Abstract
The article investigates the dynamics of the dissolved oxygen (DO) content, the water temperature, the chlorophyll «a» and the under-ice irradiation in the small mesotrophic lake during the springtime heating period. All data were obtained from long-term observations using high-precision autonomous equipment. In the convectively mixed layer (CML) were recorded the DO oscillations up to 1 mgO2/l with daily frequency. Assumedly, these fluctuations are due to the daytime intensification of the photosynthesis on the background of under-ice irradiation growth (reaching 120 W/m2 when snow melts from ice surface) and the destruction of newly formed organic matter.
 The CML also exhibits the high-frequency DO fluctuations (on scales from 2—3 minutes to 3—4 hours), presumably related to convective currents and seiche activity. The involvement of oxygen-depleted water portions from underlying stratified layer to convective mixing leads to decrease of DO concentration in CML, but the total oxygen content in the water column during the under-ice convection period increases at a rate ~0.1—0.4 gO2/m2 per day due to photosynthesis enhance.
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