Toxicity tests for assessment of pesticide effects on aquatic plants

In response to concerns that current risk assessment procedures may not ade quately reflect the long-term risk of pesticides to non-target aquatic plan ts, the results of standard toxicity tests, based on Lemna minor, have been compared with responses seen in alternative tests, based on Myriophyllum s picatum, a submerged dicotyledon, Lagarosiphon major, a submerged monocotyl edon, and Glyceria maxima, an emergent monocotyledon. For this purpose, sho rt-term, glasshouse-based studies were conducted to determine the relative sensitivity of each species to selected herbicides, while a long-term, outd oor, mesocosm study is in progress to evaluate the ecological relevance of glasshouse-generated results. Large differences were observed between the s ensitivities of these species which indicated that Lemna is not the most se nsitive test species for all herbicides. Furthermore, results from the meso cosm experiment revealed long-term effects that cannot be predicted from sh ort-term tests and may have implications for species viability in subsequen t seasons. The implications for the selection of appropriate species for us e in toxicity tests will be discussed.