Microscale decoupling of sediment oxygen consumption and microbial biomass in an oligotrophic lake

Summary Sediments of aquatic ecosystems are hotspots for biological activity. Here, we address the question if, within surface sediments, oxygen consumption is linearly related to cell abundance. In addition, we identify habitat‐specific factors influencing underlying microbial processes. Sediment microcosms were established from three sites within oligotrophic Lake Ånnsjön, Sweden, to use microsensors for measuring oxygen profiles and estimate spatially resolved oxygen consumption rates at the water–sediment interfaces. To evaluate differences between habitats, we measured sediment carbon content and C : N : P as a proxy for diagenetic state and organic matter bioavailability. Epifluorescence microscopy was used to assess the microscale distribution and size of surface‐colonising microorganisms. There was no linear correlation between oxygen consumption rates and microbial cell abundances. Cell‐specific respiration rates were highest in the profundal compared to the littoral‐ and inflow‐sediment microcosms, whereas vertical variability in all these parameters was highest at the inflow, intermediate in the littoral and least variable in profundal sediments. Illumina sequencing of spatially resolved 16SrRNA genes was used to test for possible influence of bacterial diversity on spatially resolved oxygen consumption rates. Bacterial α‐diversity decreased over depth at each site, but was also lower in sediments from the most active profundal zones of the lake compared to the inflow. We suggest that bacteria in profundal sediments mainly use highly oxidised organic compounds, resulting in overall low growth yield despite high metabolic activity. In the lake inflow and the littoral, more reduced organic substrates of terrestrial origin are used at lower rates but with higher yield.