Modelling Community Response for Ecological Significance Based on Laboratory Simulations of Variable Copper Exposure

Communities in aquatic ecosystems receiving runoff contaminants from urban catchments are often preconditioned to a background contamination level during dry periods. This pre-exposure may affect the community response to subsequent storm runoff contaminants. This work presents a model to describe the response of an assemblage of estuarine periphyton to such variable copper exposure, i.e. long term at a baseline concentration, followed by short-term at higher runoff concentration. The model allows for differentiation of effects between the long-term and short-term exposures, with quantification of the enhancing and suppressing effects of copper on the community. The model was evaluated based on the response curve of a periphyton community under laboratory-simulated exposure to variable copper concentrations, with PSII quantum yield as the response measure. Model predictions are close to observed values. The model shows improved goodness of fit for positive response compared to the traditional logistic model. Diagnosis of the model identified new effect concentration points which are of ecological relevance. They include the Pivotal-Effect Concentration (PC) at maximum yield (Y _max) and the effect concentration at 50 % yield (E _0.5Y). Therefore, the model described can be a useful tool for better understanding and managing ecological impacts of runoff on receiving aquatic ecosystems.