Seasonal and spatial pattern of dissolved organic matter biodegradation and photodegradation in boreal humic waters

Abstract. Studying the competitive effects of microbial and light-induced transformation of dissolved organic matter (DOM) and trace metals is crucially important for understanding the factors controlling aquatic carbon (C), micronutrient and toxicant transformation in boreal waters. Here we determined the biodegradability and photodegradability of DOM and their effects on the behavior of dissolved trace metals in humic surface waters from the European subarctic: an ombrotrophic peat bog continuum (subsurface water–peatland pool–stream) and a stratified forest lake across seasons. Along the bog water continuum, the biodegradation rate was the highest in subsurface waters and the lowest in the acidic peatland pool. Photodegradation was similar for subsurface waters and the stream but was not detectable in the peatland pool. The waters of the forest lake exhibited a strong seasonal effect of biodegradation, which was the highest in October and the lowest in June. Overall, the biodegradation was capable of removing between 1 % and 7 % of initial dissolved organic carbon (DOC), being the highest in the forest lake in October and in the peatland pool in summer. 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. Only a few trace metals (TMs) were sizably affected by both photodegradation and biodegradation of DOM (Fe, Al, Ti, Nb and light rare-earth elements (REEs)), whereas V, Mn, Co, Cu and Ba were affected solely by biodegradation. A likely mechanism of metal removal was their co-precipitation with coagulating Fe(III) hydroxides. Compared to typical CO2 emissions from inland waters of the region, biodegradation of DOM can provide the totality of CO2 evasion from lake water surfaces, whereas biodegradation and photodegradation are not sufficient to explain the observed CO2 fluxes in the bog water continuum. Overall, these results demonstrated strong spatial and seasonal variability in biodegradation and photodegradation of DOM and organic TM complexes, and we call for the need for a systematic assessment of both processes across seasons with high spatial resolution.