The objective of our study was to identify environmental conditions that structure lake sediment microbial communities and determine whether community composition explained inter‐lake variation in potential methanogenesis rates. We performed a comparative analysis of microbial communities and methanogenesis rates in 14 lake sediments along gradients of pH and primary productivity. Variation in methanogen community composition and non‐methanogen microbial community composition was best explained by pH and sediment organic matter content. However, these regulators of methanogen community structure were not associated with differences in methanogenesis rates. Instead, variation in lake methanogenesis rates was best explained by proxies for organic matter supplied to sediments (lake chlorophyll a concentration and sediment pore‐water total phosphorus) and the composition of the non‐methanogen microbial community. Our results suggest a role for sediment bacterial community in influencing methanogenesis via the supply of growth substrates.

Terrestrial loads of dissolved organic matter (DOM) have increased in recent years in many north temperate lakes. While much of the focus on the “browning” phenomena has been on its consequences for carbon cycling, much less is known about how it influences nutrient loading to lakes. We characterize potential loads of nitrogen and phosphorus to seepage lakes in northern Wisconsin, USA, based on a laboratory soil leaching experiment and a model that includes landscape cover and watershed area. In these seepage lakes, nutrient concentrations are positively correlated with dissolved organic carbon concentrations (nitrogen: r = 0.68, phosphorus: r = 0.54). Using long‐term records of browning, we found that dissolved organic matter‐associated nutrient loadings may have resulted in substantial increases in nitrogen and phosphorus in seepage lakes and could account for currently observed nutrient concentrations in the lake. “Silent” nutrient loadings to brown‐water lakes may lead to future water‐quality concerns.

In lakes, the production and emission of methane (CH 4) have been linked to lake trophic status. However, few studies have quantified the temporal response of lake CH 4 dynamics to primary productivity at the ecosystem scale or considered how the response may vary across lakes. Here, we investigate relationships between lake CH 4 dynamics and ecosystem primary productivity across both space and time using data from five lakes in northern Wisconsin, USA. From 2014 to 2019, we estimated hypolimnetic CH 4 storage rates for each lake using timeseries of hypolimnetic CH 4 concentration through the summer season. Across all lakes and years, hypolimnetic CH 4 storage ranged from <0.001 to 7.6 mmol CH 4 m −2 d −1 and was positively related to the mean summer rate of gross primary productivity (GPP). However, within-lake temporal responses to GPP diverged from the spatial relationship, and GPP was not a significant predictor of interannual variability in CH 4 storage at the lake scale. Using these data, we consider how and why temporal responses may differ from spatial patterns and demonstrate how extrapolating cross-lake relationships for prediction at the lake scale may substantially overestimate the rate of change of CH 4 dynamics in response to lake primary productivity. We conclude that future predictions of lake-mediated climate feedbacks in response to a shifting distribution of trophic status should incorporate both varying lake responses and the temporal scale of change. Plain Language Summary Many lakes produce substantial amounts of methane, a potent greenhouse gas. Previous research has found that more methane is produced from lakes with high algal biomass. However, little is known about how methane dynamics from a single lake respond to annual changes in algal biomass. By examining lake methane dynamics and metrics of algal biomass from five lakes across 5 years, we find that within-lake responses of methane dynamics do not align with the across-lake patterns we see when comparing different lakes. Within-lake methane responses were different between lakes and were undetectable in some lakes. Understanding the temporal scale of how lakes respond to changes in algal biomas is important for predicting the role of lakes for producing methane under future environmental change scenarios.

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