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Northcote, T.G., F.R. Pick, D.B. Fillion and S. Salter. 2005. Interactions of nutrients and turbidity in the control of phytoplankton in a large Western Canadian lake prior to major watershed impoundments. Lake and Reserv. Manage. 21(3):261-276.Kootenay Lake is a large (over 392 km 2 ) fjord-type lake, part of the upper Columbia River Basin, which has undergone significant limnological changes due to a range of human activities over the past half century. We analyzed the limnological conditions of the lake during the mid 1960s, prior to major dam construction on its main tributaries. At that time, large volumes (25.4 km 3 yr -1 ) of highly turbid (up to 180 JTU) but anthropogenically phosphate-enriched water entered the south end via the Kootenay River. This interacted with smaller volumes of less turbid and much lower nutrient waters entering from the Duncan River in the north and lateral lake drainages (15.6 and 9.8 km 3 yr -1 respectively) to produce complex spatial and temporal differences in physical and chemical features (temperature, light penetration, ionic composition, pH, dissolved oxygen and nutrients) as well as in phytoplankton biomass, productivity and taxonomic composition. In the southern part of the lake, phytoplankton biomass, cell density and 14 C uptake rates were severely depressed during late spring and summer by light limitation from incoming silt turbidity, in spite of high phosphate concentrations. In contrast, phytoplankton stock and production was elevated in the middle to northern parts where transparency was high. Experimental algal bioassays using filtered lake waters demonstrated that through this period nutrient (primarily phosphorus) limitation occurred in the northern but not in the southern parts of Kootenay Lake. Watershed impoundments during the 1970s homogenized and simplified this ecosystem. On-going efforts to rebuild fisheries through restoration of the pre-dam nutrient loading may not return Kootenay Lake to the spatial and temporal complexity that once existed.Key Words: watershed pre-dam conditions, phytoplankton biomass, primary productivity, light limitation, nutrient limitation, algal bioassays, mountain lake, Kootenay Lake The relationship of phytoplankton abundance, production and community structure in large aquatic systems to changing conditions in nutrient supply and turbidity has long been a challenging research field for both oceanographers (Sverdrup 1953, Jones and Wills 1956, Lehman 1991, Yin et al. 1997 and limnologists (Talling 1957, Verduin 1965, Tilzer et al. 1976, Okuda et al. 1995. Few detailed studies exist in this field for large North American mountain lakes, the most notable exception being the extensive studies on Lake Tahoe in the Sierra Nevada, USA (Tilzer et al. 1976, Goldman 2000.In Kootenay Lake, one of the largest waterbodies (~392 km 2 ) in the mountainous upper Columbia River system of southeastern British Columbia (Fig. 1), several major shifts in both nutrient supply and turbidity have occurred within the last half century (Zybl...