Differential CO2 and O2 Benthic Community Metabolism in a Soft-Water Lake

CO2 accumulation is greater than O2 removal in the hypolimnion of Dunham Pond (zmax = 4.5 m) during the summer, and in the whole lake during winter stratification. During much of stratification no free oxygen and less than 1% surface light exist below 2.5 m. Despite low light and on-going accumulation of CO2 during stratification, CO2 uptake is found in situ in sediment surface incubations at 3.5 m. The CO2 uptake is associated with an epipelic mat of blue-green algae apparently undergoing anaerobic photosynthesis. Thus, anaerobic photosynthesis and anaerobic respiration appear to be going on simultaneously in the hypolimnion. Respiratory quotients are conventionally thought of as a characteristic of metabolism imparted by the nature of the biochemical substrate being oxidized by an organism. Respiratory quotients of mixed communities of aerobes and anaerobes depend more upon the nature of the oxidizing agent being reduced. The degree to which a community RQ is greater than expected (about 0.85) describes the amount of metabolism for which oxygen is not (yet) the terminal electron acceptor. Excess oxidation of carbon creates ambient reducing power potentially available for bacterial (anaerobic) photosynthesis and chemolithotrophy.