High seasonal and spatial dynamics of bio- and photodegradation in boreal humic waters

Abstract. Studying competitive effects of microbial and light-induced degradation of dissolved organic matter (DOM) is crucially important for understanding the factors controlling aquatic carbon (C) transformation in boreal waters. However, studies addressing both DOM and trace element (TE) behavior are limited, which does not allow assessment of coupled C – TE (including macro- and micronutrients and toxicants) biogeochemical cycles in these environmentally important settings. Here we conducted a seasonally-resolved assessment on the degree of DOM and related major and TE transformation under biotic activity and sunlight using conventional incubations of humic surface waters from the European subarctic. We studied bio- and photodegradation over 2–3 weeks in an ombrotrophic peatbog continuum (subsurface water from piezometer – small peatland pool – outlet stream) during July, and in three horizons (0.5, 5 and 10 m) of deep (30 m), a stratified forest lake from the same region during June, August and September.

Along the bog water continuum in July, biodegradation rate was the highest in subsurface waters collected via piezometer and the lowest in the acidic peatland pool (0.17 to 0.03 mg C L^-1 d^-1, respectively). Photodegradation was similar for piezometrically collected subsurface waters and the stream (about 0.3 mg C L^-1 d^-1), but was not detectable in the peatland pool. The waters of forest lake exhibited a strong seasonal effect of biodegradation, which was the highest in October and the lowest in June (0.04 and 0.02 mg C L^-1 d^-1, respectively). The photodegradation of DOM from the forest lake was observed only in June and August (0.19 and 0.07 mg C L^-1 d^-1, respectively). Biodegradation was capable of removing between 1 and 7 % of initial DOC, being the highest in the forest lake in October and in peatland pool in summer. The photolysis was capable of degrading a much higher proportion of the initial DOC (10–25 %), especially in the forest lake during June and the bog stream during July. The change of optical parameters confirmed the highest photodegradation occurs in June (Arctic summer) and demonstrates a decrease of chromophoric (aromatic) compounds during incubation, whereas biodegradation acted preferentially on aliphatic, low molecular weight compounds. Only a few trace metals were sizably affected by both photo- and biodegradation of DOM (Fe, Al, Ti, Nb and light REE), whereas V, Mn, Co, Cu and Ba were affected solely by biodegradation. Typical values of TE removal over a 2-week period of incubation ranged from 1 to 10 %. These effects were mostly pronounced in the less acidic forest lake compared to the bog waters. A likely mechanism of TE removal was their coprecipitation with coagulating Fe(III) hydroxides.

When averaged across sites and seasons, DOM biodegradation and photodegradation processes could remove 5.3 and 10.8 mg C L^-1 y^-1, respectively. Compared to typical CO₂ emissions from inland waters of the region, biodegradation of DOM can provide the totality of C-CO₂ evasion from lake water surfaces whereas bio- and photodegradation are not sufficient to explain the observed fluxes in bog water continuum. Overall, these results demonstrated strong spatial and seasonal variability in DOM and TE complexes bio- and photodegradation, which was poorly accessed until now, and call for the need of a systematic assessment of both processes across seasons with high spatial resolution.