General Comments from the Reviewers General Comments …

1 General Comments from the Reviewers General Comments from reviewer 1. Comment: This is an interesting study and the authors have collected a unique dataset using cutting edge methodology. The paper is generally well written and structured. However, in my opinion the paper has some shortcomings in regards to some data analyses and text, and I feel this unique dataset has not been utilized to its full extent. Below I have provided numerous remarks on the text as it is often vague and long-winded. In several instances I also suggested to cite more relevant and recent literature. Furthermore I made additional suggestions for more in-depth analyses of the data. Key critical points are a) the development of the NDI to chlorophyll a relationship (20 versus 60 data points, presentation of Figure 3 ), b) a lack of a discussion of effects of the distance (6m) of the AUV.

2 Sensor to the subsurface of the ice (is there any information on the chlorophyll concentration in the under-ice water, how did this distance affect signal/ noise ratios and NDI-based chlorophyll a estimates) , c) lack of information on AUV location and potential navigation errors, and c) the calculation of patch-sizes based from data with a large foot-print (of the radiometer), how were data-points for calculated. A revised manuscript might not fit the TC Brief Communication . format anymore, however a detailed Supplementary Information section might be useful and could help to keep the manuscript in a short format. Given these shortcomings the manuscript requires major revisions. Response: The authors would like to thank the reviewer for their Comments .

3 Care has been taken to improve the work and address their concerns as per the specific Comments below. As noted by the reviewer , the authors feel that the length restraint of a Brief Communication prohibits the inclusion of a Supplementary Information section. Specific Comments from reviewer 1. reviewer Comments Reply Abstract: Please focus the abstract on Efforts have now been made to address this concern while still remaining your study and your results. In within the word limit. The balance that the authors were aiming for is to particular the last two sentence are highlight the accomplishments in both the science and the engineering. vague. I would prefer to see some data on algal biomass, ice thickness and snow thickness from this study in the abstract, rather than a description of where to go next.

4 More generally, I. suggest to focus the manuscript on the scientific results rather than on the innovation in engineering. P1, L 12: to quantifying ecosystem The aim of this opening sentence is to highlight the importance of responses (quite a long shot to go understanding spatial variability of algal communities. Although the from simple observations to authors aren't suggesting that predictions be made here, it is maintained predictions: : :) that there is an importance in understanding this variability if attempts to model the community are to be made. P1, L14:.. to describe the spatial This has now been amended in the revised abstract. heterogeneity of ice algal distribution. P1, L15: rather define the ice as land- Abstract has now been amended.

5 Fast sea ice rather than first-year, P1, L15-16: These Abstract has now been amended. (Please clarify sentence). P1, L 16 -17: This a very vague Abstract has now been amended. statement. The manuscript does not provide any information how ice algal communities could be categorized or how productivity could be measured. I. suggest to delete and re-write abstract as per above Comments . Introduction: P1, L 23 -24: Please be Citations have now been updated to better reflect the material. more specific. Focus could be on the entire Southern Ocean ( Saenz &. Arrigo 2012, Arrigo 2014, Meiners et al. 2012) or Antarctic fast-ice ecosystems. Rysgaard is an older (and Arctic) reference. Maybe cite Mundy et al. 2007 and the recent papers of Campbell et al.

6 2014, 2015? P1, 28 -29: This statement is true for Amended to now read: Arctic sea ice and Antarctic land-fast Sea ice algae in McMurdo Sound, Antarctica comprise a shade-adapted sea ice, but not necessarily true for autotrophic assemblage '. Antarctic sea ice (see Horner et al. 1992, Arrigo 2014). P1, 29-31: Please clarify sentence Amended to now read: (Significance of what?) The significance of this algal community to marine ecosystems as a concentrated food resource prior to ice break-up, and its potential role in seeding pelagic production subsequent to ice break-up, are widely recognized (Arrigo & Thomas, 2004).'. P2, L1: Classic work on ice algal Citation has now been added and the authors would like to thank the patchiness in Antarctic land-fast sea reviewer for this suggestion.

7 Ice has been conducted by Swadling et al. (1997). This would be a good citation. P2, L5: Maybe re-phrase? Links The phrasing here has now been kept the same but the citations have between ice algal biomass and the been updated to reflect more current work and the expansion into remote under-ice light field are well sensing techniques. established . P 2, L 9-10: As far as I understand This sentence has now been slightly modified by removing the word NDI explain variability in the biomass spatial'. ( in a dataset used for calibration efforts), but a NDI does not explain spatial variability! Please correct this sentence. I suggest major re-write of the Changes have been made to the Introduction to improve the flow but introduction. It should provide an structural the authors feel that it was important to maintain the key ideas overview of a) the importance of ice algae / patchiness / NDI as this links to the rest of the paper.

8 As for of Antarctic land-fast sea ice studies new, emerging technologies for non-invasive sampling, this has been left and b) recent advances in technologies for the Discussion as there have been so few studies to date. to measure ice algal biomass non- invasively. Methods: P2, L 13: This study did not This has now been corrected as suggested. measure transmittance! Transmittance is defined as the ratio between incoming solar radiation at the surface of a medium and the amount of radiation at the bottom of the medium. Rather than spectral properties of light transmittance the multi-spectral under-ice irradiance was measured. Please use proper terminology. P2 L17-18: patches a few meters This has now been corrected as suggested. across and very small (1s to 10s of meters is duplicated information, I.

9 Suggest to rewrite/combine these sentences P2, L18: report snow thickness in m This has now been corrected as suggested. (SI unit) rather than cm . P2, Line 23: replace Southern Ocean This has now been corrected as suggested. with land-fast sea ice in McMurdo Sound . P 2, L25: Could you please provide a A more explicit validation of this technique using these datasets has just linear regression and an R2 for the ice been published in Lucieer et al. (2016). thickness (measured from cores). versus ice thickness (measured from Lucieer, V., Nau, A. W., Forrest, A. L., & Hawes, I.: Fine-Scale Sea Ice the sonar) relationship. This would Structure Characterized Using Underwater Acoustic greatly help to understand sonar-based Methods. Remote Sensing, 8(10), 821, , ice thickness error.

10 2016. This reference and the mean difference of m between measurement techniques has now been included. P2, L30: greater spatial precision , This has now been corrected as suggested. please provide information on the AUV positioning system and its accuracy, it might be useful to cite recent AUV (Katlein et al. 2015, JGR). here P3, L4-5: Given the speed of the While there will be some overlap at 1Hz, the data are presented as point vehicle and the downsampling to 1Hz values rather than estimating a running mean. All this would do would there will still be some overlap of the be to further smooth the signal and so wasn't presented here. area measured. Was this accounted for ( through calculating a running mean?). P3, L 10: Are there any measurements No direct measurements of Chla concentrations were made in the water.