects were extended to the year after treatment. A few species colonizing Chara were able to establish densities comparable with those found on the angiosperms in the pre‐herbicide period but in some cases the time of peak density was delayed. Data from fish gut analyses indicated that there was a change in the diet of the eel following the death of the angiosperms and that this was largely a reflection of the loss, or reduction in density, of many invertebrates associated with the angiosperms.