<i>Chaoborus</i> spp. Transport CH₄ from the Sediments to the Surface Waters of a Eutrophic Reservoir, But Their Contribution to Water Column CH₄ Concentrations and Diffusive Efflux Is Minor

Chaoborus spp. (midge larvae) live in the anoxic sediments and h-ypolimnia of freshwater lakes and reservoirs during the day and migrate to the surface waters at night to feed on plankton. It has recently been proposed that Chaoborus take up methane (CH4) from the sediments in their tracheal gas sacs, use this acquired buoyancy to ascend into the surface waters, and then release the CH4, thereby serving as a CH4 "pump" to the atmosphere. We tested this hypothesis using diel surveys and seasonal monitoring, as well as incubations of Chaoborus to measure CH, transport in their gas sacs at different depths and times in a eutrophic reservoir. We found that Chaoborus transported CH, from the hypolimnion to the lower epilimnion at dusk, but the overall rate of CH4 transport was minor, and incubations revealed substantial variability in CH4 transport over space and time. We calculated that Chaoborus transport similar to 0.1 mmol CH4 m(-2) yr(-1) to the epilimnion in our study reservoir, a very low proportion (<1%) of total CH4 diffusive flux during the summer stratified period. Our data further indicate that CH4 transport by Chaoborus is sensitive to water column mixing, Chaoborus density, and Chaoborus species identity.