The manuscript investigates the influence of spring-neap tidal cycles on carbon dynamics in two North Sea estuarine systems (Humber and Elbe), presenting a comparative analysis supported by field observations. While the study benefits from a comprehensive dataset, several fundamental issues need to be addressed before the manuscript can be considered for publication. Below are the major concerns:

Major Comments:

1. Data Visualization

Consider implementing standardized cartographic conventions for improved clarity and scientific rigor.

• The cartographic elements in Figures 1, 3i, and 5i exhibit significant technical deficiencies in their presentation. The water mass trajectory plots lack scientific justification and supporting methodological documentation.
• The box plots require statistical significance indicators for between-group comparisons.
• The tidal range representations (blue lines, Figures 3 & 5) need proper measurement annotations and methodological context.

2. Analytical Depth and Data Integration

The Discussion section currently provides only cursory analysis of parameter relationships. The study relies heavily on literature values rather than leveraging the original dataset, quantitative assessment of individual contributors to carbon flux variations is inadequately addressed. Recommend incorporating detailed statistical analysis of the collected data to derive system-specific carbon flux contributions.

3. Methodological Documentation

• The FerryBox measurement system requires more comprehensive documentation，suggest including:
  • Detailed technical diagrams of the FerryBox installation
  • Systematic workflow documentation
  • Quality control procedures
  • Calibration protocols
• The air-water CO2 flux calculation methodology should either be integrated into the results and discussion or removed entirely

4. Conceptual Framework and Literature Context

The Introduction requires structural reorganization. Current emphasis on general estuarine carbon flux heterogeneity should be condensed. Recommend expanding the discussion of spring-neap tidal influences on carbon dynamics. Strengthen the articulation of the study's novel contributions to the field.

I will save minor comments this time because the current type of this manuscript would need a significant improvement.  

Review of the manuscript egusphere-2024-2643 « Spring-neap tidal cycles modulate the strength of the carbon source at the estuary-coast interface» by Macovei et al. submitted for discussion in Biogeosciences

General comment :

This manuscript reports a detailed analysis of FerryBox measurements (on route near surface measurements from ship of opportunity)  acquired during 4 years (2015-2018) on a route in the North Sea between Cuxhaven (Germany) and Immingham (England). The FerreyBox allows to measure temperature and salinity, carbonate chemistry (pH and pCO2), oxygen and optical properties of seawater that allows to derive information on dissolved organic matter and phyto-plancton in seawater. This manuscript only focuses on the date acquired in the Elbe Estuary (Germany) and the Humber Estuary (England) which are both large temperate estuaries that are under the influence of tide.  

The temporal resolution of the dataset allows mostly to infer conclusion on the influence of the spring-neap tidal cycle. The study shows this variability regime has an effect on the biogeochemical variability, in particular in inorganic and organic carbon concentrations, on both sites and over the gradients encountered from the land-sea interface to the adjacent coastal region.  Air-sea fluxes of CO2 are estimated at the scale of the estuaries and the authors argued that an important increase of the estuarine carbon source to the atmosphere is associated with spring tide through changes in the metabolic state. They suggest that their study has strong implications for the correct estimation of carbon budgets in tidally-driven estuaries. 

This study is a significant contribution because it shows some original results at the estuary-coast interface based. In my point of view, one strength of this study is to use FerryBox data collected when the ship is leaving or entering the port, whereas this often corresponds to the moment where ferryBox instruments are disconnected in order to protect the instrument from heavily loaded waters. The manuscript is well written and the results are convincing. The figures and tables are of good quality but could be improved. I would be glad to support the publication of this manuscript after some revisions (which I believe to be minor). My major concerns (detailed in the following sections) are related to some methodological points and the structure of the manuscript.

Specific comments :

In my opinion, one major point in this manuscript is the question of how adapted the temporal resolution of the sampling is compared to the variability of the studied process. I have the feeling that this needs to be more deeply discussed in the manuscript. I believe that the information from figures S1, S3 and S4 could be synthesized  and presented as a figure in the main manuscript to discuss this question of temporal resolution / variability.  

The trajectories presented on figure 3 and 5 are not convincing. I would suggest to make a choice between two options : (1) Use the full potential of the trajectories in the manuscript  by giving more details on the calculation, presenting them with a detailed figure and using the results to support the discussion or (2) keep their use in the actual form and just present them as supplementary material.

The results section 3.1 and 3.2 are devoted to describe separately the results in both estuaries. I am wondering if a more synthetic description could be given in order to make section 3.1 and 3.2 a little bit shorter.  

The method section for  inorganic carbonate chemistry measurements is lacking precision : What type of pH is measured pHT or pH-NBS ? Values of the dissociation constant are not conventional (Dickson et al., 2007) for open-sea oceanography and should be justified if adapted to estuaries? Why are the concentrations converted to µmol.L-1 whereas the dissociation constants are usually defined with mass amount ?

The calculation of air-sea CO2 fluxes are also lacking precision : Why the Mac head station for atmospheric CO2 ? Which wind data have been used and why using average values ? Using average values can lead to strong errors, because if periods of strong seawater pCO2 can be specifically associated to stronger (or weaker) winds, this can lead to large differences in the air sea fluxes. 

Technical corrections

L182-185 : There is something not clear to me in the correction applied to AOA and Wetlab sensors for chlorophyll. This section could be clarified.

L351-352 : This last sentence of this section could be added to the conclusion rather than in the results section.

Figure 1 :The longitudinal clusters used for figure 2 and 4 could be presented by areas on the maps of the estuaries in order to help the reader to visualize this 0.1° clusters. 

Figure 1, 3 and 5 : The projection used for the maps conducts to figures that are not horizontal. Maybe another projection could be used?