Seasonal phytoplankton and water quality relationships were identified by discriminant analysis procedures for a 20 year old borrow pit lake. Diatoms and chlorophyceans were more abundant during the colder months of spring and winter, and cyanobacteria characteristic of summer and early fall. Low inorganic nitrogen levels, higher pH values, and warmer, more stable water conditions were coupled with cyanobacteria development. Diatom abundance was associated with lower water temperatures and increased nitrite, silica and oxygen levels.