Toxicity tests for assessment of pesticide effects on aquatic plants

In response to concerns that current risk assessment procedures may not adequately reflect the long-term risk of pesticides to non-target aquatic plants, the results of standard toxicity tests, based on Lemna minor, have been compared with responses seen in alternative tests, based on Myriophyllum spicatum, a submerged dicotyledon, Lagarosiphon major, a submerged monocotyledon, and Glyceria maxima, an emergent monocotyledon. For this purpose, short-term, glasshouse-based studies were conducted to determine the relative sensitivity of each species to selected herbicides, while a long-term, outdoor, mesocosm study is in progress to evaluate the ecological relevance of glasshouse-generated results. Large differences were observed between the sensitivities of these species which indicated that Lemna is not the most sensitive test species for all herbicides. Furthermore, results from the mesocosm experiment revealed long-term effects that cannot be predicted from short-term tests and may have implications for species viability in subsequent seasons. The implications for the selection of appropriate species for use in toxicity tests will be discussed.