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Physical Geography of Northern Eurasia

Rivers, Lakes, Inland Seas, and Wetlands

Lakes

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Lakes Ladoga and Onega

The cascade of Lakes Ladoga and Onega are an important component of the vast and complex drainage system of Northern Europe, which consists of over 50 000 lakes, 3500 rivers, and many vegetated wetlands (Nezhikovsky, 1957). The Ladoga catchment covers 260 000 km2, the Onega basin being a part of it. The position of the lakes on the boundary between the Fennoscandian shield and the East European platform predetermines a diversity of drainage conditions and the complex nature of the catchment. The northern location and continentality of climate result in seasonal variations in thermal and hydrochemical regimes. In their natural condition, both lakes are distinguished by stability and high water quality which in the last three decades has been adversely affected by development.

The Ladoga and Onega experience major seasonal variations in temperature. Between December and April, the lakes are usually frozen, although the central parts of the Ladoga stay free of ice approximately once in five years. The water temperature under the ice is 0°C and increases to 3°C at the bottom of the lakes (Petrov, 1990). After the break-up of the ice, the temperature inversion is destroyed in the shallow water where by early May a normal temperature distribution develops. Because the water mass in both lakes is large, heating of the deeper parts progresses slowly and the inverted temperature distribution remains in deep waters until June-July. In spring and early summer, the shallow and deep parts of both lakes have distinctive thermal regimes and water exchange is limited. In summer, a normal temperature distribution occurs, providing for the vertical stability of the water mass. In shallow waters, the temperature can reach 24°C depending on the weather. Horizontal temperature differences are typical because of differential heating and currents and during the upwelling events, which mostly occur on the Ladoga, water temperature on the opposite sides of the lake can differ by 10°C (Petrov, 1990).

The water balance of the lakes is determined by river inflow and outflow, precipitation, and evaporation. About 90 per cent of the inflow into the Ladoga is contributed by its four largest tributaries. The ground water inflow is insignificant and accounts for under 2 per cent of the total (Mandych and Shilkrot, 1995). The outflow occurs through the Neva which discharges into the Gulf of Finland. The inflow and outflow components are the most important ones for both lakes and account, respectively, for 72-75 per cent and 83-85 per cent of the total input and output (Mandych and Shilkrot, 1995). Evaluations of the Ladoga's water balance in the periods of high (1953 — 9) and low (1960 — 78) water phases have shown that their values can vary by 18 per cent (Kirillova and Malinina, 1982). However, both lakes are characterized by slow water exchange, with a water exchange coefficient (defined as the correlation between mean inflow and total water amount) of 0.08 for the Ladoga and 0.06 for the Onega. Interannual variability in inflow and outflow has little effect on the volumes of the lakes. Thus, the extreme low water year of 1940 resulted only in a reduction in 2.5 per cent of the Ladoga's volume, while the extreme high water year of 1962 resulted in a 3.5 per cent increase (Kirillova and Malinina, 1982). Such changes have little effect on the state of the lakes compared to the internal processes. However, the shallow parts of the lakes can be affected strongly.

In their natural state, both lakes have a low salt content and high water quality. The chemical composition of the Ladoga's water is determined by the discharge of its tributaries, the Vuoksa, Svir, and Volkhov. The Vuoksa drains vast areas of the Baltic shield composed by crystalline (mainly siliceous) rocks covered by glacial clays and leached soils which implies a low salt content and good water quality. In the early 1960s, before the development of the region began, the salt content in its water averaged 27 mg l-1 (Mandych and Shilkrot, 1995). The southern tributaries of the Ladoga drain the northern part of the East European plain covered by thick Quaternary deposits and distinguished by active soil formation and leaching of the underlying rocks. The salt content in these rivers is much higher. In the Volkhov, it averages 260 mg l-1 during the low water phase and 70 mg l-1 during the high water phase (Mandych and Shilkrot, 1995). The salt content in the Onega tributaries is low (between 15 and 30 mg l-1). The concentration of organic matter is of the same magnitude which is higher than in the Ladoga. Low concentrations of organic matter and low rates of its breakdown predetermine high oxygen levels which are 91-118 per cent of saturation in the Ladoga (Kalesnik, 1968) and 90-105 per cent in the Onega (Pirozhkova, 1990). In the late 1960s, the contents of nitrogen and phosphorus in the Ladoga were still low. However, by the end of the 1970s the inflow of nutrients into the Ladoga had increased considerably. Thus, the total input of phosphorus increased from about 2400 tonnes a-1 to over 6000 tonnes a-1, and the total input of nitrogen from 13 500 tonnes a-1 to 53 700 tonnes a-1 (Solovieva and Raspletina, 1973; Mandych and Shilkrot, 1995). Water pollution in Lake Onega occurs mainly in the Petrozavodsk and Kondopoga bays, where major settlements and industry are located while across most of the lake water quality still remains high. However, there has been an increase in the inflow of phosphorus from 520 tonnes a-1 in the mid-1980s to 800 tonnes a-1 in the 1990s and of nitrogen from 12 400 tonnes a-11 to 17 000 tonnes a-1. This increase is mainly accounted for by the impact of waste discharge, although the role of precipitation is important for ammonium and nitrates (Pirozhkova, 1990). The content of phosphorous has increased in all rivers flowing into the lakes indicating that human impacts are growing across the entire Ladoga basin (Mandych and Shilkrot, 1995). The local economy focuses mainly on processing mineral resources (phosphorite and bauxite) and production of metals and cellulose. The facilities are environmentally inefficient and their high water consumption and toxic waste have strong impacts on the ecosystems of the lakes, particularly the Ladoga. While the Onega is still largely oligotrophic, eutrophication is intensifying in the Ladoga, which manifests itself through the temporary increases in primary production, slower breakdown of organic matter and cyclic changes in the composition of primary producers. Many scientists believe that the contemporary state of the Ladoga is critical and, should the high input of phosphorous continue, the lake will become eutrophic and lose its role of a reservoir of high quality fresh water (Gusakov and Petrova, 1990; Mandych and Shilkrot, 1995).

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