The not-so-dead of winter: underwater light climate and primary productivity under snow and ice cover in inland lakes

As global surface temperatures continue to rise as a result of anthropogenic climate change, effects in temperate lakes are likely to be more pronounced than in other ecosystems. Decreases in snow and ice cover extent and duration and extended periods of summer stratification have been observed in temperate lake systems throughout the Anthropocene. However, the effects of changing snow and ice cover on lacustrine communities remain largely uninvestigated. We examined underwater light climate and associated primary productivity patterns under snow-covered and clear-lake ice in 6 inland lakes in Minnesota, USA, spanning gradients of water column optical properties (blue, green, brown) associated with trophic status and organic material content. In all lakes, snow cover influenced not only the intensity, but also the spectral signature of light penetrating into the water column. Specifically, the wavelength of maximum penetration was shifted towards longer wavelengths under snow cover in green (eutrophic) lakes but was shifted towards shorter wavelengths in blue and brown lakes. Volumetric primary productivity was often higher than anticipated (e.g., ∼1200 mg m⁻³ d⁻¹; Lake Minnetonka, snow-covered ice). Carbon assimilation rates were lower under snow-covered ice throughout the water column in all lake types except immediately under cleared ice in eutrophic lakes, where phytoplankton were likely photoinhibited because of the penetration of intense, short-wavelength light. These findings suggest that changes to snow and ice cover under ongoing climate change scenarios can affect patterns of phytoplankton primary productivity in sensitive aquatic ecosystems.