The Importance of Lake Littoral Zones to Arctic-Boreal Methane Emissions

Shallow areas of lakes, known as littoral zones, emit disproportionately more methane than open water but are typically ignored in upscaled estimates of lake greenhouse gas emissions. These areas are similar to wetlands in emissions per unit area, but their extent is more poorly-characterized. While lake depth is challenging to map from remote sensing, littoral zone extent may be estimated through synthetic aperture radar (SAR) mapping of emergent aquatic vegetation, which only grows in water up to ~ 1.5 m deep. To assess the importance of littoral zones to landscape-scale methane emissions, we combine airborne SAR mapping with representative field measurements of littoral and open-water methane flux. First, we use UAVSAR data from the NASA Arctic-Boreal Vulnerability Experiment (ABoVE) to map littoral zones of 4,572 lakes across four Arctic-boreal study areas and find they comprise ~16% of lake area on average, a higher amount than previous estimates, and with strong regional differences. Next, we account for these vegetated areas through a simple upscaling exercise using paired open water and littoral methane fluxes from field measurements and previously published values, and find inclusion of littoral zones nearly doubles overall lake methane emissions relative to estimates upscaled from open water areas alone. While littoral zones commonly constitute greater fractions of small lakes, the relationship between littoral zone area and lake area is quite weak and varies regionally. Measurements from remote sensing are therefore an important way to quantify littoral zone coverage, which could eventually improve lake models.