General comments:

The manuscript is like many others reporting low oxygen conditions where not measured before. The results are not particularly new given that deoxygenation occurs more and more, either because of poor water quality (usually eutrophication) or more collection of data. The idea of a turbidity maximum is presented as an issue, but there are few data that link the turbidity maximum and low oxygen. Otherwise, the usual water quality parameters are those important in the development of the low oxygen—warmer waters, sluggish circulation, organic matter decomposition, especially from dense algal blooms.

There are poor language choices, poor logic statements; several examples were pointed out. These poor choices of words generate many illogical statements and poor understanding of the important issues and their positioning in the eventual interpretation of the results. There needs to be a thorough revision of the entire manuscript to use proper English, punctuation, consistency in references cited in the text and in the list of references.

The various methods used were not necessarily the best for defining low oxygen conditions. The results are a mixture of many different studies, methods, frequency of measurements, and methods to measure duration of low oxygen. For example, probes were placed 0.5 m below the surface. This is not a reasonable method to detect low oxygen conditions through the water column. The resources of concern, migrating fish use different depths of the water column, and thus potentially disrupted at different levels. The ‘high’ frequency data are gappy, which is acknowledged, but too many conclusions are expressed in general terms rather than sound evidence, especially based on near-surface water data.

Now that the authors have used the data to make predictions into the future, the federal (or provincial) agencies should fund an adequate monitoring program for spring, summer, and early fall. Recommendations from this study could inform what the program would include. Strategically placed profiling sondes for dissolved oxygen, temperature, and salinity information through the water column in at least a portion of the estuary for a volume-placed inquiry, especially including the bottom (or near-bottom) water where dissolved oxygen conditions are assumed to be critically low.

Section 4. Concluding Remarks is a correct, concise and well-presented summary of the reasons for the study, the basic results, and the implications for the future. The bulk of the manuscript is not any of these modifiers.

Specific comments:

L 30, “these” does not identify the antecedent. The two larger estuaries, or multiple ones (not yet identified) between them.

L 31, the first oyster producing area [by aquaculture?]

L 31-33, assumes that the two other estuaries are compromised, but not yet established.

L 41-42, the fisheries comment seems peripheral, without any other substantiating information, but rather a mention of oyster production. Perhaps a broader reference to ‘aquatic fishery resources.’

L 43, …Essential for the survival of aquatic animals and plants, oxygen levels…  [plants generate oxygen and are not compromised by lower dissolved oxygen]

L 50, also from advection of adjacent higher oxygen waters.

L 52, suggest “other aerobic organisms” instead of “decomposing organisms” Are the organisms doing the decomposing, or are they decomposing themselves through organic respiration.

L 54, oxygen saturation may also be used to determine autotrophy, respiration, and overall net production…

L 59 …long-term evolution… Geologic, millennial, decades, post-industrial period? In time with increased reactive N in the ecosystem?

L 61, liquid and solid inputs from… Suggest “dissolved and particulate fluxes from…”

L63-55, long sentence with multiple ideas and no concise point

L 68-70 is not clear. This reviewer follows the meaning, but it is not well presented.

L 70-74 recognizes the difference between organic and inorganic suspended matter. Both are contained in the term SPM. The mineral turbidity or what is often considered the suspended sediments can be confounded by organic particulates in the form of phytoplankton, living and senescent, that may affect ‘turbidity.’

L 82, south-west in this sentence, versus north-east earlier in the manuscript. Are these geographic descriptors relevant to Bay of Biscay or the eastern coast of France?

L 94-95, suggest …. The tidal influence reaches 50 km upstream of the mouth but not further because of a dam at Saint-Savinien, which is opened during spring tides ….

L 99, is the word “longitudinally” necessary? The meaning is obscure, here and elsewhere.

L 102, “Bay of Marennes-Oléron” should be identified in Fig. 1. Any geographic location in the text should be identified in Fig. 1.

L 104, suggest…..The coastal fringe is more populated (80 to 100 inhabitants km^-2) than the rural interior (40 to 60 inhabitants km^-2). The largest….

L 108, words “low,” “good” for water quality, “all uses are satisfied on average…”  need to be replaced with more appropriate descriptors.

AT THIS POINT, I WILL NOT MAKE ANY MORE WORDING SUGGESTIONS, UNLESS I DO NOT UNDERSTAND THE MEANING.

L 150+ future recommendation, if a probe is available with sufficient cable, a vertical profile would be advantageous compared to 0.5 m below the surface for defining low oxygen conditions.

L 159, define concentration below which the water is considered “hypoxic.” From Fig. 1 appears to be 2 mg/l, which usually equates to 30% saturation, depending on the temperature and salinity.

L 160-165, long sentences could be shortened and more concise.

L 194, is it is difficult to understand a 82% saturation coupled with a 2.2 mg/L dissolved oxygen concentration.

L 200, are the oxygen values representative of 0.5 m below the surface?

I did not comment on the remainder of the manuscript but did read it. The comments would have continued as before. Just the main results; more concise; too many unnecessary details, unquantifiable terms. E.g., “rather similar” used twice to describe types of data, “rather dated.”

I read the comments from Reviewer 1 and responses from the authors. I agree with the authors that this article provides a new and valuable dataset and raises attention to the deoxygenation situation of macrotidal estuaries. I also agree with Reviewer 1 that authors can further improve the clarity of statements and word choices (detailed in my specific comments) and include necessary information to help readers interpret results (e.g., SPM data to show the maximum turbidity zone, water depths of monitoring stations to justify using surface data alone).   

The turbidity maximum zone (TMZ) appeared on many occasions throughout the Results and Discussion section. However, there is no data or analysis on TMZ presented in the manuscript. According to Sections 3.1 and 3.5, the concentration of suspended particulate matter (SPM) is monitored at several stations and used to conduct path-analysis. However, the data of SPM is not shown anywhere. I suggest presenting the SPM data to directly support the analysis of TMZ and the link between SPM and oxygen dynamics.

The manuscript mainly presents and discusses variations of temperature, salinity, and dissolved oxygen (DO) in Charente. It suggests that temperature is the main controlling factor of DO. What are the conditions of other water quality parameters, such as inorganic nutrients, organic matter, and chlorophyll, in this estuary? Do they contribute to the low oxygen events in the Charente?

The analysis also suggests that the downstream estuarine water has a lower oxygen level while the upstream river water has a higher oxygen level. This spatial variability doesn’t seem to be controlled by temperature. What are the other driving factors?

In addition, Figure 4 shows that the measured oxygen level is way below the saturated oxygen level during summer. This suggests remarkable oxygen depletion, which can be related to the biological and chemical processes consuming oxygen. Are they due to microbial degradation of organic matter? While the oxygen-consuming processes are also affected by temperature, there should be sources of organic matter to fuel the microbial degradation. Therefore, I think other parameters in addition to temperature and salinity are also required to better understand the variations of DO.

Specific comments

L31: the first oyster-farming area?

L35: “salinity sensors”? Do you mean conductivity sensors? And what are used to measure temperature?

L36: “will be” ->  “is”

L36: specify the sampling frequency of the “low-frequency” dataset

L37: “a degradation of oxygenation” -> “a deoxygenation trend”

L41: -> “main controlling factor of DO”

L46: “diagnosing of” -> “diagnosing”

L60: “less the attention” -> “less attention”

L60: Not sure how we can conclude that “hypoxia in estuaries has received less the attention than in coastal waters”.  

L67: “given their large impact on aquatic organisms” – can be deleted. It has been elaborated by the preceding sentences.

L81-84: please provide references for these predicted changes.

L131: the depth of the selected sites should be described to help readers better interpret the data collected at 50 cm below the surface. Maybe mark them in Figure 1.

L135: What parameters are measured through the multi-parameter sensor?

L154: consistently use “50 cm” or “0.5 m”.

L160: “Charente estuary is not known to have hypoxic situation” - is this statement based on monitoring or the geographic, physical, and/or chemical characteristics of the estuary that make it not prone to develop hypoxia?

L161-170: What methods are used to collect these low-frequency data? And at what depths are the measurements conducted? Are they comparable to those of the high-frequency data?

L175: delete “that”

181: “varies” -> “vary”

187-188: This should be divided into two sentences.

L197: insert “with” before “DO reaching …”

L200: the unit of saturation in Fig. 2(d) is incorrect. The time period of the observations at Saint-Savinien is missing.

L217: delete “even”

L230: Fig. 4a. The shaded area of discharge is covered by that of salinity from time to time. Suggest plotting the discharge using a solid line on the top of salinity.

Fig. 4d. The legend of the gray line could be modified as “DO at saturation”. “DO-100%” can be misinterpreted as the “DO saturation – 100%” with a unit in percentage.

L240: In Figure 4, where can we tell “the presence of a moderate TMZ”?

L248: considering -> consider

L251, 257: will be -> was

L250-260: Please pay attention to the tense being used. It should be consistent throughout the manuscript.

L260: Please include the unit.

L262: “..allowed TMZ to develop and persist until the autumn floods (Fig. 4).” – this cannot be inferred from Figure 4.

L270: “This validates the use of these data …” -> “The agreement between the two datasets validates the use of the low-frequency data …”

L273: “inability of the low-frequency data to …”

L282: What trends of what variables? Figure 5 only shows dissolved oxygen.

L295: Figure 5 – suggest marking the period of high and low tides on the time series.

L302: The previous paragraph (L284-286) suggests that at high tide, the advection of oxygen-poor water from downstream leads to low-oxygen condition. But this sentence suggests the opposite.

L305: This contradicts L284-290 which suggests that the downstream estuarine water has a lower DO concentration while the upstream river water has a higher DO concentration.

L310: “Oxygen” -> “oxygen”; Do you mean “localization” or “location” of the oxygen minimum zone?

L318-319: Do you mean the DO data collected in the summer of the 3 years shows a decreasing trend? The time series presented in Figure 4 is not obvious to support this statement. The data in 2018 only covers 3 weeks, which is not sufficient to represent the entire summer condition of 2018.

L336-337: We need the information on the depths of these monitoring sites. Are there any data to show how stratified the water column is?

L346: “reach borderline or even hypoxic, levels” -> “reach borderline or even hypoxic level”

L353: delete “comprised”

L354: add a “,” after “conditions”

L355: delete the second “target”

L356: -> “summer levels of DO”

L365: “However, it is clear that the number of studies is limited and rather dated. ” – not clear to me how we can come to this conclusion. The cited references in the preceding sentences are mostly between 2021 and 2023.

L382: “used the 2020 dataset, which has the interest to include also …” -> “used the dataset obtained in the year 2020, which has included …”

L384: -> “water quality parameters”

L385: Should wind speed also be considered? This parameter links to the oxygenation of shallow waters.

L381-389: Please provide a bit more information on how the path-analysis was conducted. How do we interpret the numbers shown on the arrows and the circles in Figure 8? Does the thickness of the arrows represent the strength of the correlation between the two variables? If so, please make it clearer in the caption.

L408: “less the attention” -> “less attention”

L415-417: Do you mean the existence of turbidity maximum zone promotes severe deoxygenation? Why is temperature mentioned here?

L418-420: Got confused here. Why can low summer flows (with water stored in reservoirs) limit water warming? And how can a lower summer flow increase the inflow of more oxygenated river water?