IMPACT OF GLUFOSINATE-AMMONIUM AND BIALAPHOS ON THE ZOOPLANKTON COMMUNITY OF A SMALL EUTROPHIC NORTHERN LAKE

The impact of glufosinate-ammonium and bialaphos on the zooplankton co mmunity in a small eutrophic lake was investigated using in situ enclo sures. Concentrations inducing a 20% reduction in abundance (EC20), as interpolated from best-fit, nonlinear regression analyses were simila r for both herbicides and ranged from 0.03 to 0.16 mg/L for various zo oplankton taxa. Similarly, median effective concentrations (EC50) esti mates ranged from 0.12 to 0.50 mg/L. Thus, toxicity endpoints overlapp ed and in some cases were well below expected environmental concentrat ions calculated for accidental direct overspray (1 mg/L) or drift even ts (0.25 mg/L). Significant concentration-dependent reductions were ob served within the first 2 weeks following application and for a number of taxa persisted throughout the observation period (63 d posttreatme nt). At the highest treatment level, (10 mg/L), negative impacts were still apparent in the year following treatment. The results of this fi eld study, which demonstrate significant negative effects on a variety of zooplankton taxa at environmentally relevant concentrations and re latively slow recovery therefrom, suggest a substantial risk of sustai ned adverse impacts on the zooplankton communities should these herbic ides contaminate shallow lentic ecosystems. Extensive mitigative measu res are required to protect such water bodies from potential impacts o f phosphinothricin-based herbicides.