Biogeochemical drivers of methane dynamics in the water column of Georgian Bay’s embayment, Lake Huron

Methane (CH₄) production in oxygenated freshwater systems remains an active area of research in biogeochemistry. This study investigates CH₄ dynamics in Honey Harbour’s North Bay, a stratified freshwater oligotrophic embayment of Georgian Bay (Lake Huron), by integrating microbial community analyses, and geochemical measurements. The field study was conducted in 2021, with four sampling campaigns carried out during the months of July, August, September, and October. During stable stratification, CH₄ is present in oxic surface water with a maximum of 4.69 µg/L in September. The abundance of Synechococcus in surface waters indicates that CH₄ formation may be driven by methylphosphonate (MPn) degradation under phosphorus limitation conditions. Additionally, methanogenic archaea were detected in oxic waters, indicating possible methanogenesis in oxic surface freshwaters. The CH₄ oxidation possibly was supported by Fe-and Mn-reducing bacteria such as Geothrix and Methylobacter, which increased at depths below chemocline. Redundancy analysis (RDA) demonstrated strong positive correlations between CH₄, photosynthetically active radiation (PAR), and microbes linked to CH₄ cycling. Our study shows that CH₄ production in surface waters is influenced by phototrophic and archaeal activity, whereas deeper waters exhibit CH₄ oxidation coupled to metal cycling and Fe-and Mn-reducing bacteria. This study provides novel insights into microbial and geochemical interactions regulating CH₄ emissions in oligotrophic freshwaters of Georgian Bay embayment.