Sampling of a large undisturbed wilderness park lake prior to, and after 14 years of, mine tailing input has shown a pattern of change in the phytoplankton and zooplankton communities. Nutrient levels do not appear to be causative but heavy metal concentrations have increased and reached values substantially above background levels in a control lake. Organisms intolerant of heavy metals, including several diatoms and desmid species, have either disappeared or become extremely rare. Other, more tolerant, forms, notably several diatom species, Navicula cryptocephala, Synedra acus, S. ulna, Cyclotella bodanica and C. glomerata, have substantially increased in abundance. These changes have been accompanied by a decrease in species diversity and species number.