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

Abstract

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.

Small Boreal Lake Ecosystem Evolution under the Influence of Natural and Anthropogenic Factors: Results of Multidisciplinary Long-Term Study

“…The lake exhibits a deep zone in its southern (relatively pristine) part, with minimal inlets having anthropogenic influence. Little human activity has been present near this southern part of the lake over the past 50 years, so the studied part of the lake can be considered almost pristine, similar to other lakes in the region [12]. In contrast, the northern-most bay (Maslozavod) is subjected to significant anthropogenic pollution from both local industry (milk and butter factory) and local inhabitants of approximately 30 houses, since there is no sewage system in the Rotkovetz village.…”

“…Maximal winter stratification is in March, and the highest water temperature typically occurs in July. The length of the ice-covered period of the lakes is between 195 and 201 days; the average freeze over date is 25-30th October and the average ice out date is 12-14th May [12]. Mean seasonal water temperatures over the full period of observations on the deepest stratified station of the lake are illustrated in Figure 2B,C.…”

“…The international geostandards SLRS-4 and SLRS-5 (Riverine Water Reference Material for Trace Metals certified by the National Research Council of Canada) were used to check the validity and reproducibility of each analysis (see refs. [10][11][12] for analytical pH was measured in the field using a combined electrode calibrated against NIST (National Institute of Standards and Technology, Gaithersburg, MD, USA) buffer solutions (pH = 4.00 and 6.86 at 25˝C). The accuracy of pH measurements was˘0.02 pH units.…”

“…DOC and dissolved inorganic carbon (DIC) concentrations were measured using methods routinely used in the Geosciences and Environment Toulouse (GET) laboratory to analyze boreal water samples [10,12]. The nutrient determinations were based on colorimetric assays [13,14].…”

“…The international geostandards SLRS-4 and SLRS-5 (Riverine Water Reference Material for Trace Metals certified by the National Research Council of Canada) were used to check the validity and reproducibility of each analysis (see refs. [10][11][12] for analytical details). There was good agreement between our replicated measurements of SLRS-5 and the certified values (relative difference <15%).…”