1. The impact of whole-lake lime (slaked lime, Ca(OH)(2), and/or calcite, CaCO3) addition on plankton communities was evaluated in eutrophic hardwater lakes on the North American Boreal Plain.2. Two lakes received a single treatment of lime (Ca(OH)(2) at 74 or 107 mg L-1), two lakes received multiple treatments with Ca(OH)(2) and/or CaCO3 (5-78 mg L-1), and four lakes were untreated and served as reference systems.3. Over the long-term (> 1 year), phytoplankton biomass was reduced in multiple-dose lakes, but not in single-dose lakes. Cyanobacteria typically dominated the algal community in the years before, during and after lime treatment in both single- and multiple-dose lakes.4. In the single-dose lakes, randomized intervention analysis showed no significant change in the biomass of zooplankton after lime addition.