The manuscript “Modelling river-sea continuum: the case of the Danube Delta”, by Ferrarin et al., describes a new model configuration for the river-lagoon-sea interconnections in the Danube Delta Region. The model is first validated under current conditions and then used to explore the temporal and spatial variability of oceanographic parameters and three alternative reconnection options.

Evaluation: overall the manuscript covers and interesting topic in a region and type of system that could be extrapolated to other similar systems. The manuscript also does a good job (through visuals) at showing the spatial and temporal variability of oceanographic parameters.  However, there are questions about the datasets used, the validation of the model results and the interpretation of results could be strengthened. The manuscript would benefit from some reorganisation since the findings and the implications are difficult to follow beyond the focus on the local issue. The scientific questions and relevance are not clearly articulated in the Introduction and some of them appear further down in the Methods. New results are presented in the Discussion and on the other hand the results of the study (both those in the result section and those in the discussion section) are not put into the broader context and remain descriptive. Perhaps the validation should go in supplementary materials and the result figures currently in the discussion (11-14) could be presented in the Results section highlighting how they relate to current understanding of this type of systems. The Conclusion restates the local findings and should be strengthen by highlighting why or how this piece of work is of general interest.   

Specific issues that would be good to address include:

The modelling system and Numerical experiments. Significant details on the datasets and the modelling system need to be included without these details it would be difficult to reproduce the results and to evaluate the validity of the numerical modelling: What is the bathymetric resolution of the different datasets? In coastal environments the bathymetry has a non-negligible impact on the obtained results. What is the temporal resolution of the Black Sea lateral boundary condition? Some of the datasets are only available upon request so it will be challenging to reproduce these results. What is the temporal resolution of the model outputs?

The numerical experiments: it is not clear for which period are the what-if scenarios run (L126). Please clarify whether it is 2015-2019 or 2018

L123 highlights why this study matters and yet it only appears here. This should we moved to the Introduction section

L131 you refer to four datasets not three

The model validation: my main concern is the lack of validation data in terms of the water division in the lower parts of the delta river network and salinity on coastal areas.

Fig 2. What are the red circles and the black diamonds in Fig2b and 2c? There is no legend for those. There might be a typo on the x and y axis of Fig 2b and Fig 2c, shouldn’t it be “m^3/s” instead of “m”?

The validation data for sea level and sea temperature is relatively far from the zone of interest where the manuscript focuses. Why is the validation for sea level done for mean sea level and not at the time the measurements are collected? Does the validation improve when done for mean daily values or is it that the model output are daily values? This is not clear.  Validation of sea level is quite poor at times (e.g. differences of over 0.1m for 0.7m variation in sea level Fig.3). The reasons (L171) for the poor fitting should be better explained/explored, does the bathymetry play a role as well in this? Why are results not presented as scatter plots?  

Similarly for the sea surface temperature, why is the validation for Constanta not shown in Fig. 4 while data is available? Temperature is usually a minor part in estuarine dynamics with most of the density gradient being driven by salinity differences. However, here the validation is presented in terms of temperature and yet you focus later on salinity results, are there no data available in terms of salinity? There are 6 satellite control SST points, however a single set of statistical parameters are given in L182. What are the implications of comparing averaged daily values with specific timings in the satellite data?

I think that if this validation section could go into supplementary materials to streamline the paper.

Table 1 could include the statistical analysis for all the validation datasets (i.e. also include sea level and satellite SST) and not just temperature for monitoring stations.

Water division: water division is only validated in the upper part of the delta, and only temperature values are shown for a point in the vicinity of the outlets. Without discharge measurements in the network or temperature or salinity in the outlets it is difficult to assess how well or bad the model is performing. The water division is likely to influence the coastal dynamics and the plumes observed. These results also don’t seem to be further explored in the paper.

Spatial and temporal variability of coastal dynamics

L216 please define the scales of variability you refer to in the text. Here it is implicit you are considering seasonal scales as per your figure 6.

L234 there is no correlation calculated here, suggest change “are highly correlated” to “can be explained by”

River lagoon connectivity:

Over which period are calculated the averages and estimates in L241 to L249?

what causes the difference in WRT in the different years in L253 to L256? Are the weather regimes very different? Is it the different river discharge? It would be good to further explore the reason for these differences.

L259 The sense of the gradient and the number of days seem reversed.

Assessment of lagoon-sea reconnection solutions

L270 please indicate for which period where the what if scenarios run.

You could combine Fig. 8 and Fig. 9 and present current Fig. 9 as differences with respect to the reference run instead of absolute values. That way the differences would be more easily perceived.

L277-282: please point to table 2

Fig 10, please change the colours they’re not very colour blind friendly particularly sol.A and sol. B

Discussion The discussion further presents new results (there are 4 results figures in this section) and while they are interesting, they’re not put in context or related to similar systems. I believe the discussion would be strengthened if these new 4 figures were moved into the results section and the results were further discussed and contextualised in the discussion. It is difficult to see what is of interest beyond the regional area and what learnings could be taken to other regions.

L305-309: I believed this is the first time that stratification is mentioned in the manuscript. So far, the analysis has been limited to the surface. Beside there is no reference to the different wind regimes, could you please elaborate on its influence on the circulation and vertical mixing patterns? Fig 6b and 6c correspond to surface salinity and currents.

L306 This is the first reference to the stratification of the water column and the stratification has not been shown anywhere in the paper, please point the reader to the figures as needed or rephrase.

L309 Fig6b and 6c only show surface, not vertical processes as indicated, please rephrase. The wind regimes are not shown anywhere in the paper so it is difficult to follow the reasoning.

L315 Upwelling is wind driven although it may interact with river plumes. You may wish to clarify the reasoning in this paragraph.

L325 is the first reference to salt intrusion, there is no previous information or profile to assess the type of salt intrusion. You could include a profile along the main branches in Fig., 12.

Fig 13, please change the colours. Sol. A and Sol. B are difficult to distinguish. It is not clear how or where the differences are calculated. Is this over the whole lagoon? Is it the differences between two points one in the lagoon and one at sea?

L337 please clarify what you mean by hydraulically limited here. You again refer to the wind here, but you do not present any information about the wind. You could include wind information in the figure in the same way that you include the river discharge.

L340. Fig14 would benefit from including wind regime

L345-347 please clarify what you mean by with hydraulically controlled.

Conclusions The conclusion could be strengthened by highlighting why or how this work is relevant beyond the study area at present it just restates what the paper does, and it is difficult to see why it would be of interest of the broader community beyond the study area. Could this be further explored?

L382 “four lagoon-sea reconnections” I believe you explore 3 different options.

Please find my review in the attached document. I have recommended the manuscript for major revision because I think it needss to be revised and better structured with the focus on a clear resear question and story line. Right now it presents a lot of model results without to much structured analysis. I assume the main focus of the authors is to demonstrate the general quality of the model. But this should be more clear from the structure of the paper. However, I think the authors have developed a very interesting model configuration and have carried out a thourough model study. I have no daubts that the manuscript can develop into a very interesting paper.