Optical characteristics of CDOM in meromictic water bodies (White Sea) and their relationship with salinity

Abstract

Chromophoric dissolved organic matter (CDOM), a coloured fraction of dissolved organic matter (DOM), is an important component of aquatic ecosystems, as it affects a spectral composition of sunlight passing through water. Optical characteristics of CDOM provide information about quantity, origin and processes of transformation of CDOM in aquatic systems. Inverse relation between absorption in UV range (which characterises concentration of CDOM) and salinity is usually observed in coastal areas: high values of optical density correspond to low salinity and vice versa. Optical properties of CDOM in two meromictic water bodies of Kandalaksha Gulf on the White Sea (lakes Elovoe and Trekhtzvetnoe) are studied in this work. Increase of salinity with depth is observed in these unique hydrological objects: surface water is fresh or lightly salted, relict sea water is located down below. Water samples were taken with submersible pump on various depths in summer of 2025 and 2026. Optical indexes were calculated by absorption spectra measured with spectrophotometer Solar PB 2201. While salinity increased with depth, absorbances and optical indexes, as a rule, were changing gradually, which characterises conservative mixing of fresh and sea water along the salinity gradient. However, in some cases abnormal behavior of spectral-optical characteristics of CDOM was observed. Such behavior is explained by photobleaching of CDOM, formation of autochthonous CDOM or accumulation of it in bottom layer. Understanding reasons of changeability of optical properties of CDOM is of key importance when solving problems like remote sensing, research of carbon cycle and ecological monitoring of coastal waters.

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Anastasia I. Deeva
Faculty of Physics, Lomonosov Moscow State University, Moscow, Russia
Russia

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