IMPACTS OF A HERBICIDE AND FERTILIZERS ON THE MICROBIAL COMMUNITY OF A SALINE PRAIRIE LAKE

Prairie wetlands are scattered among cultivated land where herbicides and fertilizers are regularly used. Avadex-BW(C) (active ingredient tr iallate) is one of the most popular pre-emergent herbicides used on th e Canadian prairies to control wild oats and other broadleaf weeds. Tr iallate enters aquatic systems through processes of runoff and direct overspray, while fertilizers (nitrogen and phosphorus) enter through r unoff and aeolian deposition. Microcosm studies on Redberry Lake, an o ligotrophic prairie lake, indicated that additions of triallate stimul ated bacterial production ([H-3]thymidine incorporation), metabolism ( [C-14]glucose uptake), and numbers, but only if nitrogen and phosphoru s were added. Because the microbial community is nutrient limited, the re is the possibility that triallate could bioaccumulate in this syste m. Addition of nitrogen and phosphorus (N + P) stimulated measures of phytoplankton biomass (Chl a), bacterial metabolism, numbers, and prod uction. Although triallate had little effect on bacteria, at any conce ntration tested, substantial declines in phytoplankton biomass occurre d at triallate concentrations greater than or equal to 1000 mu g.L-1. Greatest impacts of herbicides and nutrients, however, are expected in spring as a result of runoff events, aeolian deposition, and direct o verspray.