Review report

Title: Riverine dissolved organic matter responds to alterations differently in two distinct hydrological regimes from Northern Spain

This study examines the effects of anthropogenic flow alterations, primarily caused by dams, on DOM concentration and composition in Spanish rivers of the Atlantic and Mediterranean region. This research compares rivers with natural and altered flow regimes and looks at how different flow components impact the DOM regime, such that altered Atlantic rivers generally show lower DOM composition shifts compared to natural ones, while Mediterranean rivers appear more resistant to flow alterations, maintaining relatively consistent DOM characteristics.

The study is overall well conducted, relies on a sound empirical basis and uses advanced statistics to identify patterns. The authors introduce the topical background excellently. In that sense I think this is definitely publishable and interesting to the EGU readership. However, there are several issues that I think need some close attention to increase the accessibility and clarity of the study. There are, in my opinion, terminology and reasoning aspects that needs improvement. I hope my suggestions in this regard are helpful.

General comments

Regarding the study concept and abstract, and even for someone who works with DOC, the goals and findings of the study are not easy to grasp. I think this has partly to do with the comprehensive aspiration: the authors do not only want to look into DOM “regime” shifts after flow alterations, but also compare these shifts in two different river system types, and seek for the system properties that are statistically connected to response. This is tough to comprehend, and it does not help that the terminology is at times imprecise and self-defined: DOM “Turnover” is used here differently than in most other contexts (where it essentially means transformation and/or mineralization) – is “compositional shifts” not clearer? I also have problems to understand was is meant by “annual DOM composition” (L12), “temporal turnover indicators” (L256) and several other derivatives of the DOM-related language. I suggest to revisit the part of the work that introduces the terminology use in general, and specifically the analysis goals, concepts and expectations, and harmonize the language related to these. One headline in the results “Linking DOM regimes to flow regimes” could for example be used more often.

Specific comments

L30-32: “This highly reactive fraction…” a reference is needed.

L34 but also temporally (Catalán et al., 2016)… not an adequate citation in that context, because that work really looks at spatial differences of a time-reated property

L53 This rather general model of a DOM regime´s reaction to damming needs fine-tuning… quite a colloquial language for the central part of the study motivation

L55 inflowing DOM concentration: not really the concentration but the amount

L56 I don’t agree that “all” these biotic factors are “associated” with the natural flow regime

L 58, the term “compositional turnover of DOM” needs to be clearly defined, see above general comment.

L63 two naturally defined hydrological classes,.. this is the first appearance outside of the Abstract and the relevance of this concept demands appropriate introduction on first appearance

L65 We expect the effect of flow regime alterations on the DOM regime to depend on certain characteristics of the natural flow regime. … this is an unintuitive research goal, what “characteristics" could this be?

L 161-163, the authors state that the sampling dates to the centroid of a river serves as a measure of temporal turnover of DOM and it is computed as a dispersion. There is not a clear explanation of what this dispersion precicely means and how it is derived. More explanation would be useful.

L259, what are the “temporal turnover indicators”? These indicators are not explicitly defined.

L309 blurry but more encompassing… not sure I understand what you mean here

L 361, “hydropeaking” may need definition or referencing

L477-479, sentence quite long, consider dividing

Fig 2 add aM irrigation

Table 4 sample n and frequency may be a useful information here

Figure 7 flow properties: these should be introduced at one point earlier in the text. Maybe revise the image altogether because it is hard to read and unexpectedly complex for this stage of the manuscript. Why not for example instead of grouping by category, sort by influence, or withdraw from showing *all* influences and select the most significant/important ones. I believe this would increase the interaction with the information massively.

494 subscript CO2

Citations of the Xenopoulos review show up several times. Maybe it is useful to cite original study in some cases?

Unexpectedly, two appointed referees could not provide review reports even after the extended due date. Therefore, Associate Editor provides another required report to expedite the delayed review process. – Ji-Hyung Park

General comments

This study investigates the effects of natural and altered flow regimes on DOM concentration and composition in several rivers across Northern Spain. It is a well-designed study employing a combination of analytical tools and advanced statistical approaches. The key findings on the complex linkages between flow regimes and DOM are novel and intriguing. Although the manuscript is generally well written, it also requires both clarifications of several uncertainties and improvements for a more focused discussion as follows:

1. Hypotheses and key themes: This study examines the effects of altered flow regimes under two different natural settings. As the authors stated (“We hypothesize that DOM properties respond to both natural flow regimes and flow alterations. We expect the effect of flow regime alterations on the DOM regime to depend on certain characteristics of the natural flow regime.”), the concentration and composition of DOM would respond to changes in natural flow characteristics without any anthropogenic perturbations to flow regimes. Therefore, the quite general hypothesis needs to be more articulated with regard to which specific DOM characteristics would respond to which specific characteristics of flow regimes (both natural and anthropogenic). Furthermore, the dams in the studied region appear located in headwaters or upper reaches. Given the fact that flow regime changes vary with dam types and locations, dam effects, as a key theme of this study, need to be explained for the prevailing dam characteristics of the study region.

2. River classification: The authors describe as if they studied 20 different rivers, but actually Fig. 1 shows 20 river locations. Some of them, as tributaries, belong to the same river basins. Around five basins are delineated on Fig. 1. Please articulate how many river basins and their tributaries were studied; and use relevant terms to distinguish mainstems and tributaries. In the same context, the river classifications in L 78-81 need revision. Some of the river sites belonging to two different systems (Atlantic and Mediterranean) are shown shoulder to shoulder on Fig. 1, making me wonder whether these sites really belong to two different “climatic regions”. For instance, look at site Arlanzon. From the names, I expected that these rivers might discharge into either the Atlantic Ocean or the Mediterranean Sea. In my view, the use of these names needs to be confined to indicate something like “Atlantic-type flow regime” after defining it at its first use.

3. Data interpretation: According to Table 4, DOC concentrations are significantly different only between nA and Am. Associated descriptions in Abstract and Results (L 226-230) need to be consistent with the presented results. Although the means were not significantly different, inter-regime differences distinct for some of the river sites (Fig. 4) warrant descriptions in L 226-230, particularly in relation to (any common) specific hydrologic characteristics of these sites. Statistical analyses indicated the dominant role of high-flow periods (L 311-313) in explaining variations in DOM composition. Can this be related to flow-controlled variations in DOC conc.? Regarding the lack of dam effects on DOM composition indices, please refer to specific comments below.

Specific comments

-    Title: Do you mean that…two distinct hydrological regimes “of” Northern Spain?

-    Line (L) 2: What have altered flow regimes? 

-    L 3 and thereafter: Please remove the comma after the subject and check grammatical errors and typos throughout the manuscript including tables and figures (for instance, superscripts such as m2 and concentration units in parentheses).

-    L 6 “hydrological classes”: As suggested by the first reviewer, please use terms consistently; in this case “flow regimes”? Definitions are required for the key terms including flow regimes, DOM regimes, and DOM turnover.

-    L 8: Please do not divide this relatively short abstract into two paragraphs.

-    L 12: Was this turnover rate calculated per unit carbon? Higher DOC concentrations might have led to higher turnover rates, so this needs to be clarified.

-    L 14: What are “unusual DOM behavior”?

-    L 19: Please specify “these” in this complex sentence.

-    L 53 “DOM regime´s reaction to damming needs fine-tuning”: Please rewrite this vague sentence.

-    L 58 “compositional turnover”: Compositional changes, or variations?

-    L 93-94: Please provide more details about the hydroclimatic conditions for these six samplings. Later in Results and Discussion, sampling conditions need to be related to flow regimes and DOM characteristics. It would be helpful if sampling timings are indicated on Fig. 2.

-    L 99-105: Given the importance of DOC quantification and optical characterization in this study, please provide some key QA/QC measures, including blanks, verification standards, references,,,

-    L 218-211: Please describe in more detail about the meaning of and how to read out the magnitude differences. It is very difficult to follow!

-    L 226: It would help understand the temporal variations in DOC conc. if variations in hydrologic conditions among the six samplings were described briefly.

-    L 266-267: Interesting approach! The four regimes overlap considerably on Fig. 5a. Do some outlying sites also exhibit high DOC or any common outstanding hydrologic characteristics? 

-    L 307: Again this ‘turnover’ is an undefined, ambiguous term.

-    L 324-337: The dams studied in this study appear located in headwater streams. Dams in lowland or high-order streams might be quite different in terms of flow regimes. Please consider this stream-order effects.

-    Discussion is very long and repeating some results descriptions, but limited in citing other relevant studies to put the implications of the key findings in a wider regional or global context. Please consider removing repeated descriptions while focusing discussions on key implications and comparisons with previous studies.

-    L 331 (and other places): Please use two instead of 2.

-    L 351: This very long paragraph can be split here.

-    L 373-375: Please don’t repeat the descriptions already mentioned in Results.

-    L 382: not “increase” but “increasing trend”.

-    L 387: How can “hydrological averaging” can increase DOC concentrations? DOC flushing mechanisms explained by Raymond et al. assume higher DOC concentrations at terrestrial sources. Increased DOC conc. in stream downstream of reservoirs should then indicate additional supplies from autochthonous sources.

-    L 388-391 & 398-410: Given the significance of dam effects as a key theme, a more in-depth discussion, including comparison with previous studies on DOM characterization in other dam types (not citing only one review paper), is required to explain the observed patterns in aA and aM.

-    L 392-397: Does the lack of statistical differences in DOM composition indices simply translate into no dam effects on DOM? As suggested before, a more detailed look at large within-regime variations is required to figure out some key characteristics driving the link between dam-induced alterations in hydrology and DOM. For example, you are not addressing the trophic status of your reservoirs, which may be quite different from eutrophic reservoirs.

-    L 408: Yes, but which “factors related to dam nature and operation” are critical for your rivers?

-    L 413-414: Yes, temporality is critical. However, the following descriptions only focus on the statistically analyzed means. As suggested, a brief discussion can be added here to evaluate how hydrologic conditions preceding the six samplings might have (differentially) affected the observed patterns in the four systems.

-    L 455-: Please provide some data or literature information to link these trophic changes to autochthonous DOM production.

-    Section 4.4: The title is not in agreement with the text descriptions. This short paragraph is not redundant? Otherwise, provide a more proper discussion.

-    L 487 “longer residence times average out the naturally high turnover of DOM composition, and send relatively invariable DOM further downstream.”: This conclusion cannot explain the observed increasing DOC trend and is not in line with the following implication “increased metabolism of terrigenous DOM and increasingly higher CO2 emissions in rivers downstream of dams”. From my own experiences in highly eutrophic, impounded systems, CO2 emissions generally increase with the increasing supply of labile DOM (please refer to this review: https://doi.org/10.1016/j.watres.2022.119362).

-    Fig. 1: Given the significance of impoundments in distinguishing flow regime types, I wondered if dam locations could be indicated within each altered-regime subcatchment.