An ecosystem model for assessing ecological risks in Québec rivers, lakes, and reservoirs

The comprehensive aquatic systems model (CASM) was adapted for estimating ecological risks posed by toxic chemicals in rivers, lakes, and reservoirs in Québec, Canada. Populations of aquatic plants, invertebrates, and fish characteristic of these aquatic ecosystems were identified and generic food webs were constructed. Bioenergetics parameters that determine the growth dynamics of these populations were derived from published values for these same or similar species. Input values of light, water temperature, concentrations of dissolved nitrogen (N), phosphorus (P), and silica (Si) were constructed from available regional data or data from similar Canadian systems at similar latitudes. The model provides the capability to estimate the probability of changes in the biomass of multiple populations of primary producers and consumers as a function of the concentration of dissolved chemical contaminant. The CASM permits the evaluation of direct toxic effects, as well as indirect toxic effects that result from changes in competitive or predator–prey relations in complex aquatic food webs. Hypothetical risk assessments were constructed for pentachlorophenol, copper, mercury, and diquat dibromide in generalized rivers, lakes, and reservoirs in Québec. Numerical sensitivity and uncertainty analyses were used to describe the relative contributions of direct and indirect toxic effects on overall ecological risks estimated for functional guilds of producers and consumers in these ecosystems. This aquatic ecosystem model may become one component in a decision support system for assessing ecological risks.