WINTERING CHIRONOMID LARVAE MINE OXYGEN

Seasonally sampled cores of burrowed sediment containing Chironomid larvae were collected from Cooking Lake, Alberta and analyzed to: (1) establish the typical burrow architecture of Chironomid-larvae and assess their burrowing behavior; (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 (i.e. Polykladichnus-like), or Y-shaped with basal branches (small, Thalassinoides-like), and they descended to a depth of 20 cm. The larvae used a porpoising behavior (i.e. up and down from the oxygenated zone) to exploit food in suboxic and anoxic sediment. The geochemical analyses revealed that H₂S was present in the pore waters to within 1.5 mm of the sediment-water interface during the summer, when cyanobacteria generated sufficient O₂ to drive the oxic-anoxic redoxcline into the sediment, but in the winter, the H₂2S front extended upwards into the water column due to the cessation of algal/cyanobacterial activity. The prevalence of H₂S can be explained by 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 (SRBs). 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. So the burrows are, in part, an oxygen-mining structure.