Landscape-scale regulators of water transparency in mountain lakes: implications of projected glacial loss

We examined factors regulating water transparency in a set of 33 lakes located in the Canadian Rocky Mountains. Eighteen lakes had catchments that included glaciers and 15 did not. In each lake, we quantified midsummer attenuation rates for three ultraviolet wavelengths and photosynthetically active radiation and measured chromophoric dissolved organic matter ( CDOM) absorbance, turbidity, and chlorophyll a fluorescence. We also used GIS to quantify characteristics of lake catchments. Across lakes, turbidity and CDOM absorbance were arrayed on orthogonal gradients that intersected in a region of low turbidity and low CDOM absorbance. Nonglacially fed lakes had low turbidity and attenuation rates were regulated by CDOM absorbance, which increased with the percentage of the catchment covered by vegetation. Glacially fed lakes had low CDOM absorbance and attenuation rates increased with turbidity, which increased with the percentage of the catchment covered by permanent ice. Glaciers are retreating rapidly in the Canadian Rockies. As catchments become deglaciated, turbidity will likely decrease and the regulator of lake transparency is likely to switch to CDOM, which will have broad implications for lake ecosystems.