Alpine lakes are classified as lakes at high altitudes in mountainous zones, usually near or above the

tree line The tree line is the edge of the habitat at which trees are capable of growing. It is found at high elevations and high latitudes. Beyond the tree line, trees cannot tolerate the environmental conditions (usually cold temperatures, extreme snow ...

, with extended periods of ice cover. These lakes are commonly formed from

glacial A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...

activity (either current or in the past) but can also be formed from geological processes such as volcanic activity or landslides. Many alpine lakes that are fed from glacial melt have the characteristic bright turquoise or green color as a result of glacial flour, suspended minerals derived from a glacier scouring the

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of be ...

. When active glaciers are not supplying water to the lake, such as a majority of alpine lakes of the

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico ...

in the U.S., the lakes may still be bright blue due to the lack of algae growth resulting from cold temperatures, lack of nutrient run-off from surrounding land, and lack of sediment input. The coloration and mountain locations of alpine lakes attract lots of recreational activity. Alpine lakes are some of the most abundant types of lakes on Earth. In the

Swiss Alps The Alpine region of Switzerland, conventionally referred to as the Swiss Alps (german: Schweizer Alpen, french: Alpes suisses, it, Alpi svizzere, rm, Alps svizras), represents a major natural feature of the country and is, along with the Swis ...

alone, there are nearly 1,000 alpine lakes, most of which formed after the

Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Ma ...

. As global temperatures continue to rise, more alpine lakes will be formed as glaciers recede and provide more run-off to surrounding areas, and existing lakes will see more biogeochemical changes and

ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

shifts. An alpine lake's trophic state (i.e., level of

biological productivity In ecology, the term productivity refers to the rate of generation of biomass in an ecosystem, usually expressed in units of mass per volume (unit surface) per unit of time, such as grams per square metre per day (g m−2 d−1). The unit of ...

) progresses with age (e.g., low productivity after formation and increased productivity with vegetation and soil maturity in the surrounding watershed), but anthropogenic effects such as agriculture and

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

are rapidly affecting productivity levels in some lakes. These lakes are sensitive ecosystems and are particularly vulnerable to climate change due to the highly pronounced changes to ice and snow cover. Due to the importance of alpine lakes as sources of freshwater for agricultural and human use, the physical, chemical, and biological responses to climate change are being extensively studied.

Physical characteristics

Formation

Commonly, alpine lakes are formed from current or previous glacial activity (called

glacial lake A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier. Formation Near the end of the last glacial period, roughly 10, ...

s) but could also be formed from other geological processes such as damming of water due to volcanic lava flows or debris, volcanic crater collapse, or

landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environmen ...

s. Glacial lakes form when a glacier scours and depresses the

as it moves downhill, and when the glacier retreats, the depressions are filled with glacier meltwater and run-off. These lakes are usually quite deep for this reason and some lakes that are several hundred meters deep may be caused by a process called overdeepening. In mountain valleys where glacier movement has formed circular depressions,

cirque A (; from the Latin word ') is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie (from Scottish Gaelic , meaning a pot or cauldron) and (; ). A cirque may also be a similarly shaped landf ...

lakes (or tarns) may form when the water becomes dammed. When damming occurs due to debris from the glacier movement, these lakes are called

moraine A moraine is any accumulation of unconsolidated debris ( regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice sh ...

lakes. These dams of debris can be very resilient or may burst, causing extreme flooding which poses significant hazards to communities in the alpine, especially in the

Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 10 ...

Kettle A kettle, sometimes called a tea kettle or teakettle, is a type of pot specialized for boiling water, commonly with a ''lid'', ''spout'', and ''handle'', or a small electric kitchen appliance of similar shape that functions in a self-contained ...

lakes also form from glacier recession but are formed when a section of ice breaks off from the receding glacier, causes a depression, and then melts. Some alpine lakes reside in depressions formed from glaciers that existed during the last Ice Age yet are no longer proximate to any glaciers and are being sourced from snow, rain, or groundwater. Glacial alpine lakes have dramatically increased in number in recent years. From 1990 to 2018, the number of glacial lakes increased by 53% and the total glacial lake area increased by 51% due to

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

. Alpine lakes adjacent to glaciers may also result in a positive feedback due to decreased albedo of water relative to ice, creating larger lakes and causing more glacial melt. Ice worm glacier

Glacial alpine lakes differ from other glacier-formed lakes in that they occur at higher altitudes and mountainous terrain usually at or above timberline. For example, the Great Lakes of the U.S. and Canada are formed by the retreat of the

Laurentide Ice Sheet The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square miles, including most of Canada and a large portion of the Northern United States, multiple times during the Quaternary glacial epochs, from 2.58 million year ...

during the last

Ice Age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...

which scoured the flat rock surface but are not in the alpine. Conversely, Lake Louise located in the Rocky Mountains was formed from glacial debris damming meltwater (i.e., a moraine lake) from the Victoria Glacier.

Stratification

The annual cycle of stratification and mixing in lakes plays a significant role in determining vertical distribution of heat, dissolved chemicals, and biological communities. Most alpine lakes exist in temperate or cold climates characteristic of their high elevation, leading to a

dimictic A dimictic lake is a body of freshwater whose difference in temperature between surface and bottom layers becomes negligible twice per year, allowing all strata of the lake's water to circulate vertically. All dimictic lakes are also considered hol ...

mixing regime. Dimictic lakes fully mix twice a year between periods of vertical stratification in the summer and winter. Summer stratification is caused by heating of surface waters, and winter stratification is caused by cooling of surface waters below the freshwater temperature of maximum density (approximately 4 °C). Seasonal ice cover reinforces the dimictic stratification cycle of alpine lakes by insulating the lake from wind and warm air in the spring when stratification is generally weaker. Some shallow alpine lakes can become fully mixed multiple times per year through episodic wind or cold inflow events and are therefore considered cold

polymictic Polymictic lakes are holomictic lakes that are too shallow to develop thermal stratification; thus, their waters can mix from top to bottom throughout the ice-free period. Polymictic lakes can be divided into cold polymictic lakes (i.e., those that ...

. A number of meromictic alpine lakes (in which a deep layer of the lake never mixes with surface water) exist.

Lake Cadagno __NOTOC__ Lake Cadagno (Italian: ''Lago di Cadagno'') is a meromictic lake in the Piora valley (canton of Ticino), Switzerland. As one of a few meromictic lakes in Europe, it is the object of numerous scientific studies. The Piora valley - located ...

, located in the Swiss Alps, is meromictic due to natural springs which constantly feed the bottom of the lake with dense, saline water. Other alpine lakes, such as

Traunsee Traunsee () is a lake in the Salzkammergut, Upper Austria, Austria. Its surface is approximately 24.5 km2 and its maximum depth of 191 metres makes it the deepest and by volume largest lake located entirely within Austrian territory; only Lake ...

in Austria, have become meromictic due to salinization from anthropogenic activities such as mining. Recent studies suggest that climate change may impact the annual cycles of stratification in alpine lakes. High altitude regions are experiencing changing seasonal weather patterns and faster warming than the global average. The duration of ice cover on alpine lakes is sensitive to these factors, and shorter ice cover duration has the potential to shift the mixing regime of lakes from dimictic to monomictic (one stratified and one fully mixed period each year). A change in mixing regime could fundamentally alter chemical and biological conditions such as nutrient availability and the timing and duration of hypoxia in alpine lakes. In addition, the relatively small size and high altitude of alpine lakes may make them especially susceptible to changes in climate.

Hydrology

The

hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...

of an alpine lake's watershed plays a large role in determining chemical characteristics and nutrient availability. Sources of water inflow into alpine lakes include precipitation, melting snow and glaciers, and groundwater. Alpine lake inflow often has a large seasonal cycle due to precipitation falling as snow and low glacier melt over the watershed in the winter contrasted with rainfall and increased glacier melt in summer. Alpine lakes are often situated in mountainous regions near or above the treeline which leads to steep watersheds with underdeveloped soil and sparse vegetation. A combination of cold climate over alpine watersheds, shading from steep topography, and low nutrient concentrations in runoff make alpine lakes predominantly

oligotrophic An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates o ...

. Different watershed characteristics create a distinction between clear alpine lakes and glacier-fed alpine lakes (lakes with inflow from melting glaciers).'''' Clear alpine lakes have low concentrations of suspended sediment and

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...

which can be caused by a lack of erosion in the watershed. Glacier-fed lakes have much higher suspended sediment concentrations and turbidity due to inflow of glacial flour, resulting in opaqueness and a bright blue or brown color. The turbidity of alpine lakes plays a significant role in determining light availability for primary productivity and is heavily dependent on each lake's unique watershed.

Circulation

Circulation in alpine lakes can be caused by wind, river inflows, density currents,

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...

, and basin-scale waves. Steep topography characteristic of alpine lakes can partially shield them from wind generated by regional weather patterns. Therefore, smaller scale wind patterns generated by local topography, such as diurnal mountain breeze and katabatic wind, can be important in forcing circulation in alpine lakes. Wind patterns which vary spatially over the extent of the lake may create regions of

upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted ...

and

downwelling Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the ''sinking'' limb of a convecti ...

. River inflow can induce circulation in alpine lakes through momentum carried directly into the lake by rivers or streams and through density currents. If the inflowing water is denser than the water at the surface of the lake (due to differences in temperature or sediment concentration),

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...

drives the heavier inflowing water down the slope of the lake bed or into the lake interior. Such density-driven flows have been recorded in alpine lakes with velocities reaching nearly 1 m/s. Heating and cooling of alpine lakes can cause surface waters to become more dense than the water in the interior of the lake. This results in a gravitationally unstable water column, and the dense water is pulled downward from the surface causing convection. This vertical circulation is an efficient means for mixing in lakes and may play a significant role in homogenizing the water column between periods of stratification. Basin-scale waves, such as internal waves and

seiche A seiche ( ) is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbors, caves and seas. The key requirement for formation of ...

s, can also drive circulation in alpine lakes. Internal seiches in an alpine lake have been observed with attendant velocities on the order of a few centimeters per second.

Ecology

Microbial community structure

Alpine lakes are home to a unique diversity of invertebrates that are highly adapted to the colder and generally harsher conditions of these environments compared to lakes at lower altitudes. A few dominating species have adapted to the

conditions and intense UV radiation, with

chironomidae The Chironomidae (informally known as chironomids, nonbiting midges, or lake flies) comprise a family of nematoceran flies with a global distribution. They are closely related to the Ceratopogonidae, Simuliidae, and Thaumaleidae. Many specie ...

and

oligochaeta Oligochaeta () is a subclass of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms. Specifically, oligochaetes comprise the terrestrial megadrile earthworm ...

comprising almost 70% of the community in two well-studied alpine lakes in northern Italy, and also the two most prominent species (66% and 28%, respectively) in 28 alpine lakes in Austria. Phytoplankton populations are dominated by nanoplanktonic, mobile species including chrysophytes, dinoflagellates, and cryptophytes in the water column, with important contributions to photosynthesis also coming from the algae community attached to substrates, epilithon and epipelon. Viruses are also observed in alpine lakes at abundances of up to 3 x 10⁷ ml⁻¹, which nears the upper range of the general observed abundances from 10⁵ to 10⁸ ml⁻¹ in aquatic systems. When these invertebrate communities are characterized and measured alongside environmental parameters and lake morphology, significant relationships emerge. A study of 28 Alpine lakes found that with increasing elevation, macroinvertebrate abundances increase in small lakes but decrease in larger lakes and community composition shifts with increasing elevation, towards a small number of specialized species. The increasing abundances in smaller lakes as elevation increases are thought to be due to the more extreme temperature regimes characteristic of smaller bodies of water, selecting for a small subset of more robust species that end up thriving from less overall competition. Altitude may also affect community composition due to the change in availability of food resources. For example, at higher altitudes, alpine lakes experience shorter ice-free periods which places a limit on the amount of primary production and subsequent growth of food. In conclusion, both the lake's physical features, including size and substrate, and environmental parameters, including temperature and ice-cover, define community composition and structure, with one study suggesting that temperature and altitude are the primary drivers, and another presenting evidence instead for heterogeneity in lake morphometry and substrate as the primary drivers. The latter, in particular an increase in rocky substratum, was found to negatively impact abundance and species richness. All of these environmental parameters are likely to be impacted by climate change, with cascading effects on alpine lake invertebrate and microbial communities.

Vertebrate community structure

The vertebrate community of alpine lakes is much more limited than invertebrate communities as harsh conditions have an increased impact on the organisms, but can include fish, amphibians, reptiles, and birds. Despite this, many alpine lakes can still host diverse species at these high elevations. These organisms have arrived in several ways through human introductions, ecological introductions, and some are endemic to their respective lakes. The Titicaca water frog in

Lake Titicaca Lake Titicaca (; es, Lago Titicaca ; qu, Titiqaqa Qucha) is a large freshwater lake in the Andes mountains on the border of Bolivia and Peru. It is often called the highest navigable lake in the world. By volume of water and by surface area, i ...

in the high Andes is one species that is endemic, while others were introduced. Lake Titicaca is home to a wide variety of vertebrates, including the Titicaca water frog (''Telmatobious culeous)'', and the endangered

Titicaca grebe The Titicaca grebe (''Rollandia microptera''), also known as the Titicaca flightless grebe or short-winged grebe, is a grebe found on the altiplano of Peru and Bolivia. As its name implies, its main population occurs on Lake Titicaca. Lake Uru ...

(''Rollandia microptera)'' found only in the Titicaca basin. The basin is also home to a variety of bird species and is considered a

Ramsar Site A Ramsar site is a wetland site designated to be of international importance under the Ramsar Convention,8 ha (O) *** Permanent 8 ha (P) *** Seasonal Intermittent < 8 ha(Ts) **

Chilean flamingo, greater yellowlegs,

snowy egret The snowy egret (''Egretta thula'') is a small white heron. The genus name comes from Provençal French for the little egret, , which is a diminutive of , 'heron'. The species name ''thula'' is the Araucano term for the black-necked swan, a ...

Andean coot The Andean coot (''Fulica ardesiaca''), also known as the slate-colored coot, is a species of bird in subfamily Rallinae of family Rallidae, the rails, gallinules, and coots. It is found in Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru ...

Andean gull The Andean gull (''Chroicocephalus serranus'') is a species of gull in the family Laridae. As is the case with many gulls, it has traditionally been placed in the genus ''Larus''. It is found in the Andes in mountainous regions of Argentina, Bo ...

, and the

Andean lapwing The Andean lapwing (''Vanellus resplendens'') is a species of bird in family Charadriidae, the plovers and their relatives. It is found in Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru.Remsen, J. V., Jr., J. I. Areta, E. Bonaccorso, S ...

. Another important alpine lake,

Crater Lake Crater Lake ( Klamath: ''Giiwas'') is a volcanic crater lake in south-central Oregon in the western United States. It is the main feature of Crater Lake National Park and is famous for its deep blue color and water clarity. The lake partly fill ...

located in Oregon, is home to several introduced fish species, native amphibians, and reptiles. The amphibians and reptiles that can be found in Crater Lake are the mazama newt,

northwestern salamander The northwestern salamander (''Ambystoma gracile'') is a species of mole salamander that inhabits the northwest Pacific coast of North America. These fairly large salamanders grow to 8.7 in (220 mm) in length. It is found from southeastern ...

, northwestern ribbed frog, northwestern toad, cascade frog, pacific tree frog, northern mountain lizard, pigmy horned toad, northern alligator lizard, and northwestern garter snake.

Introduced species

Some alpine lakes do not hold any native vertebrate species and instead have grown their vertebrate communities through introduced species. Fish are commonly introduced by humans stocking lakes for recreational and competitive fishing. Crater Lake did not hold any vertebrate species before a stocking event between 1884 and 1941 of 1.8 million salmonids, mainly

Rainbow trout The rainbow trout (''Oncorhynchus mykiss'') is a species of trout native to cold-water tributaries of the Pacific Ocean in Asia and North America. The steelhead (sometimes called "steelhead trout") is an anadromous (sea-run) form of the coast ...

(''Oncorhynchus mykiss)'' and kokanee salmon (''O. nerka''). Other species introduced included

brown trout The brown trout (''Salmo trutta'') is a European species of salmonid fish that has been widely introduced into suitable environments globally. It includes purely freshwater populations, referred to as the riverine ecotype, ''Salmo trutta'' morp ...

(''Salmo trutta''),

coho salmon The coho salmon (''Oncorhynchus kisutch;'' Karuk: achvuun) is a species of anadromous fish in the salmon family (biology), family and one of the five Pacific salmon species. Coho salmon are also known as silver salmon or "silvers". The scientif ...

(''O. kisutch''),

cutthroat trout The cutthroat trout is a fish species of the family Salmonidae native to cold-water tributaries of the Pacific Ocean, Rocky Mountains, and Great Basin in North America. As a member of the genus '' Oncorhynchus'', it is one of the Pacific t ...

(''O. clarkii''), and steelhead salmon (''O. mykiss''). Introduced fish impact the limnetic and benthic communities, as they are the primary prey of non-native fish. Salamanders and newts found at Crater Lake also experienced encroachment on their native habitats and have been reduced or eliminated in numbers. These amphibians were also found in the stomach contents of fish stocked in Crater Lake, which has further reduced populations.

Lake Tahoe Lake Tahoe (; was, Dáʔaw, meaning "the lake") is a freshwater lake in the Sierra Nevada of the United States. Lying at , it straddles the state line between California and Nevada, west of Carson City. Lake Tahoe is the largest alpine lake i ...

, located between California and Nevada, also has several introduced fish species established in the basin due to recreational fishing, including

lake trout The lake trout (''Salvelinus namaycush'') is a freshwater char living mainly in lakes in northern North America. Other names for it include mackinaw, namaycush, lake char (or charr), touladi, togue, and grey trout. In Lake Superior, it can also ...

(''Salvelinus namaycush''), rainbow trout, brown trout, bluegill (''Lepomis macrochirus''), carp (''Cyprinus caprio''), and others. Lake trout, along with an introduced freshwater shrimp, ''

Mysis relicta ''Mysis relicta'' is a shrimp-like crustacean in the Mysida order, native to lakes of Northern Europe and to the brackish Baltic Sea. Appearance Mysis is a small, transparent shrimp-like crustacean, of less than 2.5 cm length. It has tw ...

'', have drastically changed the food web in Lake Tahoe. Nearby Cascade Lake in California, which is often closely studied with Lake Tahoe, does not have any introduced species due to highly restricted public access. Fish were also stocked in Lake Titicaca following the decimation of a native fish population after a fishing competition. Some studies have noted that recreational fishing of introduced species in alpine lakes may have negative effects on the overall ecosystem. Bringing in non-native species, especially to fishless lakes, can also carry pathogens and bacteria, negatively impacting the invertebrate community already there. Studies of two fishless Italian alpine lakes, Dimon Lake and Balma Lake, found that introduced fish brought new viruses and bacteria that were harmful to the native amphibians in the water. The studies also showed that the only way to fix the problem is to completely eradicate the non-native fish species in the lakes in any way possible. These included using gill nets, electrofishing, and continued aggressive recreational fishing.

Climate change impacts

Alpine lake invertebrates are arguably one of the most vulnerable communities of invertebrates to increasing temperatures associated with human-induced climate change, due to the expected increase in ice-free periods and to the relatively small impact of human-changed land cover that other similar terrestrial-aquatic systems have already been subjected to. Cold- stenothermal species uniquely adapted to survive in only a small range of cold temperatures, and larger sexual reproducing species slower to reproduce than smaller, asexual species under perturbations, could both be negatively impacted. Melting glaciers are presumed to increase the sizes of the alpine lakes that are glacier-fed, impacting the evidence size effect on community composition. Also, habitat structure could change in response to an increase in erosion from thawing permafrost, and lastly, an uptick in the frequency and magnitude of extreme weather events would increase water column turbidity, which is well-known to impact the processes of photosynthesis and respiration by increasing light attenuation and decreasing the size of the

photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological pro ...

. Alpine lake ecosystems are undergoing unprecedented rates of change in community composition in relation to recent temperature increases and nutrient loading. Consistent monitoring can help identify, quantify and characterize this ecological impact. One such monitoring technique employs macroinvertebrates as

bioindicator A bioindicator is any species (an indicator species) or group of species whose function, population, or status can reveal the qualitative status of the environment. The most common indicator species are animals. For example, copepods and other sma ...

s primarily to analyze the accumulation of trace elements associated with pollution and to, more generally, track changes in biological communities due to climate change.

Trace elements __NOTOC__ A trace element, also called minor element, is a chemical element whose concentration (or other measure of amount) is very low (a "trace amount"). They are classified into two groups: essential and non-essential. Essential trace elements ...

can occur naturally, but industrialization, including the consumption of fossil fuels, has accelerated their rate of accumulation into alpine lake environments. Though essential to life in low concentrations, some trace elements begin to function as contaminants with over-accumulation. After being released into the atmosphere, trace elements can become soluble through biogeochemical processes and end up in sediment and then mobilized through weathering and runoff to enter alpine lake ecosystems. Benthic macroinvertebrates often at the base of food webs, are the primary accumulators of trace elements, which are then transferred up the food chain to fish or birds via predation. The seminal study that used chironomids as bioindicators due to their abundance in alpine lakes and variety of feeding habits (collectors, shredders, and predators) found that most trace element concentrations are within limits of sediment quality targets, with the exception of Pb in both study lakes and Zn in one, and also concluded that trace element concentrations reflected the relative levels of pollution impacting each alpine lake in the study region of northern Italy. Another study, upon assessing the composition through time of chosen bioindicators and finding evidence for degraded water quality, concluded that the lake ecosystem had moved out of a "safe operational state".

Alkalinity and pH

Alkalinity Alkalinity (from ar, القلوي, al-qaly, lit=ashes of the saltwort) is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength ...

can be defined as the acid neutralizing capability of a body of water. Alkalinity in natural waters is largely due to

bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochemi ...

, the strong conjugate base of the weak carbonic acid, that is the product of rock weathering. Bicarbonate has the ability to act as an acid or a base in water, making it a buffer to resist change from acidic or basic inputs into a body of water. Alkalinity is measured in the unit μeq L⁻¹ which is determined by the concentration of an ion per liter of water multiplied by the charge of the ion or by

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...

. Alpine lakes have been well studied in regard to acidification since the 1980s largely because of the seasonal patterns of alkalinity and pH changes that they naturally exhibit from precipitation and snowmelt. These lakes experience seasonally low alkalinity (and thus low pH), making them highly susceptible to acid precipitation as a result of atmospheric

pollutant A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...

s. The water chemistry of alpine lakes is dominated by atmospheric

deposition Deposition may refer to: * Deposition (law), taking testimony outside of court * Deposition (politics), the removal of a person of authority from political power * Deposition (university), a widespread initiation ritual for new students practiced f ...

(transport of particles between the atmosphere) and catchment processes (drainage of precipitation). The weather patterns of alpine lakes include large periods of snowmelt which has extended contact with soil and rock resulting in increased alkalinity. Weathering of rocks that are calcareous or carbonate based (

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

) are the major contributors of alkalinity to alpine lakes whereas alpine lakes in regions of granite and other igneous rocks have lower alkalinity due to slower kinetics of the weathering. Lower alkalinity indicates a lower ability to buffer the water from acidic or basic inputs so alpine lakes with low alkalinity are susceptible to acidic pollutants in the atmosphere. It is generally accepted that alpine lakes with alkalinity less than 200 unit μeq L⁻¹ are susceptible to acidification.Impact Assessment Working Group. 1981. United States-Canada memorandum of intent on transboundry air pollution. Phase II Interim Working Paper. Washington, D.C.

Alkalinity and pH of the European Alps

The

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...

are the largest mountain range in Europe and home to some of the most well-known lakes. The bedrock in the Alps varies greatly and can be composed of granite, quartz, slate, dolomite, marble, limestone and much more. This diverse geological structure plays a role in the diverse alkalinity of each alpine lake. A study of 73 alpine lakes in the

Eastern Alps Eastern Alps is the name given to the eastern half of the Alps, usually defined as the area east of a line from Lake Constance and the Alpine Rhine valley up to the Splügen Pass at the Alpine divide and down the Liro River to Lake Como in t ...

determined that 85% of the lakes had low alkalinity values (< 200 μeq L⁻¹) with only 2 lakes having an alkalinity above 500 μeq L⁻¹. This study also determined pH and found a range from 7.93-4.80 with 21% of the lakes having a pH below 6.00. The pH in this region was also found to be independent of altitude. A similar analysis was done on 207 lakes, resulting in an alkalinity range from -23 to 1372 μeq L⁻¹ and an average of 145 μeq L⁻¹. The pH was also determined for these lakes, ranging from 4.6 to 9.2. Alpine lakes with a pH less than 6.0 had shown acidic effects on micro-organisms and a pH less than 5.3 was characterized as having reached severe acidification. This analysis was repeated on 107 alpine lakes in the Central Alps with bedrock of silicic and ultrabasic rocks. These lakes had an alkalinity range from 155 to -23 microequiavlents per liter, suggesting how sensitive alpine lakes with similar bedrock might be to acidic rain.

Alkalinity and pH of the Cascade Range

Total Alkalinity of Surface Waters, EPA, 1983

The Cascade Mountain Range extends from Northern California through Oregon and Washington. This region is composed of

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

and

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plat ...

rocks, has heavy seasonal precipitation, and coniferous forests. The Alpine Lakes Wilderness Area in the Washington Cascades has over 700 lakes. Alkalinity for lakes in the Cascade region vary from as high as 400 μeq L-1 to as low as 57 μeq L⁻¹, all of which are considered to be low alkalinity, and suggestive that they might be susceptible to acidification. The pH of these lakes ranged from 7.83 to 5.62, and in this region an acidified lake is considered having a pH below 4.7. The Cascade Range was further evaluated by subregions since the environments vary greatly. Lower alkalinity, 50-100 μeq L⁻¹, was observed in regions with little soil and

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies un ...

rocks, like that of Glacier Peak Wilderness and Mt. Rainier. Higher alkalinity, 200-400 μeq L⁻¹, was observed in regions composed of

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

and

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...

such as the Western Cascades.

Palaeoclimatological data

Paleoproxies In the study of past climates ("paleoclimatology"), climate proxies are preserved physical characteristics of the past that stand in for direct meteorological measurements and enable scientists to reconstruct the climatic conditions over a longer ...

are chemical or biological sources that serve as indicator data for some aspect of the climate and can help reconstruct past regional climates and the future fate of alpine environments. Alpine lakes themselves are unique reservoirs of paleoclimate data, particularly for understanding climate in the late

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million year ...

, as they collect and store

geomorphological Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or ...

and

ecological Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...

data in their

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

. These records of the past allow for a better understanding of how alpine lakes have responded to climate variability. Thus, by understanding these mechanisms of the past, better predictions can be made about the future response of alpine ecosystems to present-day climate change. The fraction of mineral

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...

(P) to organic P within lake sediments can be used to determine if the sediment deposits are sourced from glaciers ''(''higher mineral to organic P ratio'')'' or debris slopes ''(''lower mineral to organic P ratio'')''. Therefore, the sediment P content can inform glacial activity and thus climate at the time the sediment was deposited. For instance, an alpine lake in the

Coast Mountains The Coast Mountains (french: La chaîne Côtière) are a major mountain range in the Pacific Coast Ranges of western North America, extending from southwestern Yukon through the Alaska Panhandle and virtually all of the Coast of British Columbi ...

of British Columbia revealed cooler and wetter conditions due to the increased trend in mineral-rich (glacial-derived) P sediments which agrees with other findings of cooling in the early

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

. Magnetic properties of the sediment in alpine lakes can also help infer glacial activity at a high resolution. When the magnetic properties of the lake sediments match those of the bedrock, it can be deduced that there was more glacier movement, i.e., cooler temperatures. Along with the sediments being "detrital" (bedrock weathering), the sediments are more coarse-grained indicating high glacial activity associated with the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

Diatom A diatom ( Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising se ...

assemblages reveal changes in

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...

conditions and alkalinity which help infer changes in temperature and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

concentrations over time. During periods of warmer temperatures, extended growing seasons led to more benthic plant growth which is revealed by more periphytic (substrate-growing) diatom species. After the onset of the

Industrial Revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...

, diatom assemblages revealed more acidic conditions which are associated with higher carbon dioxide concentrations. Aside from the alpine lakes themselves serving as a source of paleoclimate observations, the surrounding alpine zone also contributes many useful proxies such as tree ring dynamics and geomorphological features.

References

{{Reflist Wikipedia Student Program Lakes by type Limnology