A comparison of the responses of two microcosm designs to a toxic input of copper

Two microcosm designs were compared for their sensitivity to toxic concentrations of copper. One design simulated a littoral zone, including macrophytes, sediment, and associated organisms. The other design used a periphyton community collected on polyurethane foam artificial substrata. Microcosms were dosed with copper sulfate (0–300 µg Cu 1^–1, nominal concentrations) and monitored for changes in several structural and process variables. Coefficients of variation of responses measured from the littoral microcosms were greater than from responses measured from the artificial-substrata microcosms. Effects were detected more frequently at lower concentrations of copper in the artificial-substrata microcosms than in the littoral microcosms. Lowest observable effect concentrations (LOECs) for measures of community structure ranged from 20.2–42.8 µg Cu 1^–1 in the artificial-substrata microcosms and from 24.0–98.5 µg Cu 1^–1 in the littoral microcosms. LOECs for measures of community process ranged from 42.8–310.3 µg 1^–1 in the artificial substrata microcosms. Significant differences from controls for community process were detected only at 304.7 µg Cu 1^–1 in the littoral microcosms. While there were differences between the two microcosm designs in the concentrations of copper that resulted in adverse effects, response trends were similar. Often, dose-response relationships between variables and copper concentrations were not log-linear, but showed stimulations at intermediate concentrations of copper (10–100 µg 1^–1, nominal concentrations). The choice of microcosm design should be dependent on the particular research question, as the designs differ in complexity and in the ease of construction and maintenance.