Seasonally sampled cores of burrowed sediment containing chironomid larvae were collected from Cooking Lake, Alberta, and analyzed to (1) assess and establish the typical burrowing behavior and burrow architecture of chironomid larvae; (2) record micrometer-scale geochemical profiles of O₂, H₂S, and pH in the uppermost sedimentary layers throughout a seasonal cycle; and (3) link changing geochemical conditions to changing burrowing behaviors. We observed that the larvae lived in soft, water-saturated sediment, maintained by open burrows accreted by the animal's mucous. Chironomid-larvae burrows were small and Y-shaped (e.g., Polykladichnus-like) or Y-shaped with basal branches (Thalassinoides-like) and were 20 cm deep. The larvae moved up and down from the oxygenated zone (“sounding” behavior) to exploit food in suboxic and anoxic sediment. Geochemical analyses showed that H₂S was present in the pore waters to within 1.5 mm of the sediment-water interface during the summer, when lake-bottom algae and cyanobacteria generated sufficient O₂ to drive the oxic-anoxic redoxcline into the sediment. In the winter, the H₂S front extended upward into the water column owing to the cessation of algal and cyanobacterial activity. The prevalence of H₂S results from a combination of high-dissolved-sulfate concentrations in the lake and the abundance of microbial biomass that fuels an active subsurface population of sulfate-reducing bacteria. Interestingly, burrowing behavior was not linked to seasonal changes in the sediment chemistry. This is in part due to the ability of chironomid larvae to exploit oxygen islands in the sediment: in the winter, the chironomid larvae harvest their oxygen from the uppermost photosynthetic layer in otherwise O₂-impoverished sediments.