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ABSTRACT:Recent developments in impact modelling of global warming on lakes have resulted in a greater understanding of how these vital ecosystems are likely to respond. However, there has been little quantitative analysis of this in an Irish context, despite the importance of lakes in the island's landscape. Here, we explore the impact of global warming on the hydrodynamics and thermal structure of a sentinel Irish lake under future climate scenarios. A 1D lake model, Simstrat, was calibrated and validated using water temperature data collected from Lough Feeagh, a site of long-term ecological research in the west of Ireland. Once validated, the model was then driven by daily climate model projections to generate informative thermal metrics for the time period of 2006–2099. Despite the moderating influence of the Atlantic, projections indicate that global warming will have a marked effect on the thermal structure of Feeagh, with surface water temperatures set to warm by 0.75°C under a more stringent mitigation pathway (RCP 2.6) and 2.42°C under a non-mitigation pathway (RCP 8.5). While warming was projected to be greatest in summer in the epilimnion, winter warming was greater than in other seasons in the hypolimnion. Stratification is projected to become more stable and earlier, and the growing season to be longer by 11 to 47 days, depending on mitigation pathways. Future studies could use a similar modelling workflow to investigate the possible implications of global warming on other Irish lakes, particularly those of specific societal importance or those of conservation interest.

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.

The intensity and frequency of storms are projected to increase in many regions of the world because of climate change. Storms can alter environmental conditions in many ecosystems. In lakes and reservoirs, storms can reduce epilimnetic temperatures from wind‐induced mixing with colder hypolimnetic waters, direct precipitation to the lake's surface, and watershed runoff. We analyzed 18 long‐term and high‐frequency lake datasets from 11 countries to assess the magnitude of wind‐ vs. rainstorm‐induced changes in epilimnetic temperature. We found small day‐to‐day epilimnetic temperature decreases in response to strong wind and heavy rain during stratified conditions. Day‐to‐day epilimnetic temperature decreased, on average, by 0.28°C during the strongest windstorms (storm mean daily wind speed among lakes: 6.7 ± 2.7 m s−1, 1 SD) and by 0.15°C after the heaviest rainstorms (storm mean daily rainfall: 21.3 ± 9.0 mm). The largest decreases in epilimnetic temperature were observed ≥2 d after sustained strong wind or heavy rain (top 5thpercentile of wind and rain events for each lake) in shallow and medium‐depth lakes. The smallest decreases occurred in deep lakes. Epilimnetic temperature change from windstorms, but not rainstorms, was negatively correlated with maximum lake depth. However, even the largest storm‐induced mean epilimnetic temperature decreases were typically <2°C. Day‐to‐day temperature change, in the absence of storms, often exceeded storm‐induced temperature changes. Because storm‐induced temperature changes to lake surface waters were minimal, changes in other limnological variables (e.g., nutrient concentrations or light) from storms may have larger impacts on biological communities than temperature changes.

Lake ecosystems, and the organisms that live within them, are vulnerable to temperature change1–5, including the increased occurrence of thermal extremes6. However, very little is known about lake heatwaves—periods of extreme warm lake surface water temperature—and how they may change under global warming. Here we use satellite observations and a numerical model to investigate changes in lake heatwaves for hundreds of lakes worldwide from 1901 to 2099. We show that lake heatwaves will become hotter and longer by the end of the twenty-first century. For the high-greenhouse-gas-emission scenario (Representative Concentration Pathway (RCP) 8.5), the average intensity of lake heatwaves, defined relative to the historical period (1970 to 1999), will increase from 3.7 ± 0.1 to 5.4 ± 0.8 degrees Celsius and their average duration will increase dramatically from 7.7 ± 0.4 to 95.5 ± 35.3 days. In the low-greenhouse-gas-emission RCP 2.6 scenario, heatwave intensity and duration will increase to 4.0 ± 0.2 degrees Celsius and 27.0 ± 7.6 days, respectively. Surface heatwaves are longer-lasting but less intense in deeper lakes (up to 60 metres deep) than in shallower lakes during both historic and future periods. As lakes warm during the twenty-first century7,8, their heatwaves will begin to extend across multiple seasons, with some lakes reaching a permanent heatwave state. Lake heatwaves are likely to exacerbate the adverse effects of long-term warming in lakes and exert widespread influence on their physical structure and chemical properties. Lake heatwaves could alter species composition by pushing aquatic species and ecosystems to the limits of their resilience. This in turn could threaten lake biodiversity9and the key ecological and economic benefits that lakes provide to society.

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.” 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. 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” Standard classifications of dimictic lakes do not consider how variable the initial thermal stratification can be under winter lake ice Lakes 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”