Marianne Bruus Pedersen

The Impact of a Copper Gradient on a Microarthropod Field Community

In order to evaluate the power of laboratory toxicity tests to predict the situation in the field, soil microarthropods were studied at the population and community levels in a copper contaminated field, and data were compared to the outcome of single species laboratory tests in the same soil type from a companion study. In addition to copper, the influence of several soil characteristics and plant cover data on soil microarthropod abundance and distribution was tested. Total microarthropod abundance was highest at intermediate copper concentrations, and linearly related to grass biomass. For single species populations no clear picture of abundance in relation to soil copper was seen. Two collembolan species, Folsomia quadrioculata and Folsomia fimetaria, were among the most sensitive. The Shannon-Wiener index of biodiversity decreased linearly with increasing soil copper concentrations. The application of multivariate statistics showed copper to be the parameter best fitting the distribution of microarthropod species in the field. A distinction could be made between sampling points with 50–199 mg Cu/kg soil and those with higher copper concentrations. The species composition of the microarthropod community was not correlated with plant species diversity or total grass biomass, but was affected by a nearby row of tall trees. Community analysis of field data proved as sensitive as laboratory testing, and seemed superior to single species field population data concerning detection of copper effects on soil microarthropods.

Copper accumulation and fitness of Folsomia candida Willem in a copper contaminated sandy soil as affected by pH and soil moisture

Abstract Collembolans of the species Folsomia candida Willem were exposed to copper at different concentrations (11–122 mg kg−1) and pH levels (pHCaCl2 between 3.8 and 5.4) in soil collected from a field site. Total (HNO3 extractable) soil copper was determined, and the copper fraction available to the collembolans was estimated by extracting copper from the soil by different methods (extraction with 0.01 M CaCl2, extraction with artificially composed rainwater, and centrifugation to obtain the pore-water of the soil). Results were compared with the copper content of the collembolans and the effects of copper on three fitness parameters of the collembolans, viz. adult survival, reproduction and growth. For one soil treatment, the influence of soil moisture content on the fitness of the collembolans and on the toxicity of copper was studied by applying four levels of soil moisture (30, 50, 79 and 90% of the field capacity of the soil). Desorption of copper from the soil was best described by a two-species Freundlich equation that includes the influence of pH. No effects of copper were found on F. candida at the levels tested. Total soil copper was the best predictor of copper concentrations in the adult collembolans, and the inclusion of pH as a second factor improved the predictive value. Soil moisture had a large effect on fitness parameters of and copper accumulation by the collembolans, but there was no correlation between accumulation and fitness.

Effect of a copper gradient on plant community structure

Vegetation data including plant cover, biomass, species richness, and vegetation height was sampled on a copper-contaminated field with total copper contents varying from 50 to almost 3,000 mg/kg soil. The field was covered by early succession grassland dominated by Agrostis stolonifera. Plant cover, biomass, species richness, and vegetation height generally decreased with increasing copper content, although the highest biomass was reached at intermediate copper concentrations. Multivariate statistical analyses showed that plant community composition was significantly correlated with soil copper concentration and that community composition at soil copper concentrations above 200 mg/kg differed significantly from community composition at lower copper levels. Comparison of single-species (Black Bindweed, Fallopia convolvulus) performance at the field site and in laboratory tests involving field soil and spiked soil indicates that the laboratory tests conventionally applied for risk assessment purposes do not overestimate copper effects. Interaction between copper and other stressors operating only in the field probably balance the higher bioavailability in spiked soil.

Ecological Risk Assessment of Contaminated Soil

This review has described three cases of ecological risk assessment. The cases include two heavy metals (Cu and Zn) and an anthropogenic organic chemical (DDT). It concludes that there are at least two major constraints hampering the use of laboratory tests to predict effects under natural field conditions. One key issue is bioavailability, and another is suboptimal conditions or multiple stresses in the field such as climatic stress (drought, frost), predators, competition, or food shortage. On the basis of the presented case studies, it was possible to answer three essential questions often raised in connection to ecological risk assessment of contaminated sites. 1. To what extend does soil screening level (SSL) estimate the risk? The SSL are generally derived at levels corresponding to the lowest observed effect levels in laboratory studies, which often is close to the background levels found in many soils. In the cases of zinc and especially DDT, the SSL seemed quite conservative, whereas for copper they resemble the level at which changes in the community structure of soil microarthropods and the plant community have been observed at contaminated sites. The SSL correspond as a whole relatively well with concentrations where no effects or only minor effects were observed in controlled field studies. However, large variation in field surveys can often make it difficult to conclude to what extent the SSL corresponded to no-effect levels in the field. 2. Do bioassays represent a more realistic risk estimate? Here, there is no firm conclusion. The zinc study in UK showed a better relationship between the outcome of ex situ bioassays and field observations than the SSL. The latter overestimated the risk compared to field observations. However, this would be species dependent, as the sensitivity to metals may vary considerably between recognized test species, even within the same group of organisms, such as Folsomia candida and Folsomia fimetaria or Eisenia fetida and Lumbricus sp. Furthermore, it was demonstrated that bioassays were not useful for predicting general species diversity in the field as they are strongly influenced by natural variation and other factors not related to contamination. In the case of copper, bioassays with springtails and black bindweed seemed to underestimate the risk compared to the Cu concentrations at which significant changes in the community structure of soil microarthropods and plants have been observed at the contaminated site, and this was also the case for the DDT-contaminated soils. Here, bioassays with DDT-contaminated soils showed generally very low toxicity, with EC10 values considerably higher than the levels where clear effects on single species as well as community structure have been detected in the present field study. 3. Is it possible to make sound field surveys or do we lack suitable reference situations? Large natural variation caused by other factors than contaminants were observed in most cases, and this may have particularly hampered the conclusions made in the field surveys. These factors included pH, private and military traffic, age of vegetation, shading effects, and variations in light insensitivity as well as quantity and quality of organic matter. It was therefore concluded that field studies should always be interpreted in concert with similar data from a reference situation. Conclusions should therefore be made with caution in situations where important soil conditions vary between control plots and the contaminated sites. The cases also showed that indices focusing on species richness were unreliable. Estimates of evenness or dominance were recommended instead, and most authors concluded that multivariate analysis of community structure was a sensitive and useful method superior to single-species field data. This review concludes that there is a need for a tiered approach in ecological risk assessment of contaminated soils. Generic soil screening levels are needed as a first tier. Higher tiers of ecological risk assessment should, however, contain some kind of site-specific assessment. It is furthermore important to organize the various studies in a framework or decision support system that is transparent and useful for all stakeholders. A weight of evidence approach may be an obvious choice to deal with these uncertainties. The TRIAD approach, which incorporates and categorizes information in a triangle - chemistry, toxicology, and ecology - is an appropriate tool for handling conceptual uncertainties.