EFFECTS OF CA(OH)(2) ADDITIONS ON PHYTOPLANKTON COMMUNITIES UNDER-ICECOVER IN EUTROPHIC HARDWATER LAKES

The effects of Ca(OH)(2) treatment during the open-water season on phy toplankton communities under ice cover were examined for up to three w inters (Nov. Apr.) in three eutrophic hardwater lakes in central Alber ta. Phytoplankton communities were compared between treated and untrea ted lakes, and between pre- and posttreatment periods within a treatme nt lake. In all study lakes, phytoplankton biomass tended to be high a t ice-on, decrease after November, and remain low (< 2mg/L) from Janua ry to early March, when light conditions and water temperatures were a lso low. After late March to early April, phytoplankton biomass starte d to increase exponentially because of higher light conditions and wat er temperature. Most phytoplankton species that were common under ice also occurred during the open-water season. Although phytoplankton spe cies composition was diverse under ice, the biomass of most species, e xcept for flagellates, did not increase before mid-March. Species comp osition and biomass of some phytoplankton groups varied without clear trends among winters in treated and untreated lakes. Therefore, Ca(OH) (2) treatment during the open-water season did not appear to affect wi nter phytoplankton communities.