Comments to Authors

This is an interesting dataset describing longitudinal gradients and long-term trends in POC and DIC for the Elbe Estuary.  The findings from an analysis of these data would be of interest to estuarine ecologists and to the broader community studying the global C cycle.  My main concern with the paper is that there are many aspects that are either not well explained, or insufficiently explained.  This issue pervades key components of the paper including conceptualization of the study, description of methods and inferences made from the data. 

Methods: the level of detail in explaining methods is highly uneven among the various components of the study.  For example, the means for calculating air-water CO2 fluxes is highly detailed, whereas other equally important elements (e.g., POC loads and DIC mineralization) are hardly described at all in the main body of the paper.  These fluxes, and how they are derived should be explained more fully and at an early stage.  For example, in the Abstract there is reference to the “spring internal DIC load” (line 30), but at this stage, the leader is not likely to understand what this is (remineralization of organic C) or how it was determined.  Also, I did not see an explanation of how POC was measured (perhaps I missed this).

Introduction: the first paragraph focuses on C cycling in estuaries, with a mention of eutrophication.  The second paragraph focuses of climate change, and specifically, the occurrence of drought.  The third (final) paragraph does not link the ideas presented in the first and second paragraphs in such a way as to provide a clear direction for the paper.  There should be some consideration (prediction) of how estuarine C cycles may be affected by drought.  My initial reaction was that drought would reduce external (watershed) inputs of POC and DOC to the estuary.  I was surprised that there was no consideration here, or elsewhere, of the importance of allochthonous organic matter inputs, particularly as this is the driving mechanism accounting for the excess of C mineralization relative to autotrophic fixation in estuaries (see e.g., Hoellein et al. 2013).  The third paragraph should provide some clear expectations of the direction of the paper from a conceptual point of view, which the stated objectives fail to do.  Also, there should be some consideration of allochthonous C inputs and how these may have changed over time (and in response to drought).

The reliance on Rewrie et al. in review, here, and at many points throughout the paper, is not helpful, particularly as these are vague references to “ecosystem recovery”, “major shifts in ecosystem state” and “amelioration of water quality”.  It is not clear to the readers of this paper what these changes are, and what implications they may have for C cycling in the Elbe.  There is a subsequent statement that these changes include a reduction in BOD and an increase in NPP.  The former implies that the changes may have to do with improved wastewater treatment resulting in reduced organic matter inputs to the estuary.  But if that is the case, should these fluxes not be accounted for in this paper describing trends in POC and DIC?  Also, if wastewater treatment practices have been improved, this should bring about a reduction in nutrient inputs, and potentially diminish, not enhance NPP.  I short, I found it difficult to understand the long-term trends presented in this paper while not understanding what are the changes occurring in this system that seem to be the focus of a different paper.

Results: throughout the paper, loads are presented as mass per unit of time (e.g., the total mass of CO2 leaving the estuary), which is not very helpful to facilitating cross-system comparisons (vs. presenting these as values per unit area of the estuary).  By analogy, river loads (watershed export) are more commonly normalized to watershed area (i.e., as a yield per square meter) to allow comparisons among watersheds of different size.  Readers could take the values provided in this paper and divide by the specified area of the estuary to obtain estimates for comparisons to other estuaries.  But the potential for making inter-system comparisons would be enhanced if the authors were to present their data as per unit area of the estuary.

Other Comments:

The Abstract lacks a ‘big picture’ perspective.  The overall findings of the study are difficult to discern among the details of the results.

Site Description: it would be helpful if this included an indication of salinity levels along the length of the estuary (perhaps add data to Figure 1, or at least delineate polyhaline, mesohaline, etc.).

DOC: I was surprised that in a paper on estuarine C dynamics there was virtually no mention of DOC.  Is it the case that internal production of POC and subsequent remineralization of POC are the dominant C fluxes in this system?  At a minimum, it would be useful to report the proportions of total C represented by DIC, POC and DOC in river inputs to the estuary vs. relative contributions in export to the sea (and for drought vs. non-drought conditions).

Results (line 240): there is frequent use of indirect metrics (AOU, pH) to make inferences about autotrophic activity.  Are there no primary data that can be used to support these inferences (e.g., CHLa measurements)?

Results (line 260): the statement “significant POC increases occurred...” is followed by some specified values, but it is unclear what these numbers represent (the mean concentration? the increase in concentration?  If the latter, increase relative to what?).

Results (line 278): I did not understand why the TA:DIC ratio should be of interest, or what is the significance of this ratio being <1.

Discussion (line 425): do you mean to say that mineralization rates increase linearly with POC concentrations, or that mineralization efficiency increases (i.e., that the proportion of POC that is remineralized increases)?

Table 3.2: it would be helpful to include the standard error of the slope.

Figure 4: it is somewhat confusing to use the designation “m-1” as this is much more commonly used to indicate per meter.  Perhaps “mon-1” would be the better abbreviation for monthly values?

Rewrie et al. (2023) investigated the variations in biogeochemical parameters, including DIC, POC, DO, and pH, in the Elbe Estuary over a long time period of 23 years. They discovered a significant long-term increase in DIC in the Elbe Estuary, which might be primarily influenced by a rise in POC content in the upper estuary. The researchers suggest that the increased internal load of DIC during the drought period can be attributed to an extended residence time in the estuary, allowing for a longer period of POC remineralization. The manuscript highlights the correlation between POC in the upper estuary and DIC in the mid and lower estuary, which may provide valuable insights for understanding DIC variations driven by climate change and human-induced disturbances. However, I have several concerns regarding the manuscript, and I hope the authors will carefully consider them.

Main comments

1.The authors have solely examined the correlation between POC data in the upper estuary and DIC concentrations in the mid and lower estuary. Including POC data from the mid and lower estuary in the manuscript would offer a clearer understanding of the relationship between POC and DIC. Additionally, while the authors have provided a preliminary estimate of inorganic carbon export, they have not thoroughly discussed other potential factors influencing inorganic carbon export. I am curious about the relationship between dissolved organic carbon (DOC) and DIC in the Elbe Estuary. Since labile DOC can also be consumed by microbes, it may contribute to the DIC pool. I recommend the authors add some discussions on the correlations between DOC and DIC.

2. The authors utilized the CO2SYS program to calculate TA and pCO2 using DIC and pH as input parameters. However, it should be noted that the TA model in the CO2SYS program does not account for the contribution of organic alkalinity. Previous studies have reported significant concentrations of organic alkalinity (ranging from 10 to 70 umol kg^-1) in rivers and estuaries. Ignoring the presence of organic alkalinity in the calculated TA values may introduce substantial uncertainties. Therefore, I recommend that the authors include discussions addressing the uncertainties associated with the calculated TA values.

3.This manuscript extensively references a paper (Rewrie et al., in review) that is currently under review. It is generally not considered appropriate to rely heavily on the findings of a paper that is still undergoing the review process. Additionally, it seems there is a high correlation between that paper and the current manuscript. It would be helpful if the authors could provide an introduction outlining the differences and commonalities between this manuscript and the paper they have cited (Rewrie et al., in review) in the response to reviewers.

4.The authors have utilized potentiometric pH and DIC measurements to calculate pCO2. However, it is important to note that potentiometric pH measurements may introduce significant uncertainties when measuring pH in saline waters. What is the salinity of the samples collected in the outer estuary? The authors should take into consideration this potential source of uncertainty in their analysis and discussion.

Minor comments

Abstract:

L.25 Is it possible that a portion of the gained DIC can be attributed to the degradation of DOC?

Introduction:

L.55 “source”?

L.45-60 Why do the authors use "OM" and "POC" interchangeably in this paragraph? Does "OM" include DOC?

L.55 I suggest that the authors include an introduction discussing the contribution of DOC degradation to DIC.

Method:

L.150 Please add a reference for the CO2SYS program.

L.130 what is the precision of pH, DOC, and DIC measurements？

L.190 “from May to August”?

L 220 In the study of an estuary, why did the authors use a Schmidt number (Sc) in freshwater? The gas transfer velocity parameter (k) described by Wanninkhof (2014) is generally applicable to the open ocean.

Result:

L.285 How does the physical mixing between freshwater and seawater influence DIC in the outer estuary?

Fig.2a is difficult to read. Add the legend into the plot may be better.

L. 345 Why “Such dominating heterotrophy in recent years (2018–2020) and DIC generation in

the upper region (z1), would subsequently reduce the internal DIC load in the mid to lower

estuary”?

Discussion:

L.420 According to this sentence, the author has identified a correlation between POC and DIC in another manuscript that is currently under review. I am curious about the differences between this manuscript and the one mentioned. The main focus of this manuscript appears to be the role of POC in the upper estuary in driving DIC increase in the mid and lower estuary. If this point has already been addressed in the author's other manuscript, it may not be appropriate to emphasize it again in this manuscript.

L.425 Please revise this sentence. It is not complete.

L.430 I don't believe that the statement "there are no other major sources of carbon along the estuary (Abril et al., 2002)" can directly suggest that POC was efficiently remineralized and converted to DIC by the mid-estuary. The role of DOC in this process should also be considered. It is important to investigate the potential contribution of DOC to DIC through remineralization processes in the mid-estuary.