Abstract <p>This study evaluates the potential of spectral characteristics of dissolved organic matter (DOM) as indicators of changes in the dominant type of chromophoric dissolved organic matter in Lake Imandra, including variations in the balance between allochthonous and autochthonous organic matter and the degree of humification of natural waters under the influence of anthropogenic and natural processes in the reservoir. Absorption and fluorescence spectroscopy were used to obtain the spectral characteristics of DOM. The relationships between spectral indices and bulk concentrations of organic carbon, nutrients, and heavy metals were analyzed. Particular attention was given to the effect of anthropogenic sulfate inputs on the light-absorption properties of DOM in surface and near-bottom water layers of the lake. In addition, relationships between water color, humification indices, and the relative contribution of allochthonous and autochthonous organic matter were examined. Although this approach is still relatively underutilized, it allows the degree of humification and the ratio between high-molecular-weight allochthonous humic substances transported from the catchment area and low-molecular-weight compounds formed within the reservoir during production–decomposition processes to be determined with high accuracy.</p>