Freshwater lakes

Lakes are bodies of water not directly connected to the ocean and lacking the typical flow of a river or stream. Though there is no scientifically agreed upon distinction between lakes and ponds, lakes are considered larger. Some lakes contain salt water (saline or brine lakes), but freshwater lakes are more common. Freshwater lakes are also crucially valuable to many species, including humans, that rely on a supply of potable water.

The water present in freshwater lakes makes up approximately 0.01 percent of the total water on the surface of the Earth. By volume, this is approximately 29,989 cubic miles (125,000 cubic kilometers), in contrast to the oceans, which contain 328.68 million cubic miles (1.37 billion cubic kilometers). The freshwater lake water mass is distributed among over 920 large named lakes and countless smaller lakes over all the continents of the world.

Major Freshwater Lakes

While all lakes share certain characteristics, many lakes also have features that make them unique biomes. They are typically key elements in the larger ecosystem of a region, providing habitat and resources for a wide range of plants and animals. Many of the larger freshwater lakes in the world are also vitally important to humans, including as a source of drinking water. Lakes can be formed in a variety of different ways, and depending on hydrological conditions may last for varying lengths of time on a geological time scale.

The oldest lake in the world is considered to be Lake Baikal, located in the southern Siberian region of Asian Russia and an estimated 25 to 30 million years old. The lake was formed through tectonic activity that formed the Baikal Rift; it is still active and growing at a rate of 1 inch (2.5 centimeters) per year. At its present dimensions (about 395 miles, or 636 kilometers long; 30 miles, or 48 kilometers wide; and 0.5 miles, or 0.74 kilometers deep), Lake Baikal holds the greatest volume of freshwater of any biome on Earth, at 5,670 cubic miles (23,614 cubic kilometers), which represents one-fifth of the freshwater on Earth. Lake Baikal is also the deepest lake on Earth, with a maximum depth of greater than 1 mile (1.6 kilometers). There are roughly 300 rivers that feed Lake Baikal, but only one, the Angara River, drains the lake. The lake is home to over 2,600 known species of flora and fauna, including the Baikal seal (Pusa sibrinica), which feed on the deep-living Baikal oil fish (Comephorus baicalensis and C. dybowskii). The lake is also home to the salmonid omul (Coregonus automnalis migratorius), which is an important food source to the Indigenous population of Buryat people who live on the eastern side of the lake. Lake Baikal was designated a World Heritage Site by the United Nations Educational, Scientific and Cultural Organization (UNESCO) in 1996.

Lake Tanganyika, located in Africa and surrounded by Tanzania, Zambia, the Democratic Republic of the Congo, and Burundi, is considered the longest freshwater lake in the world, with a length of approximately 420 miles (676 kilometers). Lake Tanganyika is the second-largest lake in the world, with a freshwater volume of 4,534 cubic miles (18,900 cubic kilometers). It is also the second-deepest lake in the world, following Lake Baikal at almost 1 mile (1.5 kilometers) deep. Because of its length and depth, the lake does not mix below a depth of 656 feet (200 meters), and is anoxic at these depths; therefore, all biological activity occurs above the 656 feet (200 meter) depth. Lake Tanganyika was formed tectonically as a rift lake, situated in the southwestern part of the African Great Rift Valley. It is one of seven Great Rift Valley lakes that include Lakes Victoria, Malawi, Turkana, Albert, Kivu, and Edward. Lake Tanganyika is fed by two main rivers, the Ruzizi River entering the lake from the north, and the Malagarasi River that feeds the lake from the east.

Lake Tanganyika drains into the Congo River drainage system via the Lukuga River on the western side of the lake. The lake is home to 250 species of cichlid fishes that have been the focus of studies on speciation and other evolutionary research areas. Numerous cichlid fish species are popular in the aquarium trade industry, and other fish species are important as a food source. Commercial fishing on Lake Tanganyika has been in operation since the 1950s. But as in other regions, the fishing industry has been in decline. However, attempts to manage the resources of the lake have been implemented since 2004 through a Water and Nature Initiative program organized by the International Union for Conservation of Nature (IUCN) to monitor physical, chemical, and biological conditions of the lake.

Lake Superior is the largest of the Laurentian Great Lakes of North America, and is surrounded by Ontario, Canada, from the north, and Minnesota, Wisconsin, and Michigan's upper peninsula from the south. Lake Superior is the third largest freshwater lake in terms of volume, holding about 2,800 cubic miles (11,600 cubic kilometers),of water, but it is the largest by surface area (31,800 square miles, or 82,414 square kilometers). The geologic formation of Lake Superior is from scouring of the land surface as the glacial ice sheets retreated at the end of the last ice age, also known as the Pleistocene Period, approximately 10,000 years ago. This makes Lake Superior one of the youngest major lakes in the world. The other Laurentian Great Lakes, as well as most of the major lakes of North America from the temperate latitudes northward, were formed in a similar manner at the end of the Pleistocene Period. Lake Superior is fed by approximately 200 rivers, and drains into Lake Huron by way of the St. Mary's River. There are approximately 80 species of fish in Lake Superior. The lake is considered oligotrophic because of the low nutrient content and low primary productivity in the water column. The shores of Lake Superior were first inhabited at the end of the Pleistocene Period, around 10,000 years ago, by the Plano, who hunted caribou in the region. They were followed by the Archaic, Ojibwe, Cree, and Laurel peoples. The Ojibwe called Lake Superior Gichigami, or "big water."

Lake Titicaca is the largest freshwater lake in South America, with a surface area of 3,232 square miles (8,372 square kilometers) and a volume of 214 cubic miles (893 cubic kilometers). At an elevation of 2.37 miles (3,811 meters), Lake Titicaca is the highest navigable lake in the world, and lies between Peru and Bolivia on the eastern side of the central Andes Mountain Range. The lake has five major rivers that feed it, the Ramis, Coata, Ilave, Huancané, and Suches Rivers. There are several small tributaries that also feed into the lake from the mountains; however, there are no drainages from Lake Titicaca. The lake consists of two subbasins that are connected by the Strait of Tiquina. The large subbasin is called Lago Grande, and the smaller one is Lago Piqueño. Lake Titicaca was formed during the Oligocene Epoch of the Paleogene Period, about 23 million years ago, through strike-slip movement as the Andes Mountain Range was forming. The lake is inhabited by people living on floating reed islands or Uros that can be moved throughout the lake. There are about five islands that are inhabited on the lake, some by native Quechua speakers. Lake Titicaca has about thirty endemic species of fish: twenty-eight cyprinodonts and two catfish. The Titicaca Orestias killifish (Orestias cuvien) became extinct after the introduction of the rainbow trout (Salmo gairdneri) in 1942 and the silverside (Basilichthyes bonariensis) in the early 1950s for commercial purposes. Lake Titicaca is home to large populations of waterfowl, earning it status as a Ramsar Site in 1998.

Lake Ladoga is located in European Russia, just north of St. Petersburg, and is considered the largest freshwater lake in Europe, and fourteenth-largest lake in the world. The lake has a surface area of about 6,907 square miles (17,891 square kilometers) and a volume of about 200 cubic miles (837 cubic kilometers). Lake Ladoga was formed by scouring as the glaciers receded after the last Ice Age, between 12,500 and 11,500 years ago. There are five major rivers that feed Lake Ladoga, and it drains to the Gulf of Finland to the southwest via the Neva River. In the Middle Ages, Lake Ladoga was part of the trade route between Scandinavia and Greece. In modern times, overfishing in the 1950s and 1960s led to stringent regulations to help recovering resources, which rebounded by the 1970s to 1990s. There are over forty species and subspecies of fishes in the lake, including important commercial salmonids. The lake has been exposed to anthropogenically mediated eutrophication since the 1960s, from intense industrialization and agriculture within the watershed. Lake Ladoga is home to a subspecies of ringed seal known as the Ladoga seal.

Lake Murray is the largest lake in Papua New Guinea and Oceania, with a surface area of about 250 square miles (650 square kilometers) and a maximum depth of 33 feet (10 meters), though it can expand to five times the area during the monsoon rainy season. The lake is formed in a shallow depression in the land relief and is fed by five major rivers and drains to the Gulf of Papua via the Herbert River. Lake Murray is home to many waterfowl, designating the lake a Ramsar Site since 1987. Basalt Lake is a small freshwater lake on Byers Peninsula on Livingston Island in the South Shetland Islands, Antarctica. It is formed in a depression surrounded by volcanic outcrops, and drains to the south into Bransfield Strait.

Limnology of Freshwater Lakes

Limnology is the study of inland freshwater bodies and is derived from the Greek limné, which means "lake." It includes the study of rivers and streams or lotic (flowing) systems and lakes and ponds or lentic (standing) systems. George Evelyn Hutchinson (January 30, 1903–May 17, 1991) is considered the father of limnology. He published the series A Treatise on Limnology in four volumes from 1957 until 1993. As outlined in that work, lakes can be divided into physical zones. The nearshore or littoral zone covers areas of lakes where there is light penetration to the bottom. The offshore or limnetic zone is the open water pelagic region of a lake. The profundal zone occurs in deep lakes and is the region below where light penetration occurs. The benthic zone is on the lake bottom and includes the region of water sediment interface. Limnological studies of freshwater lakes cover the physical, chemical, and biological nature of lakes and their interactions.

Physical studies in a lake are concerned with aspects of circulation, that is, vertical and horizontal structure and dynamics. These studies are important to determine the relationship between temperature and density that leads to determining vertical stratification within a lake. Depending on the surface area–depth ratio and the climatic regime of the geographic location, a lake may be permanently stratified, for example, the very deep Lake Tanganyika of the African Great Lakes; on the other hand, shallow lakes with large surface areas, for example, the central basin of Lake Erie of the North American Laurentian Great Lakes, is continuously mixed.

The vertically stratified sections of lakes can be divided into the upper epilimnion, middle metalimnion, and bottom hypolimnion. The epilimnion is where relatively warm water can mix vertically in the presence of wind shear on the surface, where the majority of the pelagic primary productivity takes place because of exposure to light penetration. The metalimnion is also known as the thermocline, where a sharp difference of a short vertical distance in water temperature will occur. This is the density boundary between the upper, warm, less dense lake water and the lower, cold, denser lake water. The cold, dense, lower lake water is known as the hypolimnion. This layer is still and stagnant, typically devoid of oxygen or anoxic, and rich in nutrients because the biological activity taking place above it in the epilimnion eventually sinks into the hypolimnion as it decomposes. If most of the microbial decomposition takes place before the organic matter reaches the hypolimnion, it oxidizes into nitrates and phosphates that can be easily recycled back into the lake biosphere; however, much of the organic material does not completely decompose to the completely oxidized inorganic form, and remains in a reduced form until exposed to oxygenated or aerobic conditions.

Monomictic lakes in temperate zone latitudes or higher, which lose heat over the winter by freezing over completely or partially, and are also considered holomictic, will mix completely from top to bottom once a year during the spring season. The mixing brings nutrient-rich bottom water to the surface and oxygen to the bottom, an important event that drives the biological activity for the lake. Dimictic lakes will mix twice a year; this typically takes place when the water temperatures are relatively low, which would be during the spring and autumn, separated by a summer stratification period. Polymictic lakes mix several times over an annual period, not necessarily based on seasonal patterns. Polymicitic lakes are typically shallow, with large surface areas that are exposed to persistent wind shear.

Chemical studies in limnology focus on biologically active chemicals dissolved in lake water. Nutrients that include nitrogen, phosphorus, and silica are important constituents in the cellular structure of aquatic plants—both vascular and single-celled organisms known as phytoplankton. The productivity of a lake is limited by the concentration of nutrients available to plants to utilize for growth. In freshwater lakes, the limiting nutrient is phosphorus. The trophic status of a lake can be defined by the concentration of phosphorus in the lake, typically measured as total phosphate, which includes both inorganic phosphate and organic phosphorus molecules in living phytoplankton cells and particulate and dissolved organic material. Oligotrophic lakes hold total phosphorus content of less than 12 milligrams per liter; these freshwater lake systems are considered pristine, healthy biomes. An example of an oligotrophic lake is Lake Superior of the North American Laurentian Great Lakes.

Mesotrophic lakes have a total phosphorus content range between 12 and 24 milligrams per liter. These freshwater lake systems are considered moderately productive and considerably healthy. Lake Ontario of the North American Laurentian Great Lakes is considered mesotrophic. Eutrophic lakes have a total phosphorus content range of 24 to 96 milligrams per liter. The process of eutrophication may occur naturally; however, in many cases these freshwater lakes are influenced by anthropogenic activity through agriculture or industry. For example, Lake Ladoga in Russia has been eutrophicated since the 1960s, likely because of intense industrialization and agricultural efforts within the watershed. Hypereutrophic lakes have total phosphorus content greater than 96 milligrams per liter. These lakes have intense phytoplankton blooms, low water clarity, and are mostly devoid of oxygen near the bottom. The phytoplankton blooms in hypereutrophic lakes are usually cyanobacteria, such as Aphanizomenon flos-aquae, as found in Upper Klamath Lake in southern Oregon.

Other important chemical aspects of freshwater lakes include salinity and acidity. Various processes can raise the salt content of otherwise freshwater lakes, and studies have shown that human activity—particularly the salting of roads to melt snow and ice—has greatly increased lakes' salinity in some regions. This change has major environmental implications, including the contamination of drinking water sources. Another concern is increasing acidity of freshwater bodies, which is linked to rising carbon content associated with anthropogenic climate change. While rising acidity is also a concern in saltwater bodies, a 2018 study found that the rate of acidification in freshwater lakes was triple that of the oceans.

Biological studies in limnology include plankton dynamics in limnetic zones of lakes; these include phytoplankton and zooplankton, their dynamics and interactions, and their importance in the freshwater lake food web. In the littoral zone, higher aquatic plants are important constituents of the freshwater lake biome as shelter and food sources for aquatic organisms from aquatic insects to fishes, waterfowl, and aquatic mammals. Benthic organisms occupy the bottom habitat from the littoral zone to the deep dark regions of lakes. These habitats are occupied by mollusks, annelids, crustaceans, and benthic fishes.

Freshwater Lake Inhabitants

Freshwater lake biomes can contain species of phytoplankton from all major groups. The Chlorophyta or green algae include desmids and flagellated Chlamydomonas; most are considered edible and nutritious for zooplankton. The Chrysophyta are divided into the flagellated golden Chrysophyceae and diatoms or Bacillariophyceae, which are considered highly edible by zooplankton. Members of the Chrysophyceae can be colonial such as Dinobryon or singular cells like Ochromonas. The Bacilliariophyceae or diatoms are either centric or pinnate in shape. An example of a centric freshwater diatom is Cyclotella and a pinnate is Navicula. The Pyrrophyta or dinoflagellates are flagellated unicellular organisms that are considered mixotrophic, with the ability for autotrophic or heterotrophic production, for example, Peridinium. The Eugleophyta can be autotrophic or phagotrophic, and some classify the group as Euglenozoa. An example of a euglenid with autotrophic capability is Euglena. The Cryptophyta are flagellates that are considered highly edible by zooplankton, for example, Cryotomonas. The Cyanobacteria are considered primitive autotrophic bacteria, without a cellular nucleus. This group can cause nuisance blooms under high phosphorus conditions and low nitrogen concentrations, where species like Anabaena can fix nitrogen from dissolved N2 gas.

The zooplankton community of the freshwater lake biome consists of Protozoa, Rotifera, and Crustacea that include Cladocera and Copepoda. The Protozoa include small and large ciliates that are important in the microbial loop part of the food web, and mostly feed on bacteria and microbially processed particulate organic matter. The Rotifera are herbivores that feed on the small phytoplankton species, and are an important food source for carnivorous zooplankton. An example of a rotifer is Asplanchna. The dominant zooplankton group in freshwater lakes is the Crustacea. Dominant within the Crustacea are the cladoceran daphnids, which are herbivores. An example of a cladoceran daphnid is Daphnia. There are carnivorous clodecerans, such as Bythotrephes longimanus. The Copepoda include herbivore and carnivore species among this group. An example of an herbivore calanoid copepod is Diaptomus. An example of an omnivorous cyclopoid copepod is Cyclops.

The benthic fauna consists of macrobenthic organisms, including mollusks such as clams (Corbicula) and mussels (Dreissena). These are important filter feeders and have the ability to influence the trophic structure and dynamics of a lake by reducing the standing crop of the phytoplankton community. The meiobenthic community consists of organisms that range in size from 0.1 millimeter to 1 millimeter in length, and include crustaceans like harpacticoid copepods and ostracods, as well as nematodes. The meiobenthos are important in the microbial loop and contribute in the processing of organic materials into inorganic nutrients. Microbenthos organisms are key players in the microbial loop and remineralization process. These organisms are less than 0.1 millimeter in size and include bacteria, diatoms, ciliate, amoeboid, and flagellated protozoans.

Water in its freshwater form is an important resource to humans and other organisms on the major global continents. The freshwater lake biome is critical to the ecology and sustainability of life within the terrestrial environment. The pattern of distribution of this resource is dependent on the major global climatic regimes. Since the global system is dynamic, the climatic regimes are constantly in flux and shifting, and these shifting patterns will cause shifts in the distribution of freshwater resources.

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