Can time-weighted average concentrations be used to assess the risks of metsulfuron-methyl to Myriophyllum spicatum under different time-variable exposure regimes?

We tested the effects of the herbicide metsulfuron-methyl on growth of the submerged macrophyte Myriophyllum spicatum under laboratory conditions using different exposure scenarios. The exposures of each scenario were comparable in the concentration × time factor, viz., the same 21-d time-weighted average (TWA) concentrations but variable in peak exposure concentrations (ranging from 0.1 to 21000 ng ai L⁻¹) and exposure periods (1, 3, 7, 14 or 21 d). To study recovery potential of the exposed M. spicatum plants we continued the observation on shoot and root growth for another 21 d in herbicide-free medium so that the total observation period was 42 d. Non-destructive endpoints, length and number of new shoots and roots, were determined weekly from day 14 onwards. Destructive endpoints, dry-weight (DW) of main shoots, new shoots and new roots, were measured at the end of the experiment (t=42 d). Metsulfuron-methyl exposure in particular inhibited new tissue formation but was not lethal to main shoots. On days 21 and 42 after start exposure, EC10/EC50 values for new tissues expressed in terms of peak concentration (=measured concentration during exposure periods of different length) showed large differences between exposure scenarios in contrast to EC10/EC50 values for days 21 and 42 expressed in terms of 21-d and 42-d TWA concentrations, respectively. At the end of the experiment (day 42), 42-d TWA EC(x) values were remarkably similar between exposure scenarios, while a similar trend could already be observed on day 21 for 21-d TWA EC(x) values. For the macrophyte M. spicatum and exposure to the herbicide metsulfuron-methyl the TWA approach seems to be appropriate to use in the risk assessment. However, the data from the toxicity experiment suggest that on day 21 also the absolute height of the pulse exposure played a (minor) role in the exposure - response relationships observed.

Effects of four fungicides on nine non-target submersed macrophytes.

We tested the sensitivity of nine submersed macrophyte species to the fungicides chlorothalonil, pentachlorophenol, fluazinam, and carbendazim. Endpoints determined 3 weeks after the start of the treatment were based on shoot and root growth in water. Carbendazim proved not or only moderately toxic to these macrophytes. Pentachlorophenol and chlorothalonil were more toxic than fluazinam. Taking all endpoints into consideration, toxicity levels differed very substantially. Although root endpoints reflecting root growth were in some cases more sensitive than shoot endpoints, shoot growth endpoints like relative growth turned out to be more reliable than the root growth endpoints. Due to the large differences in the type of mode of action between fungicides, it is very difficult to predict their potential effect in the environment or even to predict whether non-target organisms like macrophytes are likely to be sensitive. Ideally, therefore, the registration of fungicides requires an extensive risk-assessment procedure, which also covers non-target groups like macrophytes.

Phosphorus leaching from cow manure patches on soil columns.

The loss of P in overland flow or leachate from manure patches can impair surface water quality. We studied leaching of P from 10-cm-high lysimeters filled with intact grassland soil or with acid-washed sand. A manure patch was created on two grassland and two sand-filled lysimeters, and an additional two grass lysimeters served as blanks. Lysimeters were leached in the laboratory during 234 d with a diluted salt solution, and column effluent was passed through a 0.45-microm filter, analyzed for pH, dissolved reactive P (DRP), and total dissolved P (TDP). At the end of the experiment lysimeter soil was sampled and analyzed for pH, available P, and oxalate-extractable P, Fe, and Al. The concentration of TDP in the effluent from the sand column increased to 25 mg L-1 during the first weeks and remained above 10 mg L-1 during the rest of the percolation. In effluent from grass + patch lysimeters TDP gradually increased to 4 mg L-1. Both in the manure and in the effluent of the sand lysimeter P was found mainly in the form of DRP, but in the effluent from the grass lysimeters was found mainly as dissolved unreactive P (DUP=TDP-DRP). Earthworm activity was responsible for decomposition of the manure patch on the grass lysimeters. Manure patches and their remains were found to be a long-term source of high concentrations of P in leachates. Spreading of patches after a grazing period could reduce their possible negative impacts on the environment.