Herbicide effects of metazachlor on duckweed (Lemna minor and Spirodela polyrhiza) in test systems with different trophic status and complexity.

Growth of common duckweed Lemna minor under optimal standard test conditions was compared to growth of L. minor exposed to nutrient-poor water in both a modified standardised test and in oligo- to mesotrophic indoor pond mesocosms in order to test the impact of trophic conditions and test system complexity on the effect of the herbicide metazachlor (2-chloro-N-(pyrazol-1-ylmethyl)acet-2',6'-xylidide) on aquatic macrophytes. In the mesocosms L. minor was replaced by greater duckweed Spirodela polyrhiza after 3 weeks due to high mortality even in the controls. The pond systems contained other macrophytes and biota as well as sediment and were thus more complex than standard test systems. For L. minor front area, the ErC(50) (50% effective concentration related to growth rate) was 2.8 microg L(-1) metazachlor in the standardised and 4.7 microg L(-1) in the modified laboratory test after 7 days (4.9 microg L(-1) and 52.9 microg L(-1) metazachlor when using front number). In the oligo- to mesotrophic pond mesocosms, similar sensitivities to metazachlor (ErC(50) 4.5-6.4 microg L(-1)) were noted for S. polyrhiza after 21 and 28 days of exposure. In comparison with dicotyledonous macrophytes, duckweed species are more sensitive for irreversible enzyme inhibitors of growth such as metazachlor independent of trophic status and complexity of the test system.