Effect of permafrost thawing on the organic carbon and trace element colloidal speciation and microbial activity in thermokarst lakes of Western Siberia

Abstract. In order to understand the mechanisms of carbon mobilization and biodegradation during permafrost thawing and to establish the link between the organic carbon (OC), and other chemical and microbiological parameters in forming thermokarstic (thaw) lakes, we studied biogeochemistry of OC and trace elements (TE) in a chronosequence of small lakes that are being formed due to permafrost thawing in the northern part of Western Siberia. About 20 lakes and small ponds of various sizes and ages were sampled for dissolved and colloidal organic carbon and metals and culturable heterotrophic bacterial cell number. We observed a sequence of ecosystem stages corresponding to the evolution from peat thawing and palsa degradation due to permafrost subsidence in small ponds to large, km – size lakes subject to drainage and, finally, the khasyrey (drained lake) formation. There is a systematic evolution of both total dissolved and colloidal concentration of OC and TE in the lake water along the chronosequence of lake development that may be directly linked to microbial mineralization of dissolved organic matter and liberation of the mineral part (Fe, Al, TE) from organo-mineral colloids. In this chronosequence of lake development, we observe a clear decrease of the relative proportion of <1 kDa (1 kDa~1 nm) OC concentration along with concentration of total dissolved (<0.45 μm) OC. This is accompanied by an increase of the small size organic ligands (probably autochtonous exometabolites produced by the phytoplankton) and a concomitant decrease of the proportion of large-size organic (humic) complexes having allochtonous (soil) origin. This evolution may be due to the activity of heterotrophic bacterioplancton that uses allochtonous organic matter and dissolved nutrients originated from the peat lixiviation. Most insoluble TE demonstrate a systematic decrease of concentration during filtration (5 μm, 0.45 μm) exhibiting a similar pattern among different samples. At the same time, there is an increase of the relative proportion of large size particles over <1 kDa fraction for most insoluble elements along the chronosequence of lake evolution. TE are likely to be bound to colloidal OC but also coprecipitated with the mineral (Fe, Al) part of colloids. Upon progressive consumption of dissolved OC by heterotrophic bacteria, there is a liberation of Fe, Al, and insoluble TE in the water column that may be subjected to coagulation in the form of particles or large-size mineral colloids.