Waters

Only part of the water incoming to Ladoga accumulate in the lake basin proper. The rest of the water (less minor losses due to evaporation and groundwater outflow) is discharged through the river Neva to the Gulf of Finland. Mean annual discharge from the lake through the Neva is about 74 000 10⁶ m³.

Water temperature in Lake Ladoga fluctuates greatly through the year. The highest mean water temperatures are recorded in August (+15.7^oC), the lowest (equal to zero) - from February to March.

According to the temperature-based classification Lake Ladoga belongs to the so-called dimictic lakes with two full mixing periods, in the spring and autumn, as well as direct stratification in the summer and reverse stratification in the winter.

Most of the Lake Ladoga catchment area falls within the taiga zone dominated by podzolic soils. Considerable areas are water-logged. This makes the lake water yellowish and predetermines its relatively low transparency. Mean water transparancy in Lake Ladoga is 2.5 m.

In spring the greater part of Lake Ladoga is involved in the general cyclonic circulation, with the exception of the shallow southern part with two anticyclonic gyres. Similar situation is observed in the summer and autumn. Maximum surface current velocities reach 25 cm^.sec^-1. As the lake grows deeper the general circulation direction remains the same, whereas the current velocity decreases. At the 50-m depth the maximum current velocity in spring is only 3-4 cm^.sec^-1.

Density-driven currents are also active in the lake. They form along the thermobar front and cover the whole of the lake. During the thermobar period runoff moves anticlockwise (Filatov, 1991). By the end of June - beginning of July however anticyclonic circulation forms in the southern part of the lake, which promotes the spread of runoff from southern tributaries in the clockwise direction (to the Neva river headwaters).

With regard to the dominant salt components the water in the lake can be classed as predominantly bicarbonate-calcium. Sulphate and chloride waters, which are less typical for freshwater bodies, are common in the near-shore zones. Most of the waters in the bottom layer belong to the chloride-magnesium and sulphate-sodium types.

The average nitrogen content in the Lake Ladoga water in spring is 0.59-0.83 mg^.l^-1, in summer - 0.53-0.74 mg^.l^-1, in autumn - 0.55-0.66 mg^.l^-1. Phosphorus content in various zones of the open Ladoga in the ice-free period ranges from 12 to 28 µg^.l^-1 with a mean currently being 17 µg^.l^-1. Phosphorus content in much enclosed bays in the northern skerries area, exposed to heavy anthropogenic load, is notably higher (up to 70 µg^.l^-1)

Some papers describing measurements of the chlororganic compounds and pesticides in the lake water indicated high concentrations of man-made toxic substances both in the near-shore and the deep-water parts of the lake. The results of the latest investigations suggest however that previous analyses were based on insufficiently reliable data which took natural substances, primarily the products of phytoplankton function, for anthropogenic toxins.

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