Recent limnological changes in southern Kootenay Lake, British Columbia

n response to a significant abatement of phosphate loading and the construction of a dam on its major inflow (the Kootenay River), southern Kootenay Lake experienced a number of limnological changes between 1969 and 1974. Water temperatures in 1974–1975 were similar to those previously reported for 1966–1969. However, water transparency has increased, particularly during the spring months.Nitrate concentrations have remained unchanged since 1969, and ranged from 2 to 8 μM between 6 June 1974 and 22 May 1975. However, ammonium levels have been reduced in the winter months when mean concentration was about 1 μM. Dissolved phosphorus levels in 1966–1969 were consistently around 3 μM, but never reached 1 μM in 1974. Phytoplankton populations in 1974–1975 (measured as chlorophyll a) were lower than those during the 1966–1969 period, and evidence is presented that suggests that major shifts have occurred in the species composition of the phytoplankton. Whereas Kootenay Lake experienced blue–green and green algal blooms during the 1960's, no such phenomena were observed in 1974–1975 when the plankton was dominated by diatoms. Population maxima of the three most common zooplankters (Cyclops bicuspidatus, Diaptomus ashlandi, andDiaphanosoma leuchtenbergianum) have increased since 1966–1969.