Lake Baikal

Lake Baikal is a rift lake in southern Siberia, Russia, lying between the Republic of Buryatia and Irkutsk Oblast. It is generally described as the oldest and deepest lake on Earth and the largest by volume of fresh water. Its age is usually given as 20 to 25 million years, its maximum depth as roughly 1,620 to 1,700 metres, and its volume as about 23,000 cubic kilometres, close to one-fifth of the planet's unfrozen surface fresh water (^[2]; ^[3]; ^[1]). The lake occupies the deepest part of an active continental rift, and its long isolation has produced a fauna in which most species are found nowhere else, including the world's only exclusively freshwater seal. UNESCO inscribed it on the World Heritage List in 1996 under all four natural criteria, describing it as "the most outstanding example of a freshwater ecosystem" (^[1]).

Baikal is a long, crescent-shaped body of water about 636 kilometres long and 79.5 kilometres wide at its widest point, with a surface area of roughly 31,500 square kilometres (^[8]; ^[4]; ^[2]). The basin is asymmetrical, with steep slopes on the western shore and gentler slopes on the eastern, and the indented shoreline runs for about 2,100 kilometres. The lake holds some 45 islands and islets, the largest being Olkhon at about 700 square kilometres, and mountains rise more than 2,000 metres above the water on several sides (^[2]).

Published depth figures differ. Britannica gives a maximum depth of 1,620 metres; the UNESCO listing rounds to 1,700 metres; the United States Geological Survey says "over 1,600 meters" (^[2]; ^[1]; ^[3]). The discrepancy reflects different surveys and rounding rather than a dispute about the lake's rank: each source describes Baikal as the deepest lake in the world. The mean depth is about 758 metres (^[4]).

Baikal fills the central and deepest section of the Baikal Rift Zone, a place where the Eurasian continent is slowly pulling apart. The USGS describes the rift as "a modern analogue for formation of ancient Atlantic-type continental margins" — an early stage of the process by which continents separate and new ocean basins form (^[3]). Beneath the water, sediments accumulated over the lake's lifetime "reach thicknesses in excess of 7 kilometers," and the rift floor may lie 8 to 9 kilometres below the surface, "making it one of the deepest active rifts on Earth." The shallowest sediments may contain the only known freshwater occurrence of natural gas hydrates (^[3]).

The rift is still tectonically active. Hot mineral springs occur along crustal fractures, and severe earthquakes happen occasionally; in 1862 an earthquake inundated about 200 square kilometres in the northern Selenga delta, creating a new bay known as Proval Bay (^[2]).

Because Baikal's sediments were never scoured by continental ice sheets, they preserve an unusually continuous record of past climate. Joint Russian–American coring in the early 1990s recovered cores more than 100 metres long, and analyses of biogenic silica in those cores track glacial and interglacial cycles over roughly 250,000 years, with the potential to extend the record several million years further back (^[3]). See rift valleys for the broader tectonic setting.

More than 330 rivers and streams flow into Baikal, the largest being the Selenga, Barguzin, Upper Angara, Chikoy, and Uda; the lake has a single outlet, the Angara River, a tributary of the Yenisey (^[2]). The USGS states that the lake "contains as much fresh water as the Great Lakes of North America combined" (^[3]).

The water is exceptionally clear and low in dissolved minerals; from the surface an observer can see to about 40 metres, and the water is so fresh that calcium carbonate does not survive in the sediment record (^[2]; ^[3]). Unusually for such a deep lake, the water column is well oxygenated all the way to the bottom, which allows animal life at every depth (^[3]).

The lake's climate is milder than the surrounding land, but winters are long. The surface freezes in January and thaws in May or June; surface water in August is around 10 to 12 °C, reaching about 20 °C in sheltered shallows (^[2]). Photographs from the International Space Station in late April 2014 show the southern basin still largely ice-covered with open water beginning to appear (^[8]). The annual ice is not incidental: it structures the breeding cycle of the lake's seal and the seasonal movement of its fish, as described below.

Baikal's age, depth, oxygenated deep water, and long isolation have produced what UNESCO calls "one of the world's richest and most unusual freshwater faunas," the reason for its nickname "the Galapagos of Russia" (^[1]). Britannica counts 1,500 to 1,800 animal species living at different depths and hundreds of plant species, with the majority endemic (^[2]). A 2016 commentary by the Limnological Institute in Irkutsk and co-authors, summarising the specialist literature, puts the figure higher: more than 2,660 animal and more than 1,000 plant species and subspecies have been described, with roughly 60 percent of the animal species endemic, and it calls Baikal "the most species-rich lake on Earth" (^[5]). The two counts are not contradictory so much as drawn from different taxonomic baselines; the higher figure comes from the institute that maintains the lake's species inventory.

There are about 50 fish species in seven families. The most numerous are 25 species of gobies (sculpins). The omul, a whitefish, is the most heavily fished; grayling, lake whitefish, and sturgeon are also economically important. Unique to Baikal is the golomyanka, or Baikal oilfish, of the family Comephoridae, a translucent deep-water fish that gives birth to live young (^[2]). The two golomyanka species rise from depths of 100 metres or more by day to feed at 10 to 25 metres at night, a vertical migration tracked by the seals that prey on them (^[4]).

The lake's only mammal is the Baikal seal or nerpa (Pusa sibirica), "an exclusively freshwater species" confined to the lake apart from short excursions into its rivers (^[4]). How a seal came to live 1,600 kilometres from the sea is unresolved: the traditional hypothesis is colonisation from the Arctic via rivers and glacial dam lakes 300,000 to 400,000 years ago, but DNA-based estimates put the divergence of the genus Pusa at around 2 million years, "which suggests a complex history" that the paleogeography has yet to explain (^[4]).

The seal's year is governed by ice. Pups are born on the ice from late February to early April in dens that females build and maintain; the ice breaks up in May and June, when the seals moult and gather in groups that can number 1,000 to 3,000 animals on the remaining northern ice. The IUCN assessed the species as Least Concern in 2015, judging the population to be at the ecosystem's carrying capacity of about 80,000 to 100,000 individuals, while noting that reported survey figures range from 82,500 to 115,000 and may carry errors of about 30 percent (^[4]). A canine distemper outbreak killed roughly 6,500 seals in 1987–1988; the virus still circulates. Subsistence hunting of the seal goes back at least 9,000 years, commercial hunting began in the second half of the eighteenth century, and a legal hunt continues, with a 2013–2014 quota of 2,500 animals. As a landlocked, ice-breeding species that cannot relocate, the seal is regarded as vulnerable to future warming that shortens the ice season (^[4]).

Much of Baikal's biodiversity lives on the shallow rocky bottom, where endemic sponges and a large radiation of amphipod crustaceans are conspicuous. The 2016 commentary notes that most of the diversity of ancient lakes is concentrated in their coastal zones, and that at Baikal the greatest species diversity is on substrate in shallow water from about 1 to 50 metres deep (^[5]). More than 320 bird species are recorded in the Baikal area (^[2]).

The lands south and east of the lake are the homeland of the Buryat, the northernmost of the major Mongol peoples, traditionally nomadic pastoralists with a religious life combining shamanic and Buddhist elements. Their territory was ceded by China to the Russian Empire under the Treaty of Nerchinsk in 1689, and more than 440,000 Buryat live in Russia today, many in Buryatia (^[10]). Human use of the lake is far older: archaeological evidence of seal hunting on Baikal extends back to the Neolithic (^[4]).

Russian expansion reached the region in the seventeenth century, with the fortified town of Irkutsk founded in 1652 to the west of the lake (^[11]). The decisive change came with the Trans-Siberian Railway. Construction began in 1891, and the Central Siberian section reached Irkutsk in 1899, but the lake itself remained a barrier: "cargo and passengers had to cross the lake by ferry until a rail line around Lake Baikal was put into operation in 1905." The main line today runs along the lake's southern shore between Irkutsk and Ulan-Ude (^[9]).

Scientific study of the lake is anchored by the Limnological Institute of the Siberian Division of the Russian Academy of Sciences at Listvyanka and the Irkutsk State University hydrobiological station at Bolshiye Koty. Formal protection began with the Barguzinsky Nature Reserve in 1916, followed by further reserves and two national parks in 1986; a coastal protection zone covering about 88,000 square kilometres was created in 1987 (^[2]).

The lake's best-known pollution controversy concerned the Baikalsk Pulp and Paper Mill on the southern shore. Built in 1966, it "drew strong environmental protests from Soviet scientists and writers because its wastes were polluting the water," prompting a 1971 Soviet decree on protecting the lake, though industrial waste at the site remained a concern into the late 1990s (^[2]). The mill halted in 2008, was allowed to reopen in 2010, and in February 2013 the Russian government announced that "a decision has been made to shut down the Baikalsk Pulp and Paper Mill" (^[7]). The shutdown began in September 2013 and was completed in October. Reporting at the time noted that the official grounds were financial rather than ecological, that the mill had employed more than 2,000 people at its peak, and that about 6.2 million tons of sludge remained in thirteen storage reservoirs around the site, with clean-up estimates ranging from 8 to 42 billion rubles (^[6]). UNESCO's inscription decision in 1996 had already noted "concern over a number of integrity issues including pollution" (^[1]).

A different and more recent problem is confined to the shallows. In 2016, scientists led by the Limnological Institute reported that "ecological degradation of the benthic littoral zone is an emerging, urgent problem," describing within the preceding five years the proliferation of benthic algae, death of endemic sponges, mass wash-ups of dead algae and macrophytes, and blooms of toxin-producing cyanobacteria at multiple sites. They attributed the change chiefly to coastal eutrophication from untreated sewage, and stressed that it "has occurred while little change has occurred offshore" — the deep, open lake was not showing the same deterioration (^[5]). Because most of Baikal's endemic species live precisely in this shallow zone, the authors framed the issue as a biodiversity risk rather than a water-quality nuisance. This is a single research group's assessment, published as a commentary; independent long-term monitoring of the littoral zone was one of its explicit recommendations.

Warming that reduces the extent and duration of winter ice would affect the seal's breeding habitat and the lake's food web; the IUCN lists this as a potential future driver of decline while noting the species has persisted through past climate variation (^[4]).

The World Heritage Committee inscribed Lake Baikal in 1996 on the basis of natural criteria (vii), (viii), (ix), and (x), citing its status as the oldest and deepest of the world's lakes, its outstanding variety of endemic flora and fauna "of exceptional value to evolutionary science," and the surrounding system of protected areas. The inscribed property covers 3.15 million hectares, and its boundaries correspond to the core areas defined in Russia's Baikal Law, excluding five urban developed areas (^[1]). The site is one of Russia's most prominent entries among UNESCO World Heritage Sites.

• Baikal seal — the lake's endemic freshwater seal
• Rift valleys — the tectonic setting that created the basin
• Buryatia — the republic on the lake's eastern and southern shores
• Trans-Siberian Railway — the line that opened the region
• UNESCO World Heritage Sites — the listing framework

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