Review: "Turbulence-Enhanced Nutrient Supply: A Key Driver of Algal Growth in the Arctic" 

Castellani et al.

In this work, a new parametrization of ocean-ice nutrient fluxes is presented which combines molecular diffusion and turbulent exchange, with a transition between these two processes dependent on the under-ice flow regime. Authors tested this parametrization with 3 cases studies, comparing simulations with observations. With a sensitivity analysis, N fluxes across the ocean-ice interface are shown to vary strongly with bottom ice roughness. This research concludes that turbulent exchange of nutrients allows for higher ice algal growth than with molecular diffusion alone, and therefore sea ice biogeochemical models should include turbulent exchanges of nutrients.

The manuscript is well written and the interpretation of results and the main conclusions are supported by the model experiments. Some issues should be addressed, although these likely do not impact the main conclusions.

Specific comments:

1. Different values for parameter alpha_s are used in the 2 models, resulting in SIMBA having an order of magnitude lower turbulent flux. (Maybe numerical instabilities in SIMBA could be addressed by using a smaller time step?).  Almost no information is given on this parameter, how uncertain is it? 

This difference in parameters would explain the differences in N flux between the 2 models (e.g. Figures 4 and 5), but no mention of this anywhere.  This makes any comparison between the 2 models difficult to interpret, and authors should at least discuss the implications of this difference in parameters.

2. For MOSAIC, the fact that the sea ice and snow thickness data doesn't correspond to the same location than the ice algal Chl and ice N data location makes the assembled dataset maybe not very well suited to assess a new parametrization. But, there is at ice thickness data from ice cores (points in Figure 2c?), and it compares well with observations from the ice buoy data used for forcing (line in Figure 2c ?). Moreover, it appears that snow thickness data was recorded from the cores for ice N (see https://doi.org/10.1594/PANGAEA.971385). Why was this data not used? Authors should at least use this data to compare with the simulated snow depth.

3. Simulations reproduce poorly the timing of measured ice algae Chl for the MOSAIC and Resolute case studies, and as a consequence the timing of N uptake and maximum ocean-ice N flux is also not well simulated. This has little influence on some results (e.g. the sensitivity of the N flux to the roughness parameter hs) but it limits the assessment of the proposed parametrization. The authors base their confidence in the new parametrization and the need for turbulent exchange in part from the fact that simulations with turbulent exchange reach the observed Chl levels. But as the timing is off, other factors could be at play, with ice algae growing later when light is higher responsible for high growth.

I think this study could be strengthened by a minor tuning of parameters, likely the photosynthesis parameter alpha, to adjust the timing of blooms. Authors state (L218) that they do not tune parameters "to ensure compatibility", but it isn't clear why and what compatibility the authors want. There are little comparisons between the different test cases and comparisons between the 2 models is problematic (see point 1).  Also, the ice algae in the different sites are likely very different as they grow in different conditions, so it does not make sense to use the same photosynthesis parameters. If the purpose of using observations is to evaluate the new parametrization, I would think that simulations should be a close as possible to observations.

4. The discussion states (L275) that in the MOSAIC case, ice N equilibrates with ocean N but in the CICE simulation, ice N equilibrates to a lower value because the vertical resolution causes CICE to take longer to equilibrate. However, this explanation does not make sense to me: (1) N flux in CICE is greater than in SIMBA, from a greater alpha_s parameter value; (2) it has over 6+ month to equilibrate, but actually no trend is visible in ice N from December to May. The initial uptake of N is fast and ice N seems to reach an equilibrium in less than half a month. So it is not clear why ice N is lower for CICE.

It would be interesting to show ocean N concentrations along with ice N concentrations, for comparisons. 

5. Parametrization: A few details on the description of the parametrization could added for clarity. 

- what is the formula for a and b (equation 3)?

- where does the relation hs=30 z (L93) come from? This isn't used in the rest and no information seems to be given on what this surface roughness parameter corresponds to or if it is used by the models.

6. Giving N fluxes in per day might be more insightful, e.g. for comparison with the stock of nutrients in ice. Considering the values given L177, the N flux would be 0.108 mmol m-2 d-1, N in bottom ice is 1 to 3 mmol m-3 (figure 2) and you consider a 10cm bottom ice layer so an areal concentration of 0.1 to 0.3 mmol m-2. This indicates that the N flux replenishes the nutrient stock in 1 to 3 days.

​Technical corrections:

abstract L15 reference to brine drainage affecting model simulations doesn't appear in main text.

L25 " After the onset of algal bloom" missing an or the

L46 missing a verb 

L82 Is this value of the drag coefficient correct? In the given reference (Hunke et al., 2015) it is 0.00536 and typical values reported elsewhere are between 1*10^-3 and 24*10^-3 (Table 1 in Lu et al. 2011, https://doi.org/10.1029/2010JC006878). Also consider adding the formula used for friction velocity.

Figure 1 caption typo: "Soncpetual" 

Figure 2 line 3 "Starts in panel b" Probably a typo, should be panel c? are referring to initial conditions used to simulate ice thickness here?

L265 What is laminar layer

L325 " chl a max specific growth rate" I think it should be the maximum chl-specific growth rate

L345 typo "sued"

Global Model Development (Ms. No. egusphere-2025-5384)

Title: Turbulence-Driven Nutrient Supply Sustains Algal Growth in the Arctic: A Modeling Approach

Authors: Giulia Castellani, Karley Campbell, Sebastien Moreau, and Pedro Duarte

#Summary

The authors incorporated a new parameterization to calculate sea ice–ocean nutrient exchanges and examined its impact on Arctic ice-algal growth in three regions using two numerical models. The presented overall theory and story are basically reasonable, while I still have some concerns before recommendation for publication in the Global Model Development. Besides, more editorial improvement is also necessary.

#Major Comments

I guess that sea ice freezing/melting also directly influence nutrient fluxes. For example, meltwater discharge directly flushes out nutrients from sea ice column and enhances stratification just beneath ice–ocean interface. The stronger stratification may restrict turbulence even under the same flow condition. Is any relationship between the simulated ice–ocean nutrient exchanges and sea ice freezing/melting considered in these simulations and analyses? Is any contrastive feature between freezing and melting phases detected? I expect more clear description and discussion on this issue.

The authors indicated that the turbulent flux in addition to molecular diffusion has a significant impact on nutrient supply from ocean surface to ice-algal habitat. Sensitivity experiments regarding ice–bottom roughness supported their implication., My concern is potential strategies to represent spatiotemporal variability in ice-bottom roughness when this process is incorporated into traditional 3D sea ice-ocean models. I expect that the authors will provide realistic approach for advanced 3D modeling.

#Detailed Comments

= Title =

I suggest that “Algal Growth in the Arctic” will be reworded to “Ice-Algal Growth in the Arctic Ocean”, because there are algae living in the Arctic lakes. I guess that target of GMD covers wide research communities (not only oceanographers).

= Abstract =

>Lines 3, 4, and many others

I suggest to use “bottom” instead of “surface” throughout the manuscript, because the latter generally reminds of “top”.

= 1. Introduction =

>Line 57

Should “Bottom algae” be “Bottom Algae” to stand for “SIMBA”?

= 2. Methodology =

>Line 81: possible instabilities

Is this “numerical instability” (not turbulent instability)?

>Line 102 and others

The link of “https://https:” should corrected.

>Lines 135–137 and Table 1

I suggest to add a spatial map showing these locations (e.g., cruise tracks).

>Line 140

This exact name of MOSAiC should be introduced at its initial sentence.

>Line 158 and Table 1

“x” should be inserted between “0.6” and “10^{-3}”.

>Figure 2

“Hi” and “Hs” should be “ice thickness” and “snow depth”, respectively.

Specifically, “hs” is already used as a different meaning.

“Starts in panel b)” should be “Dots in Panel c)”.

= 3. Results =

>Line 170

“Section” should be inserted before “2.1”.

>Figure 3 and others

I suggest to relocate a unit (e.g., m s^{-1}) above a left axis such as Figure 2.

>Line 123: “during ice growth and melt, flushing”

I have same concern as my first major comment for this sentence.

= 4. Discussion =

Lines 239–243

The sentence of “This may be …” is too long. I suggest to separate it.

>Line 242– 243

Since Watanabe et al. (2019) is a model-intercomparison paper, it is better to cite Watanabe et al. (2015) in this context. Besides, macro-algae are also considered, but not a main focus in that study. Uptake of seawater nutrients is dominant only for specific condition (i.e., larger biomass).

>Line 249

“McPhee, 2008, e.g.” should be “e.g., McPhee, 2008”.

>Line 252

“(Long et al., 2012)” should be “Long et al. (2012)”.

>Line 273

“MOSaiC” should be “MOSAiC”

>Line 339

“a” of “Chl” should be italic.

>Line 343

I suggest to rephrase “with thick snow light limitation” by “light limitation owing to thick snow”.

>Line 349

“, preprint” should be “(Preprint)” like other parts.

= 5. Conclusions =

>Lines 364–366

The sentence of “We argue” is complex. I suggest to revise it.

= Appendix A =

>Line 372

“Bottom algae” should be “Bottom Algae” to stand for “SIMBA”.

>Line 386

Should “k_CN” be “k_N”?

= References =

There are many strange repetition such as “https://doi.org/https://doi.org/”.

I suggest to recheck details of all reference lists.

>Line 626

Reference of Tedesco et al. (Preprint) should include its journal name.