This manuscript aims at applying a model that describes dissolved organic carbon dynamics at the Changjiang Estuary, in order to identify the different processes affecting DOC dynamics in this particular site.

In general the manuscript is well written and easy to follow in all its parts. Although I am not an expert on modeling, I was able to follow the description and the application of the model that the authors used. I cannot comment on the specifics on the model used, as I am not an expert in this field, but it was clear enough that the model was properly validated by observational data over a large time scale.

In my opinion however, the authors need to work on the discussion part of the manuscript to highlight the importance of this study. Some important questions need to be answered, such as: why is this model important? what new information it gives that improves the knowledge on this region? how these results/model can be of use in other regions?

The discussion needs therefore to be re written taking these questions in mind. In the current form the discussion contains mostly a literature review on previous studies, not properly compared to the results of this study.

Also the introduction strongly needs a revision in order to have an updated bibliography. There are too many outdated references, sometimes as the only ones, that can be updated with much more recent studies

Specific comments

Line 15: salinity has no units. To be checked throughout the entire manuscript.

Lines 23-24: “DOC is defined as the nonliving organic matter in the ocean that can pass a 0.5µm pore size filter…”. This definition is not correct. DOC is operationally defined as the organic carbon that passes through a 0.2 µm pore size filter. Sometimes also 0.77 µm pore size filter is used. Also the references here needs to be adjourned.

Lines 27-28: “DOC in estuaries is predominantly derived from river inputs and marine production”. In situ production processes should be mentioned too here. Also because they are mentioned later in the manuscript.

Line 78: SPM needs to be defined.

Lined 89-92: Here the authors introduce DOM, however in the manuscript only DOC is discussed. Either the authors mention only DOC or it should be clarified the link between DOC and DOM.

Line 96: Why the turnover time scales taken into account are limited to 70 years. Refractory DOC has a turnover time that is much larger than 70 years. For turnover time of DOC see Hansell, 2013 (https://doi.org/10.1146/annurev-marine-120710-100757).

Line 145: Table 1. Some data miss the data source. Where are these data coming from?

Line 159: Reference to figure S1. There should be also figure S2. These two figures are useful to understand the comparison between the observational data and the model data. However being two different figures and being the images (surface/bottom, summer/winter) displaced in a different order a comparison is difficult to make. I would suggest to merge these two figure together and displace the observational/modeled data next to each other.

By looking at these figures, I think it would be interesting to also highlight where there is least correspondence between the observations and the model and discuss that too.

Lines 167-168: “the seasonal average of the model results over the years 2013-2017 was calculated for all four seasons”. Why only summer and winter were chosen?

Line 181: In the caption of Figure 4 there is no details on the 6 panels. Also it is not specified if these plots are from observational or model data.

Line 242: “The released mDOC from bacteria is converted into semi-refractory mDOC”. This statement needs to be supported by a reference.

Line 250: Figure 5 is difficult to read, especially the dotted lines, which are too similar as color and too thin

Yao et al used a physical mixing coupled with biogeochemistry model to investigate DOC cycling in the Changjiang Estuary along the land to coastal ocean continuum. They find with their model that in the summer, DOC distributions are dependent on biogeochemical processes as well as physical mixing, while in the winter, physical mixing predominates. The study found that terrestrial DOC is not susceptible to bacterial consumption in the estuarine system, but highly susceptible to marine bacteria consumption. Finally, the estuary is a source of DOC to the coastal ocean.

I found the study interesting and suitable for publication after moderate revisions, and have a few major comments for the authors to consider, listed below:

1. I think the authors need to go more into depth about their model set up. Why is it that refractory DOC, which makes up a large portion of the total pool, is not accounted for in the model at all? Would that simply be grouped together with the semi-refractory pool? Please clarify.
2. It seems like the semi-refractory pool in the model is a closed loop and not connected to the semi-labile/labile pools of DOC, which the model in Figure 1 suggests are impacted mostly by phyto- and zoo-plankton. However, bacteria play a role in the conversion of labile to semi-labile DOC and semi-labile to semi-refractory (and so on). It also looks like from the arrows in the model setup in Figure 1 that bacteria are taking up non-photolabile DOC and converting it into the semi-refractory pool.
3. How does the model consider past work that DOM that has been photochemically altered can become more biologically labile? The authors mention something about this in the text (how aromatics are biologically resistant, but photochemically susceptible), but don’t really go into detail, as far as I can tell. Photo-transformed, newly biologically labile DOM may also enhance bacterial productivity. Please at least discuss this in the paper (papers from Medeiros https://doi.org/10.1002/2014GL062663, Mopper, and Zhou may provide some insights) and relate to where this may fit into the model. 
4. While I like Figures 4 and 5 a lot, I think an additional figure or subplot showing DOC vs. salinity and a conserved mixing line would be informative to this study to show where and when it diverges from mixing.
5. There could be additional discussion. I think there is a lot to be discussed relating the findings to past studies that look at priming of DOM (Bianchi work), and how DOM may be refractory in one location / time, but when transported to a different location / time /  set of environmental conditions, it may be more labile (Shen & Benner, 2018, among others). A more detailed discussion of why in the winter DOC is conserved and in the summer it isn’t would be valuable as well.
6. The English is overall good but I noticed several typos.

Specific comments:

Line 24: “non-living” organic matter. That’s not necessarily true because DOC is between 0.2-0.77 microns and small bacteria can pass through 0.77 microns (so their biomass would technically be a part of the DOC pool).

Line 31: Marine DOC is not mainly derived from local production. At the surface ocean, maybe 30-50% of it is autochthonous; the other >50% is refractory and allochthonous.

Line 58: Anthropogenic activities are mentioned here as being a part of the study but then aren’t referred to again.

Line 91- why would aromatic compounds precipitating with metal ions have anything to do with photo-oxidation? That line comes out of place.

Lines 93-95: I understand for simplicity sake to keep the model as two types of DOC, but recent studies have shown that DOC that is photodegraded can become more biologically labile. So does then this photodegraded DOC move into the other pool of DOC? (see major comment above)

Figure 2b. The sampling points are very difficult to see on top of the bathymetry background (those points for 2006 are virtually impossible to distinguish). I suggest changing the way the points look (maybe filling in the color, increasing the contract).

Line 143: “757 data points are for the bottom water” – What is the depth range of this bottom water?

Line 175: “significant contract” – is it statistical? If not please avoid using that term. Also, this is referring to DOC, right?

Lines 239-240 : Please re-orient the reader to Figure 1 when discussing phyto-/zooplankton and bacteria.

Line 280: Rates are reduced in winter. Makes sense. Please tie together biogeochemically / seasonally better why in the winter it’s mostly just physical processes, whereas in the summer there are biogeochemical influences.