Seasonal studies of the main organic matter components (fulvic and humic acids, carbohydrates, lipids, proteins, urea) were carried out in 2016–2017. The research was conducted in the oligotrophic and oligohumic central part of Lake Onego, mesotrophic and mesohumic Petrozavodsk Bay, and mouth of the eutrophic and mesopolyhumic River Shuya. The concentration, seasonal distribution and ratio of the main organic matter components were considered depending on the different trophic and humus levels. It was found that the main component of organic matter in Lake Onego was humic substances,whose concentrations ranged from 7.2 to 33.4 mg/l and increased with a rise in the humus status. Their content was minimal in the central part of Lake Onego (68 % on average), and maximal in the Shuya River mouth (81 % on average). The second largest component of organic matter was carbohydrates, which contributed on average 10 %, and their concentration was 0.8–3.0 mg/l. The concentrations of lipids (0.04–0.28 mg/l), proteins (0.02–0.13 mg/l) and urea (0.02–0.09 mg/l) were significantly lower, and their contributions to total organic matter were ~1 %, ~ 0.4 % and ~ 0.3 %, respectively. It was
found that the major constituent of autochthonous organic matter was free carbohydrates (30 %); the proportion of lipids was much lower (11 %), as well as those of proteins (3 %) and urea (2 %). Humic substances were the main component of allochthonous organic matter and fulvic acids prevailed in their composition. The fulvic/humic acids ratio averaged 6:1, and was in congruence with data from other water bodies.