Fate and effects of triallate in a prairie wetland
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Extensive use of pesticides in the Canadian prairies, where cropland is interspersed with wetlands, increases the potential for contamination of aquatic ecosystems. Triallate is a widely used herbicide for the control of wild oats and it is frequently detected in aquatic systems. Because triallate is toxic to aquatic organisms and has the potential to persist in sediment, its impact on natural systems needs to be evaluated. The purpose of this research was to determine the fate of triallate in wetlands and its effect on selected aquatic organisms representing different trophic levels. Four littoral enclosures were built in a prairie pond; each was divided in half creating paired treatment and control cells. Triallate was applied at a concentration of 250 µg/L, representing a worst-case contamination by runoff. Time-specific concentrations of triallate in water, sediment and biota were measured. Triallate disappeared quickly from the water and partitioned to the sediment. Volatilization and uptake by organisms may have contributed to decreasing water concentrations. Levels in water decreased sharply in the first 3 days following application coupled with rising sediment concentrations. Levels in amphipods and aquatic plants also increased. Effects of triallate were evaluated for phytoplankton, periphyton, zooplankton (Daphnia pulex) and amphipods (Hyalella azteca). Biomass of phytoplankton and periphyton was not affected by triallate. Daphnia pulex and Hyalella azteca suffered reduced body size and reproduction. However, some of the results were inconsistent among enclosures. Microcosm and laboratory experiments were performed to complement the field study and demonstrated the toxicity of triallate to phytoplankton and D. pulex. Sublethal effects of triallate were manifested by reduced body size and reproduction in D. pulex exposed to 125 µg/L of triallate. These findings agree with results obtained in the field study. The present interim guideline (0.24 µg/L) for the protection of aquatic life appears to be adequate. In natural systems, triallate binds to dissolved and particulate organic matter and partitions quickly to sediment, reducing the amount present in solution and therefore the exposure to aquatic organisms. However, triallate persists in sediment for a longer time period and the toxicity of triallate associated with sediment has not been evaluated.