Size Fractionation of Trace Elements in a Seasonally Stratified Boreal Lake: Control of Organic Matter and Iron Colloids

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???m) 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?C0.22???m), suggesting a negligible amount of particulate Fe, OC, and associated trace metals. Large colloids (0.025?C0.22???m) 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 low-molecular weight fractions (<1?kDa) that dominate the speciation of divalent heavy metals in surface horizons. The colloidal status of As (30?C60%), 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.