We thank the reviewer for positive evaluation of our work and we provided specific, point by point responses in the attached pdf file.

This paper studies the competitive effects of microbial and light-induced degradation of DOM and trace element (TE) behavior in boreal rivers and lakes. This study addresses the biogeochemical C and TE cycles in boreal aquatic system, which is important for CO2 emission. There are some interesting results found in this study, but I do not think this paper is ready to publish in Biogeoscience at this stage.

We thank the reviewer for evaluation of our work. We carefully revised the manuscript following his/her comments and questions and provided detailed answers to all inquiries.

Overall comments:

1. This paper addresses two parts, C and TE dynamics in biological and photochemical processes. In my opinion, in this paper, no obvious connection can be found between these two parts. The authors did not integrate these the C and TE cycle well. The authors emphasize that there is no previous assessment of coupled C-TE biogeochemical cycles. But I did not find the meaningful to put these two cycles in one paper. What is the connection?

We understand the concern of the reviewer. First of all, we have to state that the part on trace elements (TE) partitioning during bio- and photodegradation represents a strong novelty of the present study. The link between DOC and TE is as following: in humic surface waters of peatlands studied in this work, most TE, which include divalent transition metals (Cu, Ni, Co, Zn, Mn) and toxicants (Be, Cr, Cd, Pb), trivalent and tetravalent hydrolysates (Al, Ga, Y, REE, Ti, Zr, Hf, Th), except probably some alkalis and oxyanions, are strongly (> 80%) complexed to DOM (Pokrovsky et al., 2012, 2016). As a result, any DOM transformation processes, be it bio- or photo-degradation, may directly control the concentration pattern of TE. From the other hand, some TE may be photosensitive (Mn, Fe), toxic (Al, Cu, As, Cd, Pb), or potentially limiting micronutrients (Zn, Co, Ni, Mo) for the bacteria and hence affect the temporal pattern of DOC concentration via affecting the overall rate of photo- or bio-degradation.

Our working hypothesis was that removal of DOM via photo- or bio-degradation will inevitably change the partitioning of trace elements, which are strongly bound to DOM (such as divalent transition metals, micronutrients and toxicants), or incorporated into organo-mineral (Fe, Al) colloids (such as trivalent and tetravalent hydrolysates). The former might either remain in solution (during photodegradation), hence not modifying their total dissolved concentration, or being taken up by growing bacteria during bio-degradation. Both mechanisms of DOM transformation are capable of co-precipitating with Fe and Al hydroxides hence removing relevant trace elements from the aqueous solution.

Note that the entire section 4.3 of the manuscript is devoted to discussion of the impact of DOM bio- and photo transformation on trace element cycling. In case of possibility of revision, this section will be reorganized and restructured around the general principles of trace metal – DOM interaction, following our working hypotheses.

1. The “Abstract” was too long and hard to follow. I am not sure whether Biogeosciences has word limitation for the abstract.

Good point. We shortened our Abstract as following, so it fits the usual format of the journal:

“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) and trace element (TE) transformation in boreal waters.  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: along an ombrotrophic peatbog continuum  and in a stratified forest lake from the same region. Along the bog water continuum, biodegradation rate was the highest in subsurface waters collected via piezometer and the lowest in the acidic peatland pool. Photodegradation was similar for piezometrically collected subsurface waters and the stream, 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. Overall, the 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. 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. A likely mechanism of TE removal was their coprecipitation with coagulating Fe(III) hydroxides.

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 bio- and photodegradation of DOM and organic TE complexes, and call for the need of a systematic assessment of both processes across seasons with high spatial resolution.”

1. P12: What methods you have used for microbial cell counting? At least give the name of the methods, only give the reference is not enough.

Good point; the method is described as following:

Active bacteria number count (colony forming units, CFU mL^-1) was performed using Petri dishes inoculation (0.1 to 1.0 mL of lake water in three replicates) performed in a laminar hood box immediately prior the experimental incubation start and upon each sampling. Samples in duplicates were inoculated on Nutrient Agar (5 g L^-1 beef extract, 5 g L^-1 gelatine peptone, 15 g L^-1 bacteriological agar, pH=6.8±0.2 at 25 °C) to determine the total number of heterotrophic bacteria. Difco@ agar (granulated powder, Lot No 6290083) inoculation was used to assess the number of oligotrophic bacteria. Inoculation of blanks was routinely performed to assure the absence of contamination from external environments.

1. Figure 1: no longitude and latitude in the map.

This is schematic map where only the general boundaries of NW Europe are shown. Detailed position of sampling point is provided in Table 1 as following:

Lake Temnoe: N64.47683°       E041.74533°

1. The language of this paper is not easy to understand, please revise. Some sentences are too long. For instance, P23 Line 568 “Assuming the entire water column studied in this work (10 m depth of Lake Temnoe) participates in DOM biodegradation and CO2 emission, the integral flux amounts to 0.2-0.4 g C-CO₂ m^-2 d^-1 across the seasons”.

We corrected the language in main occasions in the manuscripts. Please note that the text received full proofread by an English speaking scientist (Chris Benker). We do agree that the above given sentence is poorly constrained and we revised it as following:

“Therefore, integral flux from 10 m deep water layer amounts to 17 – 33 mmol CO₂ m^-2 d^-1 across the seasons”

1. There is a input error in P27 Line 661, The ar should be are.

Thanks for catching this; will be fixed at the revision stage.