Preamble

Based on an admittedly subjective selection from the enormous literature on climate change, the development and impact of global warming on Alpine lakes are traced since approximately the 1980s in as many facets as possible. This article concentrates on the European Alps but also includes examples from elsewhere.

Short History of Temperature Records From Alpine Lakes

As climate warming implies, temperature is the main driver of changes in Alpine lakes. Therefore, the focus here is on early lake temperature records.

The Swiss naturalist de Saussure was the first to measure temperatures in Alpine lakes (Barry, 1978). His first measurements in Lake Geneva, Switzerland, on August 13, 1767, after a sunny period revealed a surface temperature of 22.5–25°C and a temperature of 10.5°C at the depth of 26.5 m. Later, he measured 5.4°C at 113 m depth and concluded that the temperature at the bottom of the lake is permanently between 4° and 5°C. In July 1788, he extended the measurements to lakes on Col du Géant (3,360 m above sea level [a.s.l.]) and on mountains near Chamonix (1,050 m a.s.l.). Similar measurements were made by Morozzo Comte (1790) in Lago Maggiore, Lago Mergozzo, and Lago di Orta in Italy from 1785 to 1789. De la Bèche (1819) extended the temperature measurements at depths near the sediment over the entire area of Lake Geneva, and Fischer-Foster de and Brunner (1849) followed the seasonal cycle in Thunersee, Switzerland, from March 1848 to February 1849. Monthly observations of temperature profiles in Lake Geneva were executed by Forel (1880a, 1880b), further consolidating the understanding of the vertical thermal structure and dynamics of lakes.

The first temperature profile from a lake in Austria was published by Simony (1850) from Traunsee on August 20, 1848. Analyzing temperature profiles from Wörthersee from 1889 to 1891, Richter (1891) discovered the thermocline, also called metalimnion, as the discontinuity layer separating the upper warmer water layer from the colder deep water.

The Austrian Hydrographic Service started hydrological measurements in 1893 and included surface water temperature readings from lakes at gauge stations starting in 1898 on Zeller See, Austria. These records are still collected; thus, they are some of the longest surface water temperature records available (Matulla et al., 2018). Water and sediment temperatures have been recorded in Upper Lunzer See, Austria, situated at 1,117 m a.s.l., since 1905 (Mulley et al., 1914).

Turnowsky and Steinböck were pioneers in high Alpine lake research in Austria. Remarkable are the early observations from lakes in the Hohe Tauern mountains (Turnowsky, 1946) and from Schwarzsee/Sölden in Austria at 2,792 m (Steinböck, 1948). Noteworthy are also those by Gutmann (1961) in Mölsersee in the Tuxer Alps at 2,238 m from the years 1952 and 1953. Both lakes, the Schwarzsee and the Mölsersee (Figure 1), were inverse stratified only in winter under the ice cover. Advances in technology allow high-frequency evaluations of in situ temperature profiles (Doubek et al., 2021; Meinson et al., 2016) as well as remote estimates of lake surface temperature (Riffler et al., 2015).

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Figure 1. Air and water temperature in Mölsersee, Austria, in 1952 and 1953 according to the data of Table 2 in Gutmann (1961). Ice cover is defined as ice plus snow cover.

Types of Alpine Lakes

Alpine lakes are not restricted to the Alps; they are also located in several mountain ranges throughout the world. A precise definition of an Alpine lake, however, is not unequivocally available. These lakes are often classified as lakes at high altitudes, starting at approximately 1,500 m; in another definition approximately 3,000 m above sea level or above the tree line. Those definitions only refer to high Alpine lakes and exclude true Alpine lakes and pre- or subalpine lakes distinguished by formation and elevation (Dokulil, 2005). All lakes considered here were formed by glacial activity in the past. In Europe, Alpine lakes are usually associated with the Alps, but they are located at various altitudes in Europe, varying from the coastline in northern Norway to above 2,000 m in the Alps (Weyhenmeyer et al., 2009). Therefore, Alpine lake types are rather diverse. Alpine lakes sensu stricto are glacial valley lakes, fjord-type lakes, or finger lakes (Figure 2) such as Lake Constance in Germany or Lago di Garda in Italy (Hutchinson, 1957; Timms, 1992; Wetzel, 2001). Pre- or subalpine lakes at low elevation are termed piedmont lakes. Examples are Wallersee and the Trumer Seen in Austria. High Alpine lakes are often small lakes formed by glacial scour and referred to as cirque or tarn lakes. In many cases, they are headwater lakes (see Figure 2). Kettle lakes originating from ice left deposited in depressions can be formed at any elevation (e.g., Osterseen, Germany, at 588 m a.s.l.). High Alpine lakes may also be formed by glacial moraine damming. The retreat of glaciers due to climate warming increases the number of glacial lakes on, in, below, or at the fringe of a glacier or dammed by moraine or ice (Buckel et al., 2018; Cooke & Quincey, 2015; Shugar et al., 2020).

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Figure 2. Peri-alpine region with major lakes indicated. Morphometric data of these and other lakes can be retrieved from Dokulil (2005, Table 6.4, p. 162).

Source: Map created using StepMap.

All the lakes mentioned so far are natural lakes. If we widen our concept to include man-made lakes, storage reservoirs and pump-storage reservoirs for hydropower production (Anghileri et al., 2018; Hirsch et al., 2017), as well as mountain reservoirs for artificial snow production, must be considered (Evette et al., 2011; Fait et al., 2018). Lakes of the peri-alpine region of the Alps are mainly considered in this article (see Figure 2). For other Alpine biographical regions in Europe, see Weyhenmeyer et al. (2009).

The lakes in the Alps are divided into a northern and a southern group by the Alpine watershed running from west to east. The southern group is made up of Lake Geneva and the Insubrian lakes (Maggiore, Lugano, Como, and Garda). Parts of the northern lakes (Lakes Neuchâtel or Neuenburger See, Luzern, Zürich, Constance, Chiemsee, and Attersee) are situated in the foothills of the Alps or even some distance beyond (Ammersee and Trumer Seen).

The lakes in the Alps vary largely in area, volume, and depth (Dokulil, 2005, Table 6.4; Jung, 1994). According to Meybeck (1995), lakes in the size class 0.01 to 0.1 km² are most abundant. The number of lakes increases largely if the size class smaller than 0.01 km² is considered. In Carinthia, lakes smaller than 0.01 km² at altitudes >1,000 m a.s.l. are nine times more abundant than those in the 0.01- to 0.1-km² range (Sampl, 1976).

Many Alpine lakes are ice covered with a clear inverse thermal stratification in winter and a well-defined thermal summer stratification, making them dimictic. High Alpine lakes are usually clearer than lakes at lower elevation due to the colder Zeilewater, ultraoligotrophic status (poor in nutrients) because of generally small catchment areas, and little local anthropogenic disturbance.

Climate in the Peri-Alpine Region

The climate of the peri-alpine region is a result of dynamic long-distance weather systems, high complexity, and interactions between mountains and the atmosphere (Plaut & Simonnet, 2001). The Alps, situated along the north–south axis between approximately 44°N and 48°N, divide the moderate Central European from the Mediterranean climate. The mountain ridge effectively affects, modifies, and separates weather conditions south and north of the Alps. Similarly, the oceanic-humid conditions in the west change to dry-continental climate in the east. The moist and cool periphery contrasts with the warm and dry central alpine provinces. Moreover, climatic parameters strongly vary with altitude forming a definite elevation gradient. As a consequence of Alpine orography and global warming, elevation-dependent warming has been observed in the European Alps, conveniently defined as the linear regression of the time series of temperatures against elevation, which reaches several tenths of a degree per 1,000 m elevation per decade (Kuhn & Olefs, 2020).

Overall air temperature has increased by more than 2.0°C and sunshine duration by approximately 15% since the 1980s (Stangl et al., 2021). Warming in the Alps has a greater amplitude and exceeds the average global warming of 0.85°C above the 20th-century average (Beniston et al., 1997; Gobiet et al., 2014).

The large-scale weather patterns in the Alps are largely controlled by three air-pressure zones: the Iceland low, the Azores high, and the Siberian high or low. Together with the distinct climatic gradients in all three dimensions and the perennial snow and ice cover in the upper reaches, characteristic weather patterns are recurrently formed (Gobiet et al., 2014). Changes in these patterns are associated partly with variations in the North Atlantic Oscillation and, to a large extent, with the transition zone that shifts northward in summer and southward in winter, referred to as European Climate change (Giorgi & Coppola, 2007, 2009). Projections for the 21st century indicate increases of very dry seasons over Southern Europe in summer and extreme hot seasons throughout Europe, both affecting the Alps and their lakes as well. A comprehensive overview of climate shifts and future projections in the European Alps between 1800 and 2100 is provided by Rubel et al. (2017) and can be seen animated on the World Maps of Köppen–Geiger Climate Classification website.

Climate warming has increased the frequency and intensity of extreme weather events such as heatwaves, droughts, heavy precipitation, storms, etc. (Stott, 2016). An example is the early summer of 2018, whose extreme weather situations were caused by meanders in the jet stream of the Northern Hemisphere (Kornhuber et al., 2019). The same pattern also caused the heatwaves of 2003, 2006, and 2015.

Peri-Alpine Lakes Since the Mid-20th Century

Temperatures in the Alps and peri-alpine region have increased at twice the rate as that of the Northern Hemisphere. Warming thus potentially affects Alpine lake ecosystems via reduced ice cover, prolonged and intensified thermal stratification, and altered food webs or processes in the catchment.

Long-term observations of ice cover of lakes show a decrease in duration or a complete loss (Dokulil, 2014b; Kainz et al., 2017). Analysis of data from 513 lakes in the Northern Hemisphere (Imrit & Sharma, 2021; Sharma et al., 2019) revealed significant disturbances of ecosystem function in ice-free winters. Due to the loss of the winter ice cover, dimictic lakes can convert to monomictic state (Ficker et al., 2017). At the same time, summer temperature stratification starts earlier in the year and ends later. Thus, the stagnation period in the summer lasts much longer, which may lead to oxygen deficits in deep water (Luger et al., 2021; Woolway et al., 2021).

Long-term series of monthly mean lake surface water temperatures (LSWTs) from Alpine lakes indicate unequivocally a regime shift in the 1980s leading to enhanced rates of climate warming (Dokulil, 2014a; Woolway et al., 2017). The timing of the observed rate modifications is in accordance with shifts in several climatic parameters globally (Reid et al., 2016). Lake surface temperature depends on air temperature and is therefore affected by the North Atlantic Oscillation, particularly in winter (Livingstone & Dokulil, 2001). Like LSWTs, monthly mean deep-water temperatures tend to increase but at much lower rates (Dokulil, Jagsch, et al., 2006; Dokulil, Teubner, et al., 2006). Whereas rates for LSWTs range from 0.4° to 0.7°C per decade, hypolimnetic water temperatures (HWTs) increase by 0.1° to 0.2°C per decade. Ambrosetti and Barbanti (1999) reported HWTs increased by 0.3°C per decade for several large deep Italian lakes south of the Alps, while Livingstone (1997) described deep-water warming and cooling episodes for Swiss lakes.

Most publications on temperature change of lakes use averages per year, seasons, months, or specific periods of the year (O’Reilly et al., 2015). Trends of minimum and maximum temperatures have been published more recently, which consistently indicate significant increases (Dokulil et al., 2021; Woolway et al., 2019).

Annual minimum LSWTs, influencing ecological and biochemical processes, warmed at an average rate of 0.35°C decade^–1 between 1972 and 2014 in eight European lakes (Woolway et al., 2019). The Carinthian pre-alpine lake Wörthersee had the highest warming rate of 0.51°C per decade, which may substantially influence the ecosystem. Food web structure could be disrupted, and cold stenothermic species might be lost.

Annual maximum LSWT influences rates of metabolic activities, species survival, and biogeography. Long-term changes in 20 Alpine lakes of Austria indicate three different categories: 4 lakes with temperature increase of >0.5°C per decade, 5 lakes without any significant trend, and 11 lakes with warming rates of 0.2–0.4°C per decade (Table 1). The absolute maximum LSWT of 27.5°C was reached in Wörthersee. Even more important is the trend of prolonged periods exceeding a potential critical temperature of 20°C each year (Dokulil et al., 2021). Within this European-wide study, periods exceeding 20°C got longer over the 50 years analyzed in 2 Alpine lakes. In Mondsee, average days >20°C increased from 16 days in the years 1976–1980 to 72 days in the 2001–2005 period (Figure 3a). The depth at which water temperature exceeded 20°C in the epilimnion increased from <1 m to >6 m in Mondsee over the 50 years studied (Figure 3b). Lakes south of the Alps are always warmer than lakes in the northern Salzkammergut-region. Wörthersee, for instance, exceeded 20°C in almost all years. The average for the same periods rose from 72 to 108 days. Extended periods of maximum LSWT affect almost all organisms in the system, alter biogeochemical processes, and also modify lake trophic status, imposing changes on ecosystem functioning. Consequently, the habitable environment will become increasingly restricted for many organisms that were adapted to historic conditions.

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Figure 3. (a) Box whisker plot of number of days when the temperature exceeded 20°C in Mondsee for each 5-year period from 1976 to 2015. The horizontal dotted black line is the average of 54 days for all years. In each box, the solid black line is the median, and the dotted red line is the average. Whiskers are minimum and maximum. The solid line is a third-order polynomial to visualize the development over time. (b) Mean depth of temperature >20°C in Mondsee for each 5-year period from 1976 to 2015. The horizontal dotted line is the average of 3.5 m for all years. LSWT, lake surface water temperature.

Source: Modified from Dokulil et al. (2021).

Evaluating data from 40 years, Dokulil and Teubner (2011) were able to show that the depth of a population of Planktothrix rubescens in the metalimnion depends essentially on climate signals and the environmental conditions in the water column. Due to climate warming, the taxon is favored, especially during spring. Longer stratification periods have little effect on the development of the biovolume. The impact of the 2015 heatwave on phytoplankton mainly affected P. rubescens, which shifted its biovolume center from a depth of 11 m to 16 m (Bergkemper & Weisse, 2017).

Atmospheric chemistry and global mobility of key macronutrients have been significantly changed by anthropogenic activities. Global patterns in nitrogen (N) and phosphorus (P) emissions drive large hemispheric variation in precipitation chemistry. Deposition of N and P is therefore reflected in the water chemistry of naturally oligotrophic (high alpine) lakes (r² = 0.81, p < 0.0001). Due to increased dust and biomass burning emissions, P deposition has increased globally by 1.4 times the preindustrial rate and hence has become a major driver of Alpine lake trophic status. The impact on primary productivity will be large because a stoichiometrically deposition of 1 unit of P has 16 times the influence of 1 unit of N deposition. These results suggest a major impact of atmospheric phosphorus deposition on the water quality of naturally oligotrophic lakes (Brahney et al., 2015).

Investigations on impacts from catchments and the atmospheric nutrient deposition regime on the taxonomic and functional composition of phytoplankton communities in high mountain lakes were reported from six French Alpine lakes by Jacquemin et al. (2019). Lakes with the smallest rocky catchments showed the lowest functional richness of phytoplankton communities no matter the nutrient deposition regime. Lakes with larger vegetated catchments had phytoplankton assemblages with more diverse strategies in utilizing nutrients. Under high N/P deposition regimes, photoautotrophic taxa dominated, whereas mixotrophic taxa were superior under low N/P deposition regimes particularly in lakes with large, vegetated catchments. Climate-related changes in catchment characteristics and airborne contaminants must be considered when climate changes are evaluated in high-altitude lakes.

Invertebrate communities inhabiting alpine water bodies are sensitive indicators of environmental variability and climate change (Füreder et al., 2006). The sensitivity of the faunal species composition was tested by Fjellheim et al. (2009) using sampled variables from 126 Alpine lakes in seven different Alpine lake districts in six European mountain regions. Chironomids dominated the oligotrophic—ultraoligotrophic species assemblages with up to 60% individuals. Trends in the composition were found in the altitudinal gradient and lake water chemistry between districts. An even larger data survey from 350 remote high-altitude and high-latitude lakes was reported from 11 different mountain regions throughout Europe by Kernan et al. (2009). Across the lake districts, location of the lake largely explained the variation in species composition, producing a strong geographical signal, but lake “types” cannot be extrapolated throughout Europe. Response due to climate differences between regions varied. The authors identified three limno-regions: Nordic (Scotland and Norway), sub-Arctic (northern Finland), and Alpine (Pyrenees, the Alps, and Eastern Europe ranges).

Pighini et al. (2018) established Alpine lakes as emitters of greenhouse gases (GHGs). In the 40 lakes studied at altitudes between 200 and 1,500 m a.s.l., dissolved CO₂ and CH₄ could be detected in average concentrations of 1.10 ± 1.30 and 36.23 ± 31.15 μ‎mol L⁻¹, respectively. A dependency on the altitude was not detected. The authors concluded from their results that Alpine lakes can act as a source of GHGs.

Greenhouse gas emissions were estimated for peri-alpine and Alpine hydropower reservoirs (Diem et al., 2008). Eleven reservoirs located at different altitudes in Switzerland were net emitters of CO₂, with an average of 1,030 ± 780 mg m⁻² d⁻¹, and of methane, with an average of 0.20 ± 0.15 mg m⁻² d⁻¹. GHG emissions from reservoirs in the Alps seem to be minor contributors to global GHG emissions.

Alpine lakes at high altitudes are extremely sensitive to ice-cover duration modulated by temperature. Therefore, summer epilimnetic temperatures may rise by more than 10 degrees with further climate warming (Thompson et al., 2005).

Short-wave ultraviolet radiation (UVR) penetrates considerably deeper into clear high Alpine lakes compared to lakes in the lowland. Diffuse attenuation coefficients of radiation of different wavelengths are presented in Table 2 for Gossenköllesee, a transparent high Alpine lake at 2,314 m a.s.l. in Austria. Data were taken from Sonntag et al. (2011) and derived values calculated. The depth of the euphotic zone calculated as the 1% light intensity is far below the deepest point of the lake (see Table 2, column 4). Calculated radiation at 9.9 m, the greatest depth in the lake, was 18% photosynthetic radiation and approximately 1% of the shortest UV radiation at 305 nm. Because UV radiation penetrates the entire water column, it largely affects all types of biotas in such systems. However, this is not necessarily applicable to high mountain lakes in general. Lakes at lower altitude or glacier-fed lakes have much lower UV penetration because of greater organic or inorganic turbidity.

Comparing data from 1998–1999 and 2010–2011 from five high Alpine lakes, Weckström et al. (2016) showed that lake water temperature increased, ice cover duration decreased, and ionic composition remained stable. While dominant diatom species and types of Chrysophyte cysts definitely changed in the two lakes with the largest decrease in ice-cover duration, responses of the whole ecosystem assemblage were less evident. Chironomid communities appear to be less sensitive. Climate warming will certainly increase productivity in high Alpine lakes (Karlsson et al., 2005; Slemmons & Saros, 2012).

Plankton dynamics in remote oligotrophic, close-to-pristine state Alpine lakes often depend on glacial origin and on the lithological configuration of their watersheds, as Tiberti et al. (2013) concluded for two lakes in the western Italian Alps. These lakes were naturally fishless and not sensitive to acidification. Different mixing regimes and maximum depth of the two lakes triggered spatial variation of pico-prokaryotes, phytoplankton, and zooplankton. The rapid succession of events in extreme ecosystems confirms the prominent role of external environmental factors such as the duration of the ice-free season.

A European-wide study of 24 lakes, including Alpine lakes, revealed that warm-water fish benefit from a warmer epilimnion in Lake Constance, a reduction in Arctic charr and increase in whitefish and roach in Lake Geneva because of raised mean temperature and phosphorus concentrations, and a reduction in cold-water species and increase in warm-water species in Lago Maggiore due to significant temperature increase in both the epi- and the hypolimnion (Jeppesen et al., 2010, 2012).

Commercial landing statistics were used to describe the long-term changes in the abundance of exploited fish species and to identify mechanisms responsible for fish population changes of Lake Geneva (Anneville et al., 2017). Phosphorus concentrations proved significant for structural changes in the composition of the fish community. Rising temperature in spring increased growth rates of whitefish larvae and improved whitefish recruitment. Climate warming and declining phosphorus concentrations can have synergistic effects on the variability of fish assemblages.

Specific Impacts Through Climatic Change

How locally different extreme weather events and the divide by the Alps can affect lakes are illustrated for the two Austrian lake districts in early summer 2018. The summer 2018 was classified as the warmest and driest year on record, but with large regional differences. The prolonged drought north of the Alps since April 2018 led to record low lake levels in the Salzkammergut region, as illustrated in Figure 4. In the pre-alpine lake Wallersee, the level dropped to 68 cm below the long-term average; in a fjord-type lake in the mountains, the level dropped by only 7 cm. Lake levels in the north declined on average by 34.9 cm. In contrast, the lakes of the Carinthian Lake District south of the Alps were marginally affected. Average lake level there was +8.3 cm, ranging from +31 to –6 cm (see Figure 4).

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Figure 4. Map of Austria with the two lake districts Salzkammergut, north of the Alps, and Carinthia, south of the Alps. The lake levels of the larger lakes are shown as a positive (blue) or a negative (red) deviation (in centimeters) from the long-term average in early summer 2018. The Alpine ridge is symbolically indicated.

Compared to dry periods, flooding seems to occur more frequently in recent years. Floods higher than 1 m above average (150 cm) occurred in 1987, 1991, and 2013, as deduced from maximum and minimum monthly water levels for the period 1976–2020 in Mondsee (Figure 5). Flooding due to extreme precipitation has occurred several times during the past 100 years in Mondsee (Swierczynski et al., 2009), often with large external input as evident from sediment cores.

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Figure 5. Time series of minimum (blue) and maximum (red) monthly lake levels for the years 1976–2020 in Mondsee, Austria. Floods larger than 1 m and their respective month and year are indicated, as well as minimum levels below –25 cm. The low level reaching almost –1 m and lasting from April to December 2018 is clearly visible.

The sensitivity of the annual pattern of species succession of Cladocerans to the heatwave of 2003 differs markedly in two peri-alpine lakes (Anneville et al., 2010). The annual pattern of succession observed in 2003 is not different from the one usually observed in Lake Annecy, whereas the annual pattern in Lake Geneva is uncommon and typified by the persistence of herbivorous Cladocera during summer.

The impacts of extreme temperature events on cyanobacteria were analyzed in five deep peri-alpine lakes covering the entire trophic gradient using a synoptic approach by Gallina et al. (2011). Extreme air temperatures were observed to alter the biomass of the cyanobacteria community. In general, extreme hot events are associated with high biomass, whereas extreme cold events are characterized by low biomass. However, the assessed air temperature extremes did not lead to a dominance of cyanobacteria over the other phytoplankton groups, which also showed responses in relative biomass change during extreme events. Both extreme hot and extreme cold events were seen to generate a loss of diversity among cyanobacteria.

Understanding the effects of storms requires approaches that account for the relations between storm, lake, and catchment, as well as the preceding period and the scale of the meteorological impulse. For Alpine lakes, Perga et al. (2018) have shown that the weather conditions prior to the event are more important than the strength of the storm itself.

Alpine Regions Outside Europe

Based on long-term limnological and meteorological data from the Canadian Rockies, Parker et al. (2008) showed that Alpine lake ecosystems at altitudes >2,000 m a.s.l. are responsive to interannual variation in climate. Although they analyzed years with colder winter temperatures, higher snowfall, later snowmelt, shorter ice-free seasons, and dryer summers, relative to the 1990s, the lakes became clearer, warmer, mixed to deeper depths, and became nutrient-poor. Phytoplankton biomass significantly declined, and small mixotrophic algal species appeared. Further increase in climatic variability in the future potentially poses complications for biodiversity and ecosystem function of high-elevation lakes.

An extensive review of lakes and ponds in the Rocky Mountains of Canada and the United States was provided by Redmond (2018); it includes temperature variability, UV radiation, organic pollutants, nutrient deposition, and biological invasions. Each stressor was evaluated individually and in combination with increasing water temperature. The magnitude of multiple interdependent stressors on Alpine systems may lead to increased primary productivity. The cumulative effects of climate change, multiple stressors, and environmental response are summarized in Figure 6.

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Figure 6. Flow diagram showing the additive effects of increased temperature and drier conditions (orange). Indicated are stressors (light grey) as driving factors of environmental responses (yellow) and community composition and function of alpine lakes (dark blue). Plus signs (+) represent potential non-additive synergistic interactions. Minus signs (–) represent potential non-additive antagonistic interactions.

Effects of climate warming on complex food webs indicate that habitats are affected differently and that mobile consumers react differently to changes. Food chain length increases as air temperatures rise. Cold-adapted predator behavior drives decoupling effects across the climatic range, reflecting flexible food web architecture that may potentially threaten the sustainability of ecosystems (Tunney et al., 2014).

Combined effects of variables on lakes in the southern Rocky Mountains in the United States were modeled by Christianson et al. (2020). Air temperature and alterations in inflow had the largest effect on lake thermal conditions. The least effect came from changes in wind conditions. Larger lakes experienced more than double the increase in lake stability compared to smaller lakes. The lowest stability was found in clear, high-inflow lakes. The largest increase in thermal stability occurred in larger lakes with low inflows and high turbidity. Predicted lake stability using combined changes in variables will increase by 208% when air warms by 2°C and by 318% with a 5°C increment. Under these conditions, dimictic lakes may become more strongly stratified, and polymictic lakes will prolong stratification. Longer and more intense stratification can then result in hypoxia in deep waters.

High-elevation aquatic ecosystems are highly vulnerable to climate change in any region. Preston et al. (2016) showed that ice-out dates have shifted 7 days earlier during the past 33 years in seven Alpine paternoster lakes at elevations between 3,126 and 3,620 m in Colorado. Earlier ice breakup driven mainly by snowfall during spring was associated with increases in water residence times, thermal stratification, ion concentrations, dissolved nitrogen, pH, and chlorophyll a. In essence, these changes have a potential for major shifts in the functioning of high Alpine lakes.

Increasing water temperature and nutrient availability impacted on benthic algal assemblages and ecosystem processes in Sky Pond, a high Alpine lake at 3,322 m a.s.l. in Colorado (Oleksy et al., 2021). Under natural conditions, abundance of Bacillariophyta was significantly greater than that of Chlorophyta and Cyanobacteria. Nutrient enrichment with N and P favored Chlorophyta, resulting in the highest overall algal biomass. Nutrients and warming stimulated heterotrophic microorganisms as well as primary producers.

High-frequency data from Emerald Lake, a long-term study site in the Sierra Nevada of California, were combined with summer lake temperature data from an additional 18 lakes, all typical for glacially scoured mountain lakes. Elevation, lake area, and maximum depth ranges in these lakes were 2,475–3,770 m a.s.l., 0.7–12.6 ha, and 5.1–35.0 m, respectively (Sadro et al., 2019). Average annual air temperatures in the region increased at 0.63°C decade⁻¹. Variation in snowpack size accounted for 93% of variation in summer epilimnetic temperatures among the 18 lakes. The authors conclude that

ecosystem processes in snowmelt-dominated lakes are most at risk from climatic shifts that reduce the accumulation of snow during the winter, extend summer-like conditions into the spring, and delay the onset of cooling in the autumn. As a result, complex lake thermal responses can produce years with cold lake temperatures despite record high air temperatures. (p. 16)

Tropical high-altitude Andean lakes located above the treeline (≥4,000 m a.s.l.) share intense solar UVR with temperate Alpine lakes but lack ice cover during winter and do not stratify during summer. UV-A transparency was assessed in 26 tropical high-altitude Andean lakes from three regions of the Bolivian Eastern Andes (Aguilera et al., 2013). Results indicate that tropical high-altitude Andean lakes are less transparent to UV-A (K_d range = 1.4–11.0 m⁻¹; Z_1% depth range = 0.4–3.2 m) than typical temperate Alpine lakes (1–6 m and 1.3–45 m, respectively). These lakes also differ in vertical profiles of UV-A, chlorophyll a, and temperature, suggesting that they may be a separate category of Alpine lakes.

Large changes in lake area were reported by Liu et al. (2019) for 14 lakes in Central Asia for the period 2001–2016. The total area of Alpine lakes tended to expand due to warmer and wetter climate, whereas lowland lakes declined in area possibly because of human activities and climate change. Expansion and shrinking of lakes were also reported for lakes in the Hindu Kush–Himalaya–Tibetan regions due to climate change and associated glacier retreat (Yang et al., 2019).

Glacial lakes in the Snowy Mountains of Australia and their ice cover were used by Green (2011) to trace climate change. Using a combination of observations and historic photographs, the author was able to document the development of ice breakup for five lakes, substantiating earlier breakup since the 1970 due to global warming.

An integrated understanding of tropical and temperate high-mountain lakes was provided by Catalan and Donato Rondón (2016), who highlight the scientific and social values of mountain lakes.

Which Future for Alpine Lakes?

A summary of the current understanding of lake ecosystem responses to climate change was provided by Shimoda et al. (2011) for six large lakes, including three Alpine lakes—Geneva, Constance, and Zurich. The authors presented details for thermal structure, spring phytoplankton development, summer phytoplankton communities, cyanobacterial dominance, system resilience, and sudden shifts. As a future perspective, more studies on in-lake processes will be needed to understand heterogeneity in responses among different water bodies.

According to the latest climate report (AR6) of the Intergovernmental Panel on Climate Change (IPCC, 2021), the current temperature on the Earth’s surface has increased by 1.1°C, relative to the average from 1850 to 1900. If global GHG emissions continue as before, an increase in global temperature between 2.1° and 3.5°C by 2100 is forecast. Consequently, glaciers in the Alps and other Alpine regions will largely disappear (Haeberli et al., 2019).

Deglaciation of high mountains will soon become an increasing problem. The retreat of the glaciers and permafrost degradation as a result of melting subsurface ice creates new landscapes, new lakes, and unconsolidated morainic material. Newly formed lakes may have potential for water storage or hydropower production but are at risk of sudden water outbursts due to impacts caused by rock/ice avalanches or mass movements. Magnin et al. (2020) discuss the potential formation and the risks associated with the Mont Blanc massif in the Western European Alps, and Haeberli et al. (2016) discuss opportunities and risks of new lakes. Possible future lakes in the Himalaya–Karakorum region are modeled by Linsbauer et al. (2016), and Colonia et al. (2017) discuss the potential for new lakes in the Peruvian Andes. Further new water bodies such as reservoirs, impoundments, or small ponds are created in these landscapes through anthropogenic activities, posing new challenges as aquatic resources (Saulnier-Talbot & Lavoie, 2018).

Global warming and associated extreme weather events will increasingly lead to nutrient inputs into the lakes, resulting in climate-induced eutrophication caused mainly by erosion and export from the catchment by heavy rainfalls or flooding (Dokulil, 2014b; Dokulil & Teubner, 2011). Flood frequency has been analyzed by Fink et al. (2016) for Lake Constance. Quantifying the effects of climate variability on the potential shifts in the flood regime indicates possible changes in deep-water dissolved oxygen concentrations. Warming in the Alps will likely decrease renewal of the deepest layers by approximately 27%; hence, lower oxygen concentrations are expected to degrade deep-water quality. First signs of such deterioration in the hypolimnion were described for Mondsee by Luger et al. (2021). Deep-water oxygen concentrations have significantly declined, and nitrogen concentrations increased, most likely because of terrestrial inputs from the catchment by flood events.

Investigations on warming effects in the deep peri-alpine Lago Lugano using 24 years of monitoring data revealed confined depth of vertical mixing during turnovers when winters were warmer than usual (Lepori et al., 2018). As a result, renewal of phosphorus to surface waters was reduced. This reduction of phosphorus increased the potential for nutrient limitation, but due to warmer winters and summers, chlorophyll a, as an index of phytoplankton biomass, increased because of complex direct and indirect food web effects. These climate-induced deteriorations of water quality or eutrophication processes will need further management interventions to meet the restoration targets.

Extrapolations of long-term data sets and projections into the future indicate that summer LSWTs will increase by approximately 2°C or more until 2050 in Austrian Alpine lakes (Dokulil, 2014a). Depending on the climate scenario used, Lepori and Roberts (2015) predict an increase of up to 6°C for Lago Lugano by the end of the 21st century.

Optimistic scenarios still trust that global society will be able to restrict global warming to less than 2°C with adapted countermeasures, particularly reduction of GHG emissions. The pessimistic approach assumes that most of the world population will not change their habits and lifestyles. Too many countries practice politics of avoidance, particularly those emitting most of the GHGs. Therefore, it seems unlikely that the global society can limit global temperature increase to well below 2°C above pre-industrial levels as formulated in the Paris Agreement (e.g., IPCC, 2021). Global temperatures will continue to rise as long as emissions continue. In 2020, GHG emissions in the atmosphere reached a new record, and the trend has continued in 2021 (World Meteorological Organization [WMO], 2021):

Concentration of carbon dioxide (CO₂), the most important greenhouse gas, reached 413.2 parts per million in 2020 and is 149% of the pre-industrial level. Methane (CH₄) is 262% and nitrous oxide (N₂O) is 123% of the levels in 1750 when human activities started disrupting Earth’s natural equilibrium. The economic slowdown from the COVID-19 pandemic did not have any discernible impact on the atmospheric levels of greenhouse gases and their growth rates, although there was a temporary decline in new emissions.

Conclusion

Climate warming has impacted Alpine lakes at all altitudes at least for more than three decades. The European Alps are particularly affected because mean temperature increment is twice as high as the global average. Continuing warming will certainly affect almost all aspects of any type of mountain lake ecosystem—natural lakes, reservoirs, impoundments ponds, and newly formed lakes due to glacier retreat. Depending on the reduction of GHGs realized in the near future, Alpine lakes will warm above the present level by 2–6°C until the end of the 21st century. Extreme weather situations such as heatwaves, droughts, heavy precipitation, storms, etc. are expected to further increase, and they will impact Alpine regions and lakes worldwide. Based on the expected increase in temperature and the associated impacts on almost all aspects of the ecosystem, together with increasing GHGs and extreme climatic events, the outlook for Alpine lakes of the world is far from good.