Studies investigating the use of biomarkers in pesticide risk assessment have greatly increased in recent years; however, issues concerning the ecological meaning of enzymatic responses have proved controversial. Ideally a good biomarker response should be modulated by the environmental contaminants alone and demonstrate a predictable behavior towards certain types of toxins. As these premises are rarely observed, the present study aims to outline research that has contributed to an understanding of the behavior of two widely used biomarkers, cholinesterase and glutathione‐S‐transferase, describing environmental and biotic factors that affect their response in freshwater invertebrates. Studies were performed in the main classes of aquatic invertebrates with these biomarkers and conclusions were reached concerning their behavior towards the main classes of pesticides. Links between biomarker responses and conventional endpoints were evaluated so that ecological relevance could be attributed to enzymatic responses. Toxicity of mixtures was investigated, and cases of synergism and antagonism were pointed out as factors changing the expected toxicity of aquatic systems and leading to misinterpretations of biomarker responses. Finally, the use of biomarkers as a tool for biomonitoring and in situ assays was investigated, with discussion of advantages and disadvantages of their use. Environ. Toxicol. Chem. 2010;29:5–18. © 2009 SETAC

Damming is one of the main causes of the global decline in freshwater biodiversity. Yet, many hydroelectric dams are being built (or planned) in the Neotropics, where the high species diversity and lack of basic ecological knowledge provide a major obstacle to understanding the effects of this environmental change, which has been mostly described from the perspective of taxonomic change. However, this approach does not account for biological function. Trait‐based analysis provides an alternative approach to bioassessment. We assessed the impact of dam closure on the functional structure of fish communities of a Neotropical river by applying trait‐based analyses to the response of individual traits aggregated at the assemblage level. Fish data were collected during three distinct time periods (1 year before, 1 year after and 5 years after dam closure), at eight sites located downstream of the dam, in the reservoir, transition zone and upstream. The results indicated that reproduction strategies (migration and parental care) and diet (detritivores) were the trait categories mostly affected by the dam, with the response of downstream assemblages differing from upstream of the dam. A trait‐based analysis to the impact of damming on fish communities appears to be a promising approach using an initial descriptive analysis of individual traits and regression models of multiple traits that reflect species’ adaptation to the new environment. This study provides both an alternative and complementary approach to taxonomic assessment of impacts from damming, contributing towards a more robust evaluation of the response of fish communities to dams.