Spatiotemporal nutrient loading to Cultus Lake: Context for eutrophication and implications for integrated watershed-lake management

Cultus Lake, British Columbia experiences significant anthropogenic nutrient loadings and eutrophication. If continued unabated, these stresses threaten the persistence of two resident species at risk (coastrange sculpin and Cultus Lake sockeye salmon) and the many ecosystem services provided by the lake. We constructed water and nutrient budgets for the Cultus Lake watershed to identify major sources of nitrogen and phosphorus loadings to the lake. A steady-state water quality model calibrated against current nutrient loadings and limnological data was then used to explore water quality changes in response to various scenarios of nutrient loading to the lake. Atmospheric deposition was a major source of both nitrogen and phosphorous, agriculture was a large source of nitrogen, and septic leaching and migratory gull guano were major phosphorous sources to Cultus Lake. Future loading scenarios suggest that Cultus Lake may become mesotrophic in response to unmitigated increases in projected nutrient loadings from gull guano, septic leaching, agricultural intensification, and atmospheric deposition, highlighting the need to abate phosphorus loading to the lake. Although mitigating phosphorous sources such as gull guano and septic leaching will slow the eutrophication of Cultus Lake, a reduction in atmospheric phosphorus deposition from the regional airshed will be necessary to halt or reverse eutrophication and protect the ecosystem services and species at risk habitat provided by the lake.