Boundaries

The Lena River ecoregion encompasses the drainage areas of continental Arctic rivers from Anabar eastwards to Khroma, as well as the Novosibirskiye Islands. In the west, the border of the ecoregion goes along the Syuryakh-Dyangy Range and Anabarskoye Plateau (the divide between the Popigay River [Taimyr, 607] and Anabar, and then between the Kotuy River headwaters [607] and Olenek). Further southward, the Lena drainage area is contiguous with the Yenisei drainage area along the Berezovyy Range, Ilimskiy Range, and the western spurs of the Srednesibirskoye Ploskogorie Upland. To the west of Lake Baikal, the divide between the Barguzin River (Yenisei drainage [605]) and the Tsipa River (Vitim River tributary) lies along the Ikatskiy Range. The Cherskogo Range divides the Karenga River drainage (Vitim River tributary) and the Nerch River drainage (Shilka River tributary [619]). Further in the northeast, the Cheromnyy, Zapadnyy Lyundor, and Urushinskiy ranges divide the Tungur River (Olekma River tributary) and headwaters of Shilka [619] and Middle Amur [617]. The Stanovoy Khrebet Range forms the border between the Aldan River drainage area and the Zeya River (Middle Amur [617]) and the Uda River [615]. The eastern border of the ecoregion with the Sea of Okhotsk Coast [614] coincides with the Dzhugdzhur Range, as well as the southern part of the Indigirka River drainage [609] along the Suntar-Khayata Range. Further in the north, the drainage areas of the Yana and Indigirka rivers [609] are separated by the spurs of the Cherskogo and the Polousnyy Kryazh Ranges.

Topography

The Anabar River is a large river in northwestern Yakutia. Its catchment area is situated north of the Arctic Circle. The river is 939 km in length, with a catchment area of 104,461 km².

The Olenek River is 2,270 km long with a catchment area of 219,300 km². The river network has 18 tributaries with catchment areas anywhere from 1,200 to 47,700 km². The largest tributaries are the Arga Sala (554 km in length), the Pur (501 km), the Siligir (344 km), and the Ukukit (347 km) rivers. The upper Olenek extends from the river’s source downstream 1530 km into Arga-Saly.  The middle Olenek extends from this point downstream 899 km to the confluence with the Sukhana River. 

The Lena River derives its waters from an extensive catchment area of 2,486,000 km². It is Russia’s second-largest river by volume and third largest by catchment area. Average annual discharge of the river is 488 km³. The length of the river from its outflow up to the eastern point of the delta area, Bykovskii Cape, is 4,270 km. The Lena forms a well-developed delta where it flows into the Laptev Sea.

The Lena River has around 70 large tributaries; the largest (Aldan, Vilyui, Olekma, Vitim, and Kirenga) contribute 70% of the river’s discharge.

Because of its hydrobiological characteristics and ichthyofauna composition, the Lena River is split into upper and lower parts. The upper portion of to the river runs to the Vitim River outfall through a narrow valley compressed by mountain ranges, while the lower part flows from Aldan outfall to Stolb Island. At the confluence of the Vitim River and the Lena River, the river widens to 2 km and reaches depths of 2.6 m. Upstream of Buotoma River, the Lena River enters the Yakutsk-Vilyui depression, which splits the river’s main flow into arms and anabranches. In this area, the total width of the river and its islands is 10-15 km, while the width of the main channel reaches 3 km. The Lena River receives its largest left tributary—Vilyui River—approximately 160 km downstream from its confluence with the Aldan River. A nearby region called Forty Islands gets its name from a widening flood plain, abundant shoals, deep pits, and predominance of low swampy and sandy banks covered by shrubs and trees.

While the Lena River’s depth ranges from 2 to 12 m, its average flow velocity stays fairly constant at 4 to 6 km per hour. As it flows through a wide valley towards the Latitude of Dzhardzhan, the river’s width is 2—6 km, the depth of its channel is 15 to 20 m, and the flow velocity is roughly 0.6 m per sec. The Lena River reaches a depth of 28 m in the area from Kyusyur Peninsula to Tit-Ary Island—a relatively barren region devoid of islands and developed vegetation. The delta around Stolb Island—an area nearly 45,000 km²–has pebbly or weakly silted ground cover. The Trofimovskaya River—which receives 60% of its water from the Lena River—and the Bykovskaya Channel (used for navigation) are two of the Lena’s largest anabranches.

The Yana River—the product of the Dulgalakha and Sartanga rivers—has the largest slope (15 cm per 1 km) of all the ecoregion’s rivers. The river is 906 km long with an average annual discharge of 32 km³. The Adycha, Bytantai, and Olde rivers are the largest of the Yana River’s 89 tributaries. Situated in the mountainous eastern Verkhoyansk region, the Yana River’s northern drainage is made up of tundra and lakes. In contrast, there are few lakes in the central and southern drainage regions.

Freshwater habitats

Below the steep, mountainous Ebe-Khai banks of the Anabar River, the river bottom is covered by large pebble and stones, and dominated by sand in the river’s lower reaches.  The Anabar River reaches depths of 7—10 m, and while the hade of the river channel is about 30 cm per one km, areas with rapids often display inclinations of more than 1 m per 1 km. The valley of the Anabar widens into hummocky tundra in the North Siberian lowlands, where the river inclination is less than 6 cm per 1 km. River depths vary abruptly, rising to 20 m in reaches and declining to 0.5—0.6 m on rifts.

The Olenek River flows into Olenek Bay, where—according to average summer data—ice melts completely by July 25 before re-freezing in early October. There is a distinct wind phenomenon that helps determine fish dispersal in feeding migration areas. River flow pushes against boundary sea water, and allows freshwater fish to inhabit littoral areas rich in food.

The Yana River’s ice breaks up in late May through early June, before freezing over in the first third of December. The river’s current ebbs during winter months while the majority of rifts remain frozen in the spring—leaving a chain of reaches in the river.  Downstream from the Ust-Yansk region, the Yana River splits into many anabranches and a delta with an area of 10,240 km².

Mountain and submountain rift areas exist in the upper reaches of the Lena River’s main tributaries.  These regions are characterized by high flow velocities, and a river bottom dominated by stone and pebble-stone; these areas also serve as ideal spawning grounds for many anadromous fish. As the Lena system reaches into the plain, flow velocity declines notably.  The rivers meander considerably under strong backwaters as they move through valleys with several lakes and bayous.  The amount of precipitation dictates the rivers’ regimes, and water level fluctuation peaks during autumn and spring. As a result of spring floods, shoals are warmed and favorable for reproduction of spring spawning fish. Large flood plains are actually completely flooded during periods of high water rise. When spring floods taper off, a large number of lakelets and pools persist as homes for young fish. These young will migrate into the river during the autumn floods, before a period of ice (up to 2.8 m thick in some areas) dominates from roughly October through May. During summer months, high air temperatures and long daily sun exposure favor warming waters—particularly in lakes and bays—and rapid development of phyto- and zooplankton.

Terrestrial habitats

The Anabar River begins its journey in the northern part of the Middle Siberian Plateau, and flows through a relatively narrow valley with larch forest tundra. Lakes cover only 0.67% of the land due to poor Olenek River drainage, while swamps dominate nearly 6.3% of the total land area.

The Lena River catchment area—from the mountain taiga up to the tundra—is comprised of well-developed flood plain water bodies, swampy lowlands, and delta zones.

Description of endemic fishes

Endemic fish include the salmonids Coregonus baunti and C. skrjabini, and possibly another Coregonus sp. (found in Baunt Lakes). Yakutian char (Salvelinus jacuticus) is also endemic.

Coregonus baunti is a rare species distributed only in select Baunt lakes. This lacustrine whitefish is distinct from all other European and Siberian whitefishes of the group C. lavaretus sensu lato, especially in terms of its spring spawning, plantovorous feeding habits, and certain morphological characteristics.

Coregonus skrjabini is markedly different from C. baunti by some morphological characteristics. Both Baunt whitefishes show evidence of divergence in a lake region.

Coregonus sp. is a form of lacustrine ryapushka (group of species C. albula/sardinella sensu lato) isolated in the Baunt Lakes region by some 1000 km from the nearest populations of the Lena River migratory C. sardinella. Defining features of the species are spring spawning habits, a strictly lacustrine lifestyle, and a number of unique morphological features.

Salvelinus jacuticus is an endemic char from lakes of the Lower Lena. It appears endemic, though its taxonomic status needs clarification; there are a number of lacustrine chars inhabiting lakes of the delta area.

Other noteworthy fishes

The gudgeon, Gobio soldatovi tungussicus, is an endemic subspecies typical of the Amur River drainage. It is hypothesized that this fish penetrated into the Lena basin during a glacial epoch when the basin was linked with headwaters of Amur in Transbaikalia. Further taxonomic clarification is needed.

Near-endemics include the sturgeon, Acipenser baerii chatys, and Davatchan (Salvelinus alpinus erythrinus).

Acipenser baerii chatys is one of the Lena’s most valuable fish. Both adults and juveniles inhabit the delta channels and estuarine areas with salinities below 13-15%.  Spawning follows an upstream migration in July, and occurrs over riffles near Tit-Ary Island.

The name Salvelinus alpinus erythrinus is usually reserved for deepwater chars of Siberian lakes in Transbaikalia.  The current name may encompass a number of different species, or simply a lacustrine ecomorph of S. alpinus sensu lato.

Ecological phenomena

Ichthyofauna of the ecoregion is represented by a number of ecological groups: lake, lake-river, river, anadromous and brackish water semi-anadromous. The distribution of ichthyofauna can be divided into five river zones: avandelta, delta proper, the lower reaches, the middle reaches, and the upper reaches. The avandelta zone and the freshened sea coastal areas adjoining it are inhabited by marine fishes tolerant to considerable fresh water; it serves as the feeding area for all populations of nelma, sardine cisco (Coregonus Sardinella), arctic cisco (C. autumnalis) and muksun (C. muksun). The river delta zone serves as a permanent area of habitation of nelma, sardine cisco, arctic cisco, and ecomorphs of muksun. Here, brackish water semi-anadromous and anadromous fishes develop and grow to maturity. The zone of the lower reaches (with most pronounced valley features) serves as the major breeding area of whitefishes and some populations of nelma and sturgeon. Coregonids are the most abundant, although cyprinids are not numerous in this zone. The middle reaches zone is characterized by a predominance of grayling, pike, lenok, and cyprininds, and serves as the main breeding area for nelma. The upper reaches zone is inhabited mostly by cold-loving rheophiles, lenok, grayling, Eurasian minnow (Phoxinus phoxinus), and bullheads. In the same area eurythermal cyprinids (Leuciscus leuciscus and Carassius carassius) are common.

Justification for delineation

This ecoregion shares many similarities with the ecoregions situated in the west—namely Ob [602] and Yenisei [605], which help form the Siberian district of the Arctic marine province. However, ecoregions [605], [602], and [608] are regarded as subdistricts of the Siberian, west Siberian, middle Siberian, and east Siberian districts. Drainage of the Lena River and its neighboring rivers is characterized by less-developed fauna in comparison with ecoregions [602] and [605], which have more rigorous climates.

Level of taxonomic exploration

Good