Comment on os-2021-99

The objective of the paper, thus the modelling of primary productivity and its relation to environmental parameters via their influence on the underwater light field is an interesting approach. However, I see some flaws in the methodology applied. The authors state the effect of CDOM and suspended particular material on the diffuse attenuation coefficient Kd in the introduction and discuss it to a certain extent at the end on the manuscript. However, the methodology applied is an approach that takes into account only biogenic light attenuating substances, represented by chl-a, thus an approach that is commonly used for case I water types. As the work was conducted in coastal waters, which are typically case II, this probably introduce errors in the analysis as substances like CDOM or suspended inorganic particles are ignored in the calculations, although they are known to play an important role.

The objective of the paper, thus the modelling of primary productivity and its relation to environmental parameters via their influence on the underwater light field is an interesting approach. However, I see some flaws in the methodology applied. The authors state the effect of CDOM and suspended particular material on the diffuse attenuation coefficient Kd in the introduction and discuss it to a certain extent at the end on the manuscript. However, the methodology applied is an approach that takes into account only biogenic light attenuating substances, represented by chl-a, thus an approach that is commonly used for case I water types. As the work was conducted in coastal waters, which are typically case II, this probably introduce errors in the analysis as substances like CDOM or suspended inorganic particles are ignored in the calculations, although they are known to play an important role.
Furthermore, there is a considerable lack of diligence in the manuscript, e.g. inconsistent use of variable names/terms or incorrectly displayed equations. Also, I had the impression that some bio-optical terms have been misunderstood. This makes the text difficult to read and to understand. Many different analyses have been made and the results are presented, but the reader is not well guided towards the main findings and the important connections. I would have also liked if an overall conclusion would have been presented in the end, but only the single result chapters are discussed as stand-alone parts.
I suggest that the authors improve the manuscript and hope that the following specific comments are helpful.

Introduction:
Line 15: "Increment of the mixing layer" maybe more suitable?
Line 17-18: Influencing the underwater light environment in which respect? Increase? Decrease?
Line 23: Chlorophyll-a is not equivalent to primary productivity. Although it is also not really equivalent, I think "phytoplankton biomass" might be more suitable than "primary productivity" in this context. Line 29-30: How do the changes in temperature and wind speed influence the amount of light penetrating the water? More intensive waves? Or is it rather that these conditions influence the depth of the mixing layer and thus the average amount of light the phytoplankton is receiving (which in turn influences its productivity)?
Line 34: Why do you state here explicitly the range 280-490 nm?
Line 36: Primary productivity is rather a process, not a parameter that attenuates light. "Phytoplankton" or "phytoplankton biomass" would be more appropriate.
Line 39-43: Why are you here explicitly discussing UV light decay in the water column? How is this relevant to primary productivity?
Line 50-58: The use of Kbio (=Kd-Kw) to predict chl-a values is, as far as I know, based on the assumption of a case I water type, where all optical properties co-vary with phytoplankton. The research presented here has been done in coastal waters, where, as you stated correctly, also dissolved and inorganic matter plays a role. Coastal waters are often case II water types, where the optical properties do not covary with phytoplankton. Thus, I would argue that the variability in Kd/Kbio is not only based in phytoplankton variability, especially as you state that your study area is influenced by river runoff (thus probably terrigenous CDOM), and your study site lies in the entrance of a harbor, where resuspension of sediments might be an issue. Could you please explain why you used this approach anyways?
Line 62-64: I would expect that chl-a concentrations inferred from in situ measurements and satellite remote sensing would differ, and thus also the PP/B values of the model. Why do you expect otherwise and, if you expect the same results, why do you do the comparison?
Line 64-66: It becomes not clear why you hypothesis includes these exact values. It seems that you anticipate the results that you got to formulate your hypothesis.

Materials and Methods:
Generally, check the equations in the manuscript. Many of them are incorrect. Please show more diligence regarding this matter, as it questions the way the results of the study were calculated. The things I noticed so far: In equation 2 and 3, the subscripts are not placed correctly (they should be on the depth and the irradiance, not the wavelengths). In line 99, the notation is correct. Further, the notation for logarithm is commonly not given in upper case letters. Line 86-95: Why was Kd only measured from surface down to 5 m depth? What about the rest of the water column? Furthermore, I would like to know how the instrument was deployed and if there were precautions taken regarding avoiding ship shadow etc. This might be relevant, as high quality hyperspectral light measurements are often difficult to obtain directly below the sea surface. Furthermore, you mention here that you measured from 300-700 nm, while in line 82, 280-700 nm were mentioned.
Line 110-111: Omit "radiation" in line 111, as UVR is the abbreviation for global radiation according to table 1. However, I noticed that this term is rather differently used in the manuscript. Please check this and use terms more consistent. Also, do you measure underwater global radiation? What exactly would be the spectral range of UVR?
Line 115: Give an abbreviation for "no cloud cover". I assume it is "NCD" as used later in the text (line 207)?
Line 138-139: Is it DOM or CDOM that is calculated? And it would be nice to indicate the satellite transect in Fig. 1b to see the overlap with the station measurements.
Line 144: The average +/-1x the standard deviation seems not very extreme to me. If you want to show the extreme values, why not just give min/max values?
Line 153: As I mentioned before, the approach you use is (also according to the references you cited) for the "clearest ocean waters", thus not to optically complex (coastal) waters. Why are you assuming that the difference between Kd and Kw is only phytoplankton-derived (Kbio)?
Line 156: Please rephrase: If the functions were described initially by Mobley 1994, then thy could not have been used from Morel 1988.
Line 159: In equations 6 and 7, no chl-a is included.
Line 160: Kbio is calculated, not approximated.  Table 2. Maybe it would be better to introduce it earlier as it contains also abbreviations and parameters that were used before. However, where in the table can I see that the C:Chla was limited to values >20? Furthermore, in the next line you mention that the ratio was fixed to values from 0.003 to 0.01. Where do these numbers come from?

Results:
Line 204: "Maximum above sea surface" Line 207: Where is period of peak solar radiation shown in table 3?
Line 212: Why "in contrast"? You state before that also in summer the effect of clouds has an influence on irradiance reaching the sea surface.
Line 218-219: Where can I see this in Table A1 Line 230-239: Being not specifically an expert in PCA, I noticed that both temperature and SST (sea surface temperature) are included in the analysis. Furthermore chl-a (I assume calculated from Kd as described) and Kd(490), which are closely interlinked. I was wondering if this redundancy is influencing the analysis as the parameters co-vary. Line 245-252: Here, I have several issues: When you subtract Kw from Kd, you obtain generally the attenuation coefficient of all optical active constituents in the water. In this context, you refer to it as Kbio, implying that only biogenic components are in the water. As I said before, this might be not true so close to the coast. However, of course, there is a positive significant correlation between Kd and Kbio, as one is calculated from the other. Therefore, I also do not understand the part about the two independent methods Line 257-261: Here UVR means apparently only the UV-radiation. This is in contradiction to how UVR has been used in the text before and also in table 1 (UVR = global radiation). Please be more consistent. Furthermore, of course chl-a is related to Kd (by the way, the mentioned figures C1d and C1d show the relation between chl-a and Kbio, not Kd as mentioned in the text).
Line 266: Chl-a is not a function of aw and bw, but Kbio Line 273-284: In this paragraph, you evaluate PP/B derived from satellites and say they are overestimated. In this context, the field derived PP/B estimates are the "truth"? Further, according to your findings, the waters are more productive in winter than in summer?

Conclusion & Discussion
Line 289: Why "while"? In both cases, you describe a reduction in UVR.
Line 295: Wind speed and temperature d not decrease solar radiation, but there is a correlation, as you showed. However, this is rather due to the different temperature and cloud patterns in the different seasons, right?
Line 300: What are the numbers? What is UVA / UVB in this context? Line 308: What does "5" mean here?
Line 333: Primary productivity is a process, not a light absorbing component. This would be phytoplankton. Be more precise in your phrasing. Also nutrients itself are no light absorbers (at least in the visible spectrum), but lead to increased phytoplankton biomass. Line 337-339: As you have only measured Kd in the upper 5 m, the effect of the mixed layer depth on Kd is difficult to assess, I guess. However, the fact that seasonally more particles are introduced by river runoff makes sense. And this might be also an explanation for the higher PP/B in summer than in winter. The phytoplankton biomass B represented by chl-a is overestimated as Kd (from which chl-a is derived) is enhanced by particles from the rivers.
Line 359-363: The exponential decline of light in the water column that is driven by Kd is well known: Line 378-379: Kd, thus the diffuse attenuation coefficient of water plus the constituents therein, cannot be calculated from the absorption and scattering characteristics of pure water.
Line 385-386: Kbio is not the sum of Kd and Kw, but its difference in case I waters Figure 2: What is the point of having chl-a from satellites and then calculate it again? Table 3: What is the difference between "Average from field obtained measurements (F)" and "Average value derived from field measurements (D)"?