Review of Dordoni et al. “Mineralization of autochthonous particulate organic carbon is a fast channel of organic matter turnover in Germany’s largest drinking water reservoir”

Synopsis

The central focus of this manuscript is to investigate the preferential turnover of different carbon phases into DIC in the Rappbode Reservoir in Germany. For this aim, the authors analyzed 13C and concentration of DOC, autochthonous and allochthonous POC, sedimentary OC and DIC in different seasons. Based on variations in ¹³C and concentration among different carbon phases, the authors used an isotope mass balance to calculate potential C-contribution of OM to DIC pool and compared it to DIC concentration changes to determine most plausible source of OM turnover in metalimnion and hypolimnion. The authors found that autochthonous POC is main contributor to DIC increases and further calculated its turnover rate to be 0.01 to 1.3 μmol L^-1 d^-1, within the range for oligotrophic water bodies.

Overall, this is well-organized and clearly written manuscript. The data, discussion and conclusion are intuitive for the most part. That said, I do have some minor to moderate comments and suggestions that I think will improve the manuscript. I outline these below, with line numbers where appropriate. Once these changes have been made, I fully support publication of the manuscript in Biogeosciences.

L12-13: It may be better to use “phases” instead of “sources” here. Some abbreviation seem to make me confusing, e.g., POC and ExtPOC. Could the authors just use “auto-POC” and “allo-POC” to name these two differently-sourced POC.

L14-16: I am confused about which is the “this purpose”. Does it refer to the first sentence of the Abstract? Could the authors change it to “For eliminating the influence of atmospheric exchange, we …….” ? Also, the Abstract could be improved to be more concise and logically clearer.

L32: In the introduction part, a description of characterization of metalimnion and hypolimnion seems missing, because CO2 exchanges from atmosphere and soil are also important sources of DIC.

L110: The description of d13C could be simplified, as it is a basic parameter to the audience in this field.

L130: Instead, the isotope mass model could be explained in more details, e.g., how the equation (2) was deduced？The variations of d¹³C_DOC and d¹³C_POC should be plotted in the main text, as they are important for the manuscript.

L175: The Results part looks very discrete with 6 paragraphs, even some paragraphs are only composed of 3-4 sentences. Please revise the part to be more simplified and concise.

L231: Please add several sub-titles to discussion part to make it more readable.

I think while this study is interesting the paper could do with restructure. The results are almost like bulletpoints and could be merged with the discussion to give it logic and context. I also am not convinced by the mass balance end-members. 13C isotopes vary so widely in freshwater that they overlap with terrestrial values. While the data show that there is differences between POC and DOC, there is enough overlap to reduce the conclusions that the authors state. I don't know if the endmembers the authors used for the calculations are right, and therefore, I'm not sure that the interpretation is correct. If the end-member is correct (the authors should jusitify this) and the descrition of the isotope methods was made clearer the paper could be accepted with minor revisions. If not, calculations should be revised and reconsidered after major revisions.

I would prefer if the isotopes were plotted on their own with permil axis rather than the way they are plotted - it is hard to see the real scatter/overlap

I also think that the terms that the authors use should be explained as they are not used in all countries. I have made comments in the attached pdf and hopefully these will help. Overall, I think the premise is good but I'm not convinced that the data shows what the authors conclude. some parts should be simplified and others should be explained properly. I think that the tables in the supp. information should actually be in the main article or isotopes plotted by their own.