Developing flow-ecology relationships: Implications of nonlinear biological responses for water management

Empirical relationships between stream flow and ecological responses (flow-ecology relationships) are essential for establishing environmental flows and evaluating tradeoffs between instream values and out-of-stream uses. Establishing the shape of flow-ecology relationships (i.e. slope, linearity versus nonlinearity) is particularly important to avoid crossing ecological thresholds in water management. This review focuses on ecological responses to discharge at low summer flows when out-of-stream water demand is often highest, and identifying ecological contexts where nonlinearities are most likely. Most physical attributes (temperature, dissolved oxygen, available habitat) and ecological responses (energy flow, fish survival, recruitment, community structure) show at least some evidence of nonlinear relationships with flow, although assumptions of linearity may be reasonable across limited discharge ranges which may include low flows. Nonlinearities are most likely in systems that are near existing thresholds (e.g. cold-water transitional fish communities that are close to upper thermal tolerances). The probability of nonlinearities is likely to increase under future landuse and climate change scenarios, particularly in combination with other stressors, such as eutrophication, which may greatly accelerate temperature-related decline in dissolved oxygen under climate warming. Managers need to anticipate changes in flow-ecology relationships and develop management systems that are robust to change. Field programmes to establish the slope and linearity of local flow-ecology relationships are essential for regional management, but developing generalisable flow-ecology relationships that are transferrable to regions with limited resources also needs to be a priority. Generalised relationships can be generated through meta-analysis of empirical flow-ecology relationships, and may prove especially useful if they can capture how environmental and ecological context (channel size and morphology, landuse, flow regime, antecedent conditions, habitat or taxonomic guild) affect flow-ecology relationships. For instance, linking empirical data from flow-ecology relationships to available habitat predicted by physical habitat simulation models (e.g. PHABSIM) may provide a better mechanistic basis for modelling ecological responses, while providing much needed validation for habitat simulation approaches. This would also help bridge the gap between emerging holistic environmental flow modelling approaches and more traditional habitat simulation methods.