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Lakes are significant emitters of methane to the atmosphere, and thus are important components of the global methane budget. Methane is typically produced in lake sediments, with the rate of methane production being strongly temperature dependent. Local and regional studies highlight the risk of increasing methane production under future climate change, but a global estimate is not currently available. Here, we project changes in global lake bottom temperatures and sediment methane production rates from 1901 to 2099. By the end of the 21st century, lake bottom temperatures are projected to increase globally, by an average of 0.86–2.60°C under Representative Concentration Pathways (RCPs) 2.6–8.5, with greater warming projected at lower latitudes. This future warming of bottom waters will likely result in an increase in methane production rates of 13%–40% by the end of the century, with many low‐latitude lakes experiencing an increase of up to 17 times the historical (1970–1999) global average under RCP 8.5. The projected increase in methane production will likely lead to higher emissions from lakes, although the exact magnitude of the emission increase requires more detailed regional studies. [ABSTRACT FROM AUTHOR]

Extreme wind events affect lake phytoplankton by deepening the mixed layer and increasing internal nutrient loading. Both increases and decreases in phytoplankton concentration after strong wind events have been observed, but the precise mechanisms driving these responses remain poorly understood or quantified. We coupled a one‐dimensional physical model to a biogeochemical model to investigate the factors regulating short‐term phytoplankton responses to summer wind events, now and under expected warmer future conditions. We simulated physical, chemical, and biological dynamics in Lake Erken, Sweden, and found that strong wind could increase or decrease the phytoplankton concentration in the euphotic zone 1 week after the event, depending on antecedent lake physical and chemical conditions. Wind had little effect on phytoplankton concentration if the mixed layer was deep prior to wind exposure. Higher incoming shortwave radiation and hypolimnetic nutrient concentration boosted phytoplankton concentration, whereas higher surface water temperatures decreased concentrations after wind events. Medium‐intensity wind events resulted in more phytoplankton than high‐intensity wind. Simulations under a future climate scenario did not show marked differences in the way wind events affect phytoplankton concentration. These findings help to better understand how wind impacts vary as a function of local environmental conditions and how climate warming and changing extreme weather dynamics will affect lake ecosystems. [ABSTRACT FROM AUTHOR]

Lakes are traditionally classified based on their thermal regime and trophic status. While this classification adequately captures many lakes, it is not sufficient to understand seasonally ice‐covered lakes, the most common lake type on Earth. We describe the inverse thermal stratification in 19 highly varying lakes and derive a model that predicts the temperature profile as a function of wind stress, area, and depth. The results suggest an additional subdivision of seasonally ice‐covered lakes to differentiate underice stratification. When ice forms in smaller and deeper lakes, inverse stratification will form with a thin buoyant layer of cold water (near 0°C) below the ice, which remains above a deeper 4°C layer. In contrast, the entire water column can cool to ∼0°C in larger and shallower lakes. We suggest these alternative conditions for dimictic lakes be termed "cryostratified" and "cryomictic." Plain Language Summary: Most mid and high latitude lakes are seasonally ice‐covered and have only been classified based on the thermal structure and trophic status during the open‐water season in summer. However, limited temperatures observations in these ice‐covered lakes suggest that there is a wide range of thermal structures over winter. We developed an analytical model to predict the average water temperature at the time of ice formation based on the strength of the surface winds, the area of the lake, and the maximum depth of the lake. Using both the analytical model and water temperature data from 19 different lakes in North America, Europe, and Asia, we found that the time of ice formation in lakes that are large or experience strong winds were later compared to lakes that are small or experience weak winds. The larger and windier lakes are also colder (0°C∼2°C) than smaller and calmer lakes (2°C∼4°C) at the time of ice formation. This suggests that these seasonally ice‐covered lakes can be subdivided into two additional classes during winter. The analytical model and the new categorization have important consequences for understanding fish habitat under the ice and the potential effects of climate change on these seasonally ice‐covered lakes. Key Points: Standard classifications of dimictic lakes do not consider how variable the initial thermal stratification can be under winter lake iceLakes that are shallow or windy can cool to near 0°C–1°C before ice forms and are weakly stratified, which we term "cryomictic"Deeper lakes or those with calmer winds, result in ice forming just above deeper waters of 3°C–4°C, which we term "cryostratified" [ABSTRACT FROM AUTHOR]