-This work represents a concerted effort aimed at understanding the microbiological and chemical evolution of a small boreal lake during the diurnal cycle of photosynthesis. We studied diurnal variation of y 40 dissolved macro-and trace elements, organic carbon and bacterial population dynamics in the surface and bottom water layer of the shallow Vilno Lake in the White Sea basin. Four-days continuous measurements with 6 h sampling steps both at the surface (0.5 m) and on the bottom (4.0 m depth) during no-bloom periods revealed constant concentrations (within ¡ 20-30%) of all major elements (Na, Mg, Si, K, Ca), organic and inorganic carbon and most trace elements (B, V, Cr, Fe, Cu, Ga, As, Rb, Sr, Y, Zr, Sb, Cs, Ba, all REEs, Hf, Pb, Th, U). At the same time, the concentration of some biologically important trace metals (Mo, Mn, Co, Cd) was subjected to variations partially reflecting the change of bacterioplankton concentration. This work enables two types of element behavior to be distinguished during photosynthesis in the water column -constant concentration and sinusoidal variations -depending on their speciation in solution and their affinity to aquatic microorganisms.

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The colloidal distribution and size fractionation of organic carbon and trace elements were studied in a seasonally stratified, organic-rich boreal lake, Lake Maselga, located in the European subarctic zone (NW Russia, Arkhangelsk region). This study took place over the course of 5 years in winter (glacial) and summer periods and during the spring and autumn overturn. A newly developed in situ dialysis technique (1, 10, and 50 kDa) and traditional frontal filtration and ultrafiltration (20, 10, 5, 0.22, and 0.025 lm) were used to assess element concentrations at different depths. No significant changes in element concentrations occurred during filtration through sub-colloidal pore-size membranes (20-0.22 lm), suggesting a negligible amount of particulate Fe, OC, and associated trace metals. Large colloids (0.025-0.22 lm) were found to be the main carriers of poorly soluble elements (Fe, Al, Ti, Zr, REEs, Th, and U) during the summer and winter stratification. There was also a clear change in the vertical pattern of the percentage of colloidal Al, Ti, V, Cr, Fe, and Ni during different seasons, and the greatest proportion of colloidal forms was observed during the spring and autumn overturn. This pattern is most likely linked to the dominance of soil (allochthonous) organic carbon, which complexes with trace metals during these periods. During the summer seasons, autochthonous production of small exometabolites or photodegradation increases the concentration of the lowmolecular weight fractions (\1 kDa) that dominate the speciation of divalent heavy metals Electronic supplementary material The online version of this article (in surface horizons. The colloidal status of As (30-60%), which was documented in different seasons along the full depth of the water column, is most likely linked to the presence of organic complexes. The overall results of this study suggest that changes in the colloidal speciation of trace elements with depth in different seasons depend on changes in the redox conditions, the input of soil OM, the biodegradation of plankton biomass releasing dissolved organic matter in the bottom horizons, and in upward diffusion from the sediments.

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Abatract-The results of biogeochemical and microbiological studies of three small lakes in southwestern Arkhangelsk province are presented. The lakes differ in their morphometric characteristics, thermal and oxy gen regimes, and the extent of anthropogenic impact they experience. In the periods of summer and winter stratification, anaerobic water layers with higher phosphates, ammonium, and sulfide sulfur (hydrogen sul fide) are found to form in the bottom horizon of deep water zones of the lakes. The highest concentrations of sulfide sulfur (150-210 µg dm -3 ) were recorded in the shallow Beloe Lake during winter low water period, while in summer, sulfide concentration did not differ from those obtained in other lakes (~10 µg dm -3 ). The abundance of sulfate reducing bacteria in lake bottom sediments varied from 10 to 100000 cell cm -3 , and the rate of sulfate reduction process varied from 29 to 3746 µg S dm -3 day -1 . Seasonal variations were revealed in hydrogen sulfide distribution over the water column and in the rate of sulfate reduction process in the upper horizons of bottom sediments in the examined lakes.

In the conditions of climate warming, the thawing of permafrost can provoke the formation of new thermokarst lakes and subsidence, which facilitates the removal of organic matter from thawed peat into natural waters. Hydrochemical studies of surface waters of the Bolshezemelskaya tundra have demonstrated the exponential dependence of the physicochemical parameters on the size of the water body (peat subsidence, thaw ponds, small lakes, thermokarst lakes). The hydrochemical features of thermokarst lakes of the Bolshezemelskaya tundra are determined by high content of DOC, surface runoff and the thickness of peat deposits. Measurements of concentrations and fluxes of methane showed that all studied water bodies of the Bolshezemelskaya tundra are oversaturated with CH4, and depressions, subsidence, and small water bodies (<100 m2) are characterized by the highest concentrations of DOCs. The contribution of these small reservoirs to the total coverage of the surface of the Bolshezemelskaya tundra area is significant, and their consideration can greatly change the assessment of methane fluxes from the arctic tundra.

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