Perspectives on the context-dependency of stream community responses to contaminants

Natural variation in the structure and function of aquatic communities along environmental or spatiotemporal gradients can influence how systems respond to biotic and abiotic disturbances. Although these context-dependent responses are a well-established element of basic ecology, they have received little attention from an ecotoxicological perspective. As the scope of freshwater bioassessment expands from single catchments to broad-scale, regional surveys, it has become apparent that community responses to contaminants can vary significantly among locations. This variability is generally considered a nuisance and an impediment to recognising ecological patterns; however, natural spatiotemporal variation in communities also provides opportunities to test hypotheses about context-dependent responses to contaminants in aquatic ecosystems. This study presents three examples of context-dependent responses of benthic stream communities to contaminants. We show how longitudinal variation in community composition, the development of tolerance to contaminants and exposure to multiple stressors influence the historical context of a community and hence may determine community responses to contaminants. Aquatic ecotoxicologists have made significant progress identifying abiotic factors that influence contaminant bioavailability and toxicity. A similar effort could be undertaken to identify specific ecological factors (e.g. community composition, natural disturbance regimes and life-history characteristics) that contribute to variation in susceptibility of communities to contaminants. Understanding the nature of context-dependent responses and the mechanisms responsible for variation among communities should be a key element of ecological risk assessment and a research priority in ecotoxicology.