Historical perspective is important for fisheries management: a case study from Kootenay Lake, Columbia River Basin, British Columbia, Canada

A paleolimnological approach was used to assess nutrient conditions in Kootenay Lake, British Columbia, which hosts valuable fisheries that have been impacted by anthropogenic watershed disturbances over the past ∼125 years. Diatom-inferred estimates of total phosphorus (DI-TP) and diatom production since c. 1900 follow changes in nutrient conditions related to multiple influences (dams, watershed development, and inputs from an upstream phosphorus (P) fertilizer plant). Trends in DI-TP and diatom biomass from the three sediment cores collected from Kootenay Lake are highly similar providing confidence our analyses well represent the whole lake. Peak cultural eutrophication from the fertilizer plant masked the direct influence of two impoundments (1967 and 1972) on the nutrient status of Kootenay Lake. Extensive declines in DI-TP in the 1980s, following the closure of the fertilizer plant and P retention behind dams, were coincident with large declines in kokanee ( Oncorhynchus nerka ). A fertilization program has been ongoing since 1992 to increase algal, zooplankton, and fisheries production. Targeted ‘natural P' levels based on limited measured data in 1949 were higher than pre-1930 baseline conditions derived from the diatom analyses. Diatom metrics (DI-TP, concentrations, and biomass) increase with the fertilization program whereas changes in measured lake-water TP did not. The post 1990s measured TP are below pre-1930 DI-TP but they may not represent the complex nutrient dynamics of the lake ecosystem as diatom composition and biomass all remain above baseline conditions. Nutrient management targets should incorporate algal biomass targets, but these may be complicated by changes in production associated with climate warming.