Expansion of aquatic vegetation in northern lakes amplified methane emissions

Abstract

Aquatic vegetation contributes to lake methane emissions, but changes in aquatic vegetation in northern (>40° N) lakes remain unknown, hindering evaluations of its importance in estimating lake emissions. Here we use Landsat imagery to monitor aquatic vegetation (mainly emergent and floating vegetation) in 2.7 million northern lakes from 1984 to 2021. Vegetation was observed in 1.2 million lakes, with a total maximum vegetation area of 12.0 × 104 km2, a mean vegetation occurrence of 1.68 ± 3.8% and a greenness of 0.66 ± 0.05. From the 1980s–1990s to 2010s, significant (P < 0.05) increases in maximum vegetation area (+2.3 × 104 km2) and vegetation occurrence (+73.7%) were observed and 72.5% of lakes experienced higher greenness. Vegetation expansion was affected by the temperature in sparsely populated regions, whereas lake area and fertilizer usage played vital roles in densely populated areas. The methane emission estimate that includes contributions from both aquatic vegetation and open water (1.31 [0.73, 1.89] Tg CH4 yr−1) is 13% higher than that calculated for open water (1.16 [0.63, 1.68] Tg CH4 yr−1). The long-term net increase in total methane emissions including aquatic vegetation is 125% higher than that of open water due to vegetation expansion. This highlights the necessity of incorporating aquatic vegetation in estimates of methane emissions from northern lakes. Areas of aquatic vegetation have expanded in northern lakes between 1984 and 2021, and this expansion is probably an additional climate feedback that enhances methane emissions, according to a monitoring study using Landsat imagery.