A case of thermal pollution limited primary productivity in a southwestern U.S.A. Reservoir

North Lake, a small (330 ha. surface area) southwestern U.S.A. cooling water reservoir was found to contain less phytoplankton production (104.0 mg C m⁻³ day⁻¹), lower annual mean total organic carbon (3.7 mg l⁻¹) and phytoplankton standing crops (0.9 ml m⁻³) than other local area reservoirs. Concentrations of inorganic P and N were at or below test detection limits during the study year 1973–1974.In situ ¹⁴C non-filtration primary productivity techniques demonstrated significant (≃13 percent) stimulation of planktonic primary productivity due to power plant entrainment. Optical counts showed no destruction of entrained phytoplankters. Populations of Cyanophyta were never dominant, although they frequently bloom in most other local reservoirs. Thermal loading at North Lake is thought to ultimately depress phytoplankton primary production and standing crop by causing nutrient limitation.