First estimation of the diffusive methane flux and concentrations from Lake Winnipeg, a large, shallow and eutrophic lake

Freshwater lakes are increasingly recognized as significant sources of atmospheric methane (CH4), potentially offsetting the terrestrial carbon sink. We present the first study of dissolved CH4 distributions and lake-air flux from Lake Winnipeg, based on two-years of observations collected during all seasons. Methane concentrations across two years had a median of value of 24.6 nmol L-1 (mean: 41.6 ± 68.2 nmol L-1) and ranged between 5.0 and 733.8 nmol L-1, with a 2018 annual median of 24.4 nmol L-1 (mean: 46.8 ± 99.3 nmol L-1) and 25.1 nmol L-1 (mean: 38.8 ± 45.2 nmol L-1) in 2019. The median lake-air flux was 1.1 µmol m−2 h−1 (range: 0.46–70.1 µmol m−2h−1, mean: 2.9 ± 10.2 µmol m−2 h−1) in 2018, and 5.5 µmol m−2h−1 (range: 0.0–78.4 µmol m−2 h−1, mean: 2.7 ± 8.5 µmol m−2 h−1) in 2019, for a total diffusive emission of 0.001 Tg of CH4-C yr−1. We found evidence of consistent spatial variability, with higher concentrations near river inflows. Significant seasonal trends in CH4 concentrations were not observed, though fluxes were highest during the fall season due to strong winds. Our findings suggest Lake Winnipeg is a CH4 source of similar mean magnitude to Lake Erie, with lower concentrations and fluxes per unit area than smaller mid- to high-latitude lakes. Additional work is needed to understand the factors underlying observed spatial variability in dissolved gas concentration, including estimations of production and consumption rates in the water column and sediments.