ИСТИНА

Natural waters always contain chromophoric dissolved organic matter (CDOM), which plays an important role in natural biogeochemical processes. Its composition and concentration determine the optical properties of natural water, which affects the functioning of aquatic ecosystems. From the point of view of studying composition and distribution of CDOM, the meromictic water bodies are of particular interest. The meromictic water reservoirs are those with stable vertical stratification, which occurs due to the difference in the density of water layers. The examples of such reservoirs are coastal reservoirs isolated from the White Sea, which density stratification is the result of overlapping seawater with fresh runoff. Natural CDOM effectively absorbs the UV light and emit fluorescence, therefore absorption and fluorescence spectroscopy are widely utilized for its study. The aim of this work is to compare the spectral and luminescent properties of natural CDOM in several meromictic water bodies with sulfide anoxia located on the coast of the Kandalaksha Gulf of the White Sea. These reservoirs differ in the vertical distribution of hydrological characteristics, and due to this, differences in the chemical composition and optical properties of CDOM are possible. To study CDOM, water samples were filtered through nylon filters with a pore size of 0.22 μm before spectral measurements. Absorption spectra were recorded using Solar PB2201 spectrophotometer, fluorescence spectra were measured with Solar CM2203 luminescence spectrometer at excitation wavelength λex varying from 250 to 500 nm. The absorbances decrease along with rising wavelength, with a small "shoulder" noticeable around 260-270 nm due to the presence of phenolic groups or aromatic amino acids. When excited at 270 nm, the CDOM fluorescence spectrum shows two overlapping bands: a "protein-like" band with a maximum at 300-350 nm and a humic type emission band with a maximum at 450-500 nm. The dependence of the wavelength of the spectrum maximum λmax on the λex is non-monotonic, the shift of the maximum of the emission towards shorter wavelengths with a change in the λex from 280 to 310 nm corresponds to the so-called "blue shift" characteristic of natural humic substances. With a further increase in λex, the emission maximum shifts towards longer wavelengths. An informative optical indicator for natural CDOM is the fluorescence quantum yield. Its dependences on the λex in different water bodies have a similar character (maxima are observed at 340 and 370‒390 nm), but the absolute values of the fluorescence quantum yield vary significantly in water layers taken at different horizons, which may indicate the variability in the CDOM structural characteristics. Studies of this kind are important for understanding the mechanisms of formation of CDOM optical properties in natural waters of various origin, studying the habitat conditions of microbial communities through the water column, as well as for understanding the evolution of meromictic water bodies and for developing methods for environmental monitoring of water bodies with sulfide anoxia, including relict water bodies of the Arctic zone. The study was supported by the Russian Science Foundation (grant no. 24-24-00008; https://rscf.ru/project/24-24-00008/).