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The purpose of this paper is to compare trophic characteristics of the ecosystems of small and shallow lakes with a different character of land-cover in their catchments (as exemplified by several previously unstudied lakes of the Leningrad Region, North-Western Russia, that belong to a single lake-river system).

The key limnetic parameters of four lakes are analysed. Two of the lakes are located on the territory of allotment gardens, the other two are in the forest-covered areas. A preliminary assessment was made for the production-to-destruction ratio in the ecosystems of the lakes of the study region and their vulnerability factors.

For the lakes with a largely unexploited catchment, humus of terrigenous origin can act as a “hidden” source of nutrient load (primarily as phosphates). For the lakes with a catchment occupied by allotment gardens, an elevated trophic status and intensive overgrowth by vegetation (floating forms) is driven by an increased nitrogen load.

The results can be used for planning land and water management activities in North-Western Russia and in other world’s regions with similar environmental conditions.

These results can lay a foundation for creating a region-specific model to predict trends in eutrophication and overgrowth of small shallow lakes.

There is no clear answer to the question as to what kind of humic substances (HSs) (fulvic acids (FAs) or humic acids (HAs)) is primarily responsible for colour of water in water bodies. To resolve this problem, the purpose of this paper is to compare optical properties of the samples of river water with solutions (considered as standard) containing HAs and FAs in different concentrations.

The author sampled two typical unpolluted rivers of Karelian Isthmus (Leningrad Region, North-Western Russia) with different water colours – Vyun with light-brown water and Kivioja with dark-brown water.

It is revealed that the shape of the light absorption curve for both rivers is similar to that of FAs. Among the coefficients, a 364 * is unsuitable for differentiation between HAs and FAs due to its wide transgression. In contrast, the E₅₄₀ coefficient allows one to separate these groups of HSs very well. The E₅₄₀ values of the river water samples categorise them as FAs. The values of a 440 * indicate that river waters, especially from Kivioja, contain FAs with a large molecular weight in comparison with soil FAs (up to 1.75-fold higher). It shows a distinct character of river water HSs, which is consistent with the published data.

The results outline a more direct approach to decision support in the sphere of ecological monitoring of river water.

The author has concluded that river waters are coloured by FAs.