Allochthonous and autochthonous carbon in deep, organic-rich and organic-poor lakes of the European Russian subarctic

To reveal the degree of allochthonous vs. autochthonous control on carbon dynamics in deep, boreal lakes, we studied two (one organic-rich and one organic-poor) small (~0.1 km2) and deep (~40 m), seasonally stratified lakes located in the European subarctic zone (NW Russia, Arkhangelsk region) during a 2.5-year period. The dissolved organic and inorganic carbon (DOC and DIC, respectively) concentrations were 15–30 mg l–1 and 0.5–1.9 mg l–1 in the humic lake and 0.8–4.3 mg l–1 and 18–52 mg l–1 in the organic-poor lake, respectively. The DOC profile in the organic-poor lake was sensitive to phytoplankton blooms in July–August and snowmelt in May, whereas the organic-rich lake was a highly stable system throughout the year in both the epilimnion and hypolimnion, with significant (ca. 40%) increase in DOC concentration in the hypolimnion relative to the epilimnion. The ratio of dissolved organic carbon to organic nitrogen (Corg/Norg) was significantly lower in the organic-poor lake, reflecting a strong impact of autochthonous Norg production and N diffusion from the sediment to the water column. There was a clear difference in the vertical pattern of the proportion of the low molecular weight (LMW< 1 kDa) organic carbon (OC) between the humic and the organic-poor lakes. Regardless of season, the organic-poor lake’s LMW< 1 kDa was 40%–90%, whereas that of the organic-rich lake 20%–30%. This difference was most likely linked to the dominance of peat soil and bog providing allochthonous OC to the organic-rich lake throughout the year, and possible production of LMW autochthonous organic ligands by phytoplankton and photodegradation in the organic-poor lake. The important role of allochthonous organic matter (OM) in the color properties of the lake water was confirmed by the light absorbance in the visual range and specific ultraviolet absorption measurements. Overall, deep humic (organic-rich) lakes are expected to be less sensitive to external impacts and possible climate change. The evaluation of both number and depth of humic lakes in the boreal zone is crucial for assessing their CO2 emission and carbon storage potential.