Characteristics of Kinetic Processes of Organic Matter Transformation in Mesotrophic Lakes of Karelia

In 2012–2013, studies of the biochemical oxygen demand (BOD) kinetics and the composition of organic matter (OM) were conducted on five mesotrophic lakes of Karelia — Salonyarvi, Vegarusyarvi, Valkhomozero, Syamozero and Shotozero. BOD experiments lasting up to 126 days (at 20 and 10оС) were performed in all seasons of the year. As a result, equations and values of the kinetic parameters of the BOD were obtained, characterizing seasonal changes in О2 consumption for OM oxidation in two or three stages (I, II, and L-linear). As a result of the data analysis, a number of common features characteristic of all the studied mesotrophic water bodies were revealed. In all seasons, the oxidation rate of OM at stage L was significantly lower than at stage I. The contribution of each stage to the total oxygen consumption was uneven: the L-stage had the largest (more than 50%) in all seasons except summer (33%), and the I and II stages had the smallest. In summer, due to the active production of easily oxidable OM, the total consumption of О2 at the I and II stages reached 67% of the BODtotal, which was comparable to eutrophic reservoirs. The OM oxidation rate at the I stage increased 4.0 times from winter to summer and then gradually decreased by autumn, and at the L-stage it was several times lower in all seasons than at the I. The obtained correlations between different indicators of organic matter and kinetic parameters of BOD in water from mesotrophic lakes reflect the spectrum of seasonal features of changes in the organic matter oxidation conditions. Decreasing of the labile organic matter oxidation was observed in the following series of lakes: Valkhomozero → Syamozero → Shotozero → → Salonyarvi and Vegarusyarvi which was confirmed by correlation with concentration of autochthonous organic matter.