Conventional Ecological Risk Assessment (ERA) evaluates the impact of individual chemical stressors on a limited range of taxa, assuming these structural indicators act as proxies to predict impacts on freshwater ecosystem function. However, the limited ecological relevance of this approach has been recognised, driving a call to incorporate functional indicators into ERA. We reviewed the published literature to identify measures that are potential functional indicators of down-the-drain chemical stress. We found wide variation in the use of the term ‘ecosystem function’, and concluded it is important to distinguish between measures of processes and measures of the capacity for processes (i.e. species’ functional traits). Here, we present a classification of potential functional indicators and suggest that including indicators more directly connected with processes will improve the detection of impacts on ecosystem functioning. The rate of leaf litter breakdown, oxygen production, carbon dioxide consumption and biomass production have high potential to be used as functional indicators. However, the limited supporting evidence means that further study is needed before these measures can be fully implemented and interpreted within an ERA and regulatory context. Sensitivity to chemical stress is likely to vary among functional indicators depending on the stressor and ecosystem context, so we recommend that ERA incorporates a variety of indicators relevant to each aspect of the function of interest, such as a direct measure of a process (e.g. rate of leaf litter breakdown) and a capacity for a process (e.g. functional composition of macroinvertebrates), alongside structural indicators (e.g. taxonomic diversity of macroinvertebrates). Overall, the consideration of functional indicators can add value to ERA by providing greater ecological relevance, particularly in relation to indirect effects, functional compensation (Box 1), interactions of multiple stressors and the importance of ecosystem context.

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