EFFECTS OF ALACHLOR ON AN ALGAL COMMUNITY FROM A MIDWESTERN AGRICULTURAL STREAM

The effects of a single pulse, 21-d exposure of the commonly used herb icide alachlor (2-chloro-2', 6'-diethyl-N-methoxymethyl acetanilide) o n an algal community from a typical agricultural stream in Nebraska we re studied using 18 stream microcosms located in a greenhouse, at six alachlor concentrations (0, 1, 10, 30, 100, and 1,000 mu g/L). Effects of alachlor exposure at 1.0 mu g/L were not significant (p < 0.05); h owever, at all other concentrations, alachlor had a significant negati ve effect on algal biomass. Differential taxonomic responses were obse rved, with approximately half the dominant algal taxa affected at leve ls >10 mu g/L. Some taxa recovered from exposure by day 7, while other s took longer or did not recover. A shift in the dominant algae was ob served at higher concentrations (30, 100, 1,000 mu g/L), and after 21 d these streams exhibited total algal cell densities significantly low er than the control and 1.0-mu g/L-level streams. Results of this stud y suggest that alachlor inputs can alter both algal community composit ion and biomass in agricultural streams. Consequently, potential indir ect impacts of alachlor on higher trophic levels warrant further inves tigation.