Some arguments in favor of a Myriophyllum aquaticum growth inhibition test in a water-sediment system as an additional test in risk assessment of herbicides.
At the international workshop Aquatic Macrophyte Risk Assessment for Pesticides (AMRAP), it was noted that the EU risk assessment under the directive 91/414/EEC for herbicides, based only on algae and the monocotyledonous duckweed species Lemna sp., offers no certain protection against some growth regulating auxins. Therefore, AMRAP members proposed the introduction of the dicotyledonous water milfoil Myriophyllum as additional test species. This study was aimed to compare toxicity results from three test systems (TS) with varying complexity, namely Water TS, Sediment TS and Microcosm TS using Myriophyllum spicatum as test organism. As test substances, the photosynthesis inhibiting herbicide isoproturon, the growth regulating auxins fluroxypyr and 2,4-dichlorophenoxyacetic acid (2,4-D), and the non-specific acting toxicant 3,5-dichlorophenol (3,5-DCP) were chosen. It was assessed if and why the sensitivity of M. spicatum towards the four toxicants varied in the different test systems and if the addition of sucrose to the medium used in the Water TS had an effect on the sensitivity of Myriophyllum. All TS were suitable for detecting negative effects of toxicants with different modes of action on M. spicatum. The lowest variability of endpoints was found in the Water TS with lowest experimental complexity. For auxins, the endpoint weight did not result in robust EC50 values in all TS, whereas root related endpoints, which are also ecologically relevant, turned out to be very sensitive with low variance. Sucrose in the medium of the Water TS did not seem to influence the sensitivity of M. spicatum towards isoproturon and 3,5-DCP but may have increased the sensitivity of M. spicatum roots when exposed to 2,4-D. However, the findings of all TS resulted in similar risk estimations if root endpoints were not considered.