We compared in three shallow Alaskan Arctic lakes physical properties of bulk sediment and the fine-scale (1 mm increments to 20 mm sediment depth) vertical distribution of dissolved O2, scalar irradiance, chlorophyll a, and radiocarbon assimilation by microphytobenthos to better understand the structural and functional significance of this community. Sediments showed water contents of 86–98%, and dry bulk densities of 0.012–0.146 g cm-3, depending on depth. Chlorophyll a displayed no clear vertical pattern, suggesting mixing of surface layers and showed lakewise averages of 5.1–23.7 µg cm-3. Sediments were oxic to 1.5–5.5 mm and showed attenuation coefficients of 1.17–2.07 mm-1 for photon scalar irradiance. Photosynthetic activity was localized near the sediment surface, as 57–81% of H14CO3- added to intact cores was recovered in the 0–2 mm zone. Only 26–44% of the chlorophyll a in vertical profiles was sited in the euphoric zone, but microphytobenthos in underlying, aphotic sediments immediately photosynthesized at or near rates for the surface sediment when artificially irradiated. Area-based chlorophyll a in the euphotic plus photosynthetically capable aphotic microphytobenthos was 62–105 times higher than that of the phytoplankton, pointing to the potential importance of benthic autotrophs to Arctic lake food webs.
