Effects of the herbicide metazachlor on macrophytes and ecosystem function in freshwater pond and stream mesocosms

The chloroacetamide metazachlor is a commonly used pre-emergent herbicide to inhibit growth of plants especially in rape culture. It occurs in surface and ground water due to spray-drift or run-off in concentrations up to 100 microgL(-1). Direct and indirect effects of metazachlor on aquatic macrophytes were investigated at oligo- to mesotrophic nutrient levels employing eight stream and eight pond indoor mesocosms. Five systems of each type were dosed once with 5, 20, 80, 200 and 500 microgL(-1) metazachlor and three ponds and three streams served as controls. Pronounced direct negative effects on macrophyte biomass of Potamogeton natans, Myriophyllum verticillatum and filamentous green algae as well as associated changes in water chemistry were detected in the course of the summer 2003 in both pond and stream mesocosms. Filamentous green algae dominated by Cladophora glomerata were the most sensitive organisms in both pond and stream systems with EC(50) ranging from 3 (streams) to 9 (ponds) microgL(-1) metazachlor. In the contaminated pond mesocosms with high toxicant concentrations (200 and 500 microgL(-1)), a species shift from filamentous green algae to the yellow-green alga Vaucheria spec. was detected. The herbicide effects for the different macrophyte species were partly masked by interspecific competition. No recovery of macrophytes was observed at the highest metazachlor concentrations in both pond and stream mesocosms until the end of the study after 140 and 170 days. Based on the lowest EC(50) value of 4 microgL(-1) for total macrophyte biomass, it is argued that single exposure of aquatic macrophytes to metazachlor to nominal concentrations >5 microgL(-1) is likely to have pronounced long-term effects on aquatic biota and ecosystem function.

Modification of the Zahn-Wellens test: determination of the biodegradability of poorly soluble, adsorbing and volatile substances by measurement of oxygen consumption and carbon dioxide production

Based on the Zahn-Wellens test (OECD 302 B, 1992; DIN EN ISO 9888, 1999), a test system has been developed which enables a continuous and parallel determination of oxygen consumption (pressure measurement) and carbon dioxide production (conductivity measurement). It is a closed test system consisting of a culture flask, a carbon dioxide absorption flask, a pump as well as integrated measuring and control instruments. The air circulating within the test system causes the carbon dioxide present in the test solution to be stripped out completely and directly absorbed by the absorption solution. Avoiding costly thermostatting of the test apparatus, the results of the pressure measurements were temperature-corrected arithmetically. The functional reliability of the measuring apparatus has been demonstrated exemplary in degradation experiments with selected test substances. This new test system also facilitates to test poorly soluble, adsorbing and volatile substances for inherent biodegradability and constitutes an appropriate complement to the standardised Zahn-Wellens test. The Federal Environmental Agency will use it as input to international standardisation activities ongoing within the OECD, as a draft standard.