(Fig. 2 and Supplementary Figs 2–3). In this lake, the water is typically
well-mixed throughout the lake’s depth in spring. During summer,
warmer water sits on top of the cooler water at the bottom of the
lake. Thermal resistance to mixing across this warm–cold plane (the
thermocline, about 5 mdeep at the time of sampling, Fig. 1b) partially
isolates water below the thermocline (the hypolimnion) from external
and atmospheric influences. Heterotrophic microbes oxidize energyrich
carbon compounds as they diffuse or settle down into the hypolimnion,
and the increasingly limited availability of terminal electron
acceptors for these microbes leads to vertical chemical gradients.
Reduced chemical species can be transported to oxidizing conditions
closer to the lake’s surface, fuelling additional microbial activity. The
model predicts the distribution of microbial metabolic processes and
chemical species abundance in the lake from spring to autumn,
when the thermocline breaks down and the lake mixes again.