Internal Methane Cycling in a Small Temperate Lake

Abstract:

Despite the ubiquitous methane (CH₄) supersaturation in lakes, mechanistic understanding on the internal CH₄ cycling that regulate the annual CH₄ emission from these systems remains unresolved. Multi-year measurements on vertical and horizontal variability in CH₄ concentration and isotopic (δ¹³C) signature, along with the estimates on the rates of air-water and sediment CH₄ flux from a small temperate lake (Lake Croche, South eastern Quebec, Canada) were used to reconstruct the internal CH₄ cycling in the northern temperate lakes. While flux from hypolimnetic sediments was the largest source of water column CH₄, strong, but seasonally varying CH₄ oxidation zone separated the CH₄-rich hypolimnion from surface layer. The diffusive oxygen (O₂) supply from the upper layers regulated the oxidation zone, upward expansion and storage of CH₄ in the deeper water column. Despite the strong oxidation-filter, surface water CH₄ always exceeded the atmospheric equilibrium and concentrations were varied over the space and time. Lateral advection from the epilimnetic sediments and sporadic entrainment from the sub-surface CH₄ maxima mostly explains the surface CH₄ variability during stratified period. Air-water exchange was a relatively larger sink than the oxidative loss of CH₄ in the surface layers. Using a mass-balance approach, we reconstruct the processes regulating annual CH₄ emission from northern temperate lakes.