A new OECD test guideline for the predatory soil mite Hypoaspis aculeifer: results of an international ring test.

Predatory mites are considered important biological indicators to assess potential effects of plant protection products. Toxicity testing of terrestrial mite species is required for authorisation of plant protection products in the European Union in cases where testing of leaf dwelling mites is not relevant, i.e. for defoliating herbicides, or when persistence of the chemical in soil is a concern. Since a standardised guideline for soil mites was not available in the past, an international working group developed a soil ecotoxicity test with the gamasid mite Hypoaspis aculeifer. This paper outlines the guideline development process and the principles of the protocol, and presents the results of an international ring test from which the validity criteria for the final guideline were derived. The protocol, which was published as OECD guideline 226 in 2008, is suitable for routine regulatory testing and can be used to generate data for risk assessment of soil inhabiting arthropods.

Effects of zinc contamination on a natural nematode community in outdoor soil mesocosms.

The effect of zinc on the indigenous nematode fauna of a sandy soil was determined in an experimentally contaminated outdoor field plot. The aims of the study were to describe and quantify the changes in density of separate nematode taxa and total nematodes, and the changes in the number of taxa, species diversity, community maturation, and species composition in response to zinc exposure with time, and to compare the observed responses with benchmark concentrations for soil as derived from the species sensitivity distribution (SSD) for zinc toxicity. Speciation of zinc in pore water was considered and CaCl2-exchangeable zinc concentrations were used as a measure of the bioavailable zinc fraction. After contamination, a reduction of total zinc and an increase of labile sorbed zinc over time occurred, concurrent with various changes in soil and biological characteristics. Data analyses on the nematode species revealed different sensitivity levels for several population and community endpoints to zinc exposure. Based on no observed effect concentration (NOEC) values, the most sensitive community-level response was obtained with principal response curve (PRC) analysis, which incorporates all density data in a single analysis. The PRC-based community NOECs were 56, 100, and 100 mg total Zn/kg dry soil after 3, 10, and 22 months of exposure, respectively. Based on 0.01 M CaCl2-exchangeable zinc, the community response appeared to increase, as NOECs were 4.9, 4.4, and 0.67 mg exchangeable Zn/kg dry weight. Total density was least sensitive, followed by diversity of taxa and the Shannon-Wiener index. NOECs for separate species covered a broad range from sensitive to tolerant species. This range of sensitivities was similar to the one found for other species groups tested in the field plot soil. A comparison was made between benchmark concentrations HC5 and HC50 derived from the general SSD of soil organisms for zinc and the nematode response data. These comparisons roughly confirm the predictions of the SSD model, that is, the community NOEC is in agreement with the benchmark that should protect the soil ecosystem's integrity, and large adverse effects were found at the benchmark derived for setting remediation urgency.

Influence of temperature on the regulation and toxicity of zinc in Folsomia candida (Collembola).

To generate reproducible and reliable results, most protocols for standardized toxicity tests prescribe strict limits for test conditions such as temperature and relative humidity. In the field, however, variability of climate can influence the functioning of organisms and induce changes in sensitivity. To evaluate the influence of temperature on the ecotoxicity of zinc, the springtail Folsomia candida was exposed to zinc-contaminated soil at different temperatures. In one experiment, the toxicity of ZnCl2 was determined at a constant temperature and under alternating temperature conditions. Although control performance of F. candida was influenced by the temperature treatment, effect concentrations for reproduction were not significantly different between treatments. Levels of internal zinc regulation were higher in animals exposed to alternating temperatures. Based on the observed influence of temperature on the accumulation of zinc, a second experiment was performed to determine the sensitivity of F. candida to zinc at four different exposure temperatures. A decrease in temperature led to an increase in sensitivity when sublethal parameters such as growth and reproduction were considered. However, animals were less sensitive at lower temperatures where mortality was concerned. Internal zinc concentrations in the animals increased with decreasing temperatures, and when the effect on reproduction was based on internal zinc levels, differences between temperature treatments were not significant. From the results of this study it is concluded that for F. candida, constant temperature conditions form a good basis for the extrapolation of toxicity levels to fluctuating temperature conditions. Since the average exposure temperature can alter the sensitivity of F. candida for zinc, and changes in sensitivity are parameter dependent, exposure temperature should be chosen carefully.

Development of zinc bioavailability and toxicity for the springtail Folsomia candida in an experimentally contaminated field plot.

The influence of outdoor exposure conditions and ageing of contamination on the toxicity of zinc was investigated for the springtail Folsomia candida to evaluate the validity of a standardised soil toxicity test. In three successive years, animals were incubated in an experimentally contaminated field plot. During the first months after construction of the test field, total zine concentrations of the soil decreased rapidly due to leaching of excess zinc with rainwater, while increased sorption of the remaining residues resulted in a reduced bioavailability of the metal. Although variation between replicates was substantial, the EC50s for the effect of zinc on reproduction of F. candida determined in the field experiments differed by less than a factor of two from effect concentrations obtained in laboratory tests in which the same soil was used. Expression of the EC50s on the basis of water soluble zinc allowed for a comparison with effect concentrations estimated for other soil types. EC50s were comparable with literature data, which indicates that bioavailability of zinc is the main factor determining toxicity for F. candida. It is concluded that laboratory based toxicity data are suitable to predict effects of zinc for F. candida under outdoor conditions, provided that the bioavailability of zinc is determined accurately using water soluble concentrations.