Dear authors,

thank you for submitting your manuscript on "A Global End-Member Approach to Derive aCDOM(440) from Near-Surface Optical Measurements" to Biogeosciences. I have received 2 reviews of your manuscript. Both reviewers concluded that this is an interesting study, discussing an interesting approach to estimate aCDOM(440) using a Kd-based empirical algorithm. At the same time, the reviewers made a number of comments that would need to be addressed before your manuscript can be further considered for publication in Biogeosciences. Some of these comments are addressed in the authors' response to the reviewers, while some other comments would require further revisions.

More detailed information would be needed on the dynamic range of the data used to validate the algorithm. Figures 3-8 show the range in aCDOM(440) in the data used in this study, but this could be driven mostly by changes in the amount (rather than composition or source) of CDOM. To improve the discussion on algorithm performance across a range of conditions/sources of CDOM, I suggest you provide a more complete characterization of this dataset by discussing the range in the CDOM absorption spectral slopes and/or slope ratios that are relevant to the quality and composition, rather than amount, of CDOM.

Using the terms "atypical" or "anomalous" to describe water masses, such as river plumes and wetland export, that are indeed the main (or, typical) sources of CDOM in many coastal environments has been a source of confusion in the manuscript. I suggest you revise throughout the manuscript and clarify that the 15 data subcategories mentioned in the paper are categories of "more complex" water masses that deviate from the predicted aCDOM vs Kd-ratio relationship.

Some additional information would be needed in the Methods section. For example, it is mentioned that some of the CDOM samples were frozen before further optical analysis. Did the authors assess/quantify the impact of freezing/thawing on CDOM optical properties? (see, for example, Fellman et al 2008, Sci. Total Enciron., 392, 305–312).

The manuscript is currently too long and I would encourage the authors to reduce the length of some of the sections, including 3.6 and 3.7.

Your responses to the reviewers' comments will need to be incorporated in a revised version of the manuscript that will be reconsidered for publication in Biogeosciences. Please refer to the reviewers’ comments for the revisions.

I look forward to receiving your revised manuscript.

Sincerely,

Maria Tzortziou
Associate Editor
Biogeosciences

Dear Authors,
Thank you for submitting your revised manuscript and response to reviewers' comments. I have read your revised version and I am happy to inform you that your manuscript is now accepted for publication in Biogeosciences.

Best regards,
Maria Tzortziou
Editor, Biogeosciences