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УДК: 556.1:556.55 lozovik@nwpi.krc.karelia.ru Transformation and Cycle of Labile Substances and Production-Destruction Processes in Lake Ecosystems Petr A. Lozovik, Alexandr V. Ryzhakov* Northen Water Problems Institute, Karelian Research Centre RAS 5 A. Nevskogo, Petrozavodsk, Karelia, 185030, Russia To assess the rates of chemical substances transformation the kinetic equation for lake systems was :       , where R – retention capacity, k – constant of transformation velocity, τ – flushing period. Minimal values of transformation velocity and its constant were found for large stratified lakes. This is due to specific temperature conditions in such lakes and higher labile substances transformation because of their slow flushing period. The highest transformation rates were estimated on condition τ → ∞ (νOM = 460, ν = 7, νN = 15, ν = 29, νР= 1,3 μg/l year). These values may occur in marine waters and lakes with very high τ. Average constants of rates of nitrogen cycle reactions in natural waters were estimated using kinetics of consecutive reactions of first order and field data: Norg → NH4 (k1 = 0,04), NH4 → NO2 (k2 = 0,35), NO2 → NO3 (k3 = 2,35), NO3 → Norg (k4 = 1,80 day). These constants allowed to explain typical distribution of nitrogen forms in surface waters of humid zone, where Norg form is predominate over the rest. Phosphate turnover rate was estimated by activity of alkaline and acid phosphatases, paranitrophenilphosphate was used as substrate. The determined phosphatase activity allows estimating the time of phosphorus turnover in waters of humid zone (2 – 18 hours). Considering the high rate of phosphorus cycling (several times a day) Pmin appears to be a very labile component and therefore its concentration in surface waters keeps low. Production and destruction values in water bodies determined by the new kinetic model showed that production is always higher then autochthonous organic matter (OM) destruction; difference between these parameters represents the quantity of a newly formed organic matter together with O2 emission into aquatic environment. The production constitutes 59-97 % of total destruction, while allochthonous OM destruction is 3-43 % of total destruction. The water bodies trophicity could be ranged based on primary production value (mg О2/l year) as follows: 5 – 12 – oligotrophic, 12 – 30 – mesotrophic, 30 – 75 – eutrophic, ˃ 75 hypertrophic. Generally, trophic status of a water body determined on the basis of primary production values is similar to that estimated from Рtot. – retention capacity, k – constant of transformation velocity,