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Bottom sedimentsThe natural and man-made substances found in the bottom sediments of the lake and potentially damaging to the ecosystem were cesium-137, pesticides, phenols, phosphorus and heavy metals (mercury, lead, zinc, copper, cobalt, nickel, etc.). Radioactivity of cesium-137 in the Lake Ladoga sediments varies from 0 to 17 nCi.kg-1. The fields with radioactivity values of 10 to 17 nCi.kg-1 fall into the areas with the most fine-grained sediments - argillaceous silts. According to published sources and official reports DDT is ubiquitous in the lake sediments (content is cited to range from 0.01 to 1.74 ng.g-1). The lake is thought to have several zones where DDT concentrations are over 1 ng.g-1, with their ranges coinciding with the zones where sedimentation currently proceeds rapidly. However recent investigations and calculations show the possibility to misevaluate natural substances as anthropogenic pesticides. Low phosphorus concentrations - below 0.1 % were recorded in the southern shallow areas of Lake Ladoga. The Volkhov bay showed a local abnormality with phosphorus content higher than 0.4%. This was due to phosphorus influx with the river discharge. Considerable amounts of phosphorus are carried to the lake by other tributaries as well. E.g., the river Volkhov is responsible for about 55% of total phosphorus, r. Burnaya - 16%, r. Svir - about 14%, other tributaries - 15%. The content of most heavy metals in the aleuropelitic sediments of Lake Ladoga does not exceed their average concentrations in other large lakes of the world. Total man-made pollution of the Lake Ladoga bottom landscapes generally varies from quite low to moderate reaching the heavy contamination level in some local areas (tab. 8.2.3). Primary pollution ranges concentrate in the estuary zones of rivers and smaller water courses which carry sewage from industrial and agricultural enterprises. The greatest technogenous pollution is recorded near the estuaries of the rivers Volkhov, Svir, Vuoksa, Syass. Secondary pollution areas are found in relatively deep waters and in the zones of minimum hydrodynamic activity of the bottom layer waters tending towards the sites with finely dispersed aleuropelitic sediments. Simultaneously, some areas of the lake are noted for satisfactory or good ecological status of the bottom waters and sediments. The least contaminated parts of Lake Ladoga are the Valaam ridge landscape area and a considerable portion of the West Ladoga near-shore area (fig. 3.3.1.2). |
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