Benthic photosynthesis and respiration was measured by in situ oxygen change and correlated with in situ radiation in ultraoligotrophic Char Lake Oat. 74°42′N). Photosynthesis by the moss and deep sediment zones was correlated with light but photosynthesis by the shallow rocky zone was not. Total benthic gross photosynthesis in Char Lake is 47.5 g O2m−2yr−1, or 80% of the total lake photosynthesis. Benthic respiration was 43.6 g O2m−2yr−1, in close agreement with the 40.1 g O2m−2yr−1derived from whole lake metabolism. Light input is strongly dependent on snow cover and extinction coefficient. Subsurface irradiance is 24% of the photosynthetically available surface radiation.