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Higher-tier ecological risk assessment (ERA) in mesocosms is commonly performed in lotic or lentic experimental systems. These systems differ in their physico-chemical and hydrological properties, leading to differences in chemical fate, community characteristics and potential recovery. This raises the issue of the relevance and sensitivity of community-level endpoints in different types of mesocosms. In this study, macroinvertebrate abundance and biomass estimates were used to assess the effects of a dithiocarbamate fungicide, thiram (35 and 170 A mu g l(-1)), and a petroleum middle distillate (PMD; 0.01, 0.4, 2 and 20 mg l(-1)) in outdoor stream and pond mesocosms. Streams were continuously treated during 3 weeks followed by a 2-month long post-treatment period. Ponds were treated weekly for 4 weeks, followed by a 10-month long post-treatment period. Taxonomic structure of macroinvertebrate communities was characterized using the alpha, beta and gamma components of taxa richness, Shannon and Gini-Simpson indices. Computations were based either on abundance or biomass data. Results clearly highlighted that the effects of chemicals depended on the exposure regime (for thiram) and type of system (for the PMD). Causes of the differences between streams and ponds in the magnitude and nature of effects include differential sensitivity of taxa dwelling in lentic and lotic systems and the influence of hydrology (e.g., drift from upstream) and mesocosm connectivity on recovery dynamics. This study also showed complementarities in the use of both types of mesocosms to improve the characterization of chemical effects on communities in ERA.

Micro- and mesocosms are frequently required in regulatory procedures of aquatic risk assessment for pesticides. However, many questions are still a matter of debate with regard to the use of these systems for environmental risk assessment, especially considering the inter-system variability of the measured parameters and its consequences on experimental design and data analysis. In this paper, variability of physico-chemical and biological parameters measured during two long-term experiments (8 to 9 months) in uncontaminated outdoor freshwater lentic mesocosms (8 m3) is analysed. Consequences on the design of ecotoxicity tests in mesocosms and on data analysis are also addressed. Water temperature, pH, dissolved oxygen concentration and concentration of suspended solids exhibited a very low variability whereas nutrient concentrations displayed elevated levels of variability. Among biological parameters, those measured at the individual level were less variable than those measured at the community level. Functional descriptors frequently exhibited a lower inter-mesocosm variability than structural descriptors. Aggregation of data proved to significantly reduce inter-mesocosm variability. The results indicate that univariate statistical methods may be used for physico-chemical or species-level (e.g. biometric parameters) data which exhibit a moderate inter-mesocosm variability. The use of multivariate techniques is suggested for other levels of investigation. Nevertheless, variability is not sufficient to identify useful parameters. The sensitivity towards chemicals and ecological relevance of descriptors within the experimental context must also be considered.

Although a number of biomarkers of pollutant exposure have been identified in invertebrate species, direct linkage with changes at population and/or community levels are poorly documented and, despite the ecological importance of invertebrates, there is no conclusive evidence that individual measurements of biochemical parameters may allow the effects of pollutants on populations and communities to be predicted. Among the various biochemical parameters used as biomarkers in invertebrates exposed to pollutants in the field, only those for which changes at population or community level can be suggested are discussed in the present review. At population and community levels, the development of resistance to pesticides and changes in behaviour, reproduction and development are analysed as putative consequences of biochemical and physiological alterations. Limits to the use of biochemical parameters as biomarkers of invertebrate exposure to pollutants are discussed. Future research trends and experimental approaches to the validation of invertebrate biomarkers in environmental pollution assessment are suggested.