Elevated microbial CO2 production and fixation in the oxic/anoxic interface of estuarine water columns during seasonal anoxia

Gradients of dissolved oxygen concentrations in stratified estuarine water columns directly influence microbial composition and metabolic pathways, resulting in vertical and axial chemical gradients of redox-active species. Understanding such microbial responses to changing geochemical conditions and elucidating the diversity of microbially-mediated processes are needed to comprehensively identify ecosystem functions. We tested the hypothesis that microbial metabolic processes in the vicinity of the oxic/anoxic interface in Chesapeake Bay would be elevated due to the coincidence of oxidized and reduced compounds. We measured rates of microbial redox processes associated with carbon cycles and quantified geochemical constituents. The transition from oxic to anoxic to oxic conditions was correlated with the pattern of dissolved nitrogen species, with the most reducing conditions comprising the highest concentrations of dissolved chemical compounds. The vertical distribution of community respiration was measured with changes in dissolved inorganic carbon concentrations and was related to water column stratification, with the highest median and variability (1.2 ± 1.4 μmol L−1 h−1) in the most stratified conditions. Rates of CO2 fixation under dark conditions, a measure of chemoautotrophy, were elevated at the base of oxyclines and significantly correlated with a gradient of density and some reduced compounds. Although vertical interpolation must be made with caution due to high vertical variability, chemoautotrophic production averaged 0.5 mmol m−2 h−1 from May to August and added 5.8% to autochthonous organic carbon production in the mesohaline Chesapeake Bay. Overall, our results suggest that anaerobic community metabolism and chemoautotrophy in the oxic/anoxic interface exert a small impact on estuarine carbon cycles.