Year-round metabolism data from Lakes Simcoe (Ontario, 2010-2011), Diefenbaker, Blackstrap, and Broderick (Saskatchewan, 2013-2014), Canada.

Winter, historically a largely un-monitored season, is important and changing. There is evidence of the importance of under-ice phytoplankton in temperate lakes, but it is currently unknown if the often high winter phytoplankton biomass translates to high productivity and what influence it has on year-round lake metabolism. Winters are getting shorter, but our ability to forecast change is hindered by our limited understanding of what happens under the ice. Here, we compare under-ice and open-water rates of areal gross production (AGP) and areal respiration (AR) from 3 Canadian reservoirs and one large lake using oxygen (O2) changes in light-dark bottle experiments, δ18O-O2 models, and fluorometry. During the open-water season, AGP was 81× greater than under-ice rates, with AR rates 8× higher than measured during winter. Open-water samples indicated autotrophy (P:R= 1.10). Consistent with current assumptions, the cold under-ice environment is associated with low primary productivity. Our results challenge the assumption that mean water column irradiance is lowest during the winter in dimictic water bodies; we find similar light conditions during the open-water season. Winter mean light is regulated by snow thickness; upon manual snow removal, we observe a 67 % increase in under-ice mean water column irradiance. The first-ever under-ice application of the δ18O2-method indicated that AGP responded to improvements in light. This study reveals further insights into the importance of under-ice metabolism on year-round processes in a changing climate.