In the enclosures exposed to the highest copper concentration (0.24 μM), an increased tolerance of copper was found in phytoplankton communities even after 2 days, and in the enclosure exposed to the lowest copper concentration (0.016 μM), increased tolerance was found after 12 days. In accordance with the PICT (pollution-induced community tolerance) concept, the added copper was sufficient to affect the phytoplankton community, as was confirmed by changes in species composition. Communities exposed to increased copper concentrations also showed increased tolerance for zinc, indicating common tolerance mechanisms for copper and zinc