I think this is an excellent paper, and the comments I have are so minor that I do not need to review it again.

The work is very well executed, and the authors make a strong case for the occurrence of a relatively large Hemiaulus bloom on the NES in 2019. The text itself is well written. I especially liked how the authors clearly presented their definitions of GOP, NPP, NCP and export efficiency in the introduction. Use of data from other years, to put the 2019 'anomaly' in context, was a great strength, as were the backward particle trajectory simulations.

Minor edits that I recommend for clarity:

- In the Introduction, line 86, large phytoplankton (LP) cells are defined as those > 10 um, but then a 20 um filter is used to define LP in the actual study. I suggest changing the intro to read something like "LP, defined as > 20 um in this study".

-line 117 - why is the underway system giving only km-scale resolution? Do you mean that the samples were collected AT km-scale resolution..or every km?

-line 138 - Have previous studies proven that diaphragm pumps are less damaging? (If so, I suggest adding a citation). When making any physiological measurement I usually avoid pumps completely and use water collected by Niskin or other bottle.

-Some justification should be given for the choices of C:Chl and PQ values. C:Chl, especially, can be highly variable. 

-What is the average cell size for Hemiaulus? I know they are big, but how big? Please add scale bars to Figure 2.

-Line 588 - growth rates for Hemiaulus from the dilution experiments are very similar to values for large (non-symbiont containing) Rhizosolenia species...suggest citing some of T. Villareal or T. Richardson's work.

-Line 649 - This section (4.2) was a bit confusing, and I think the point could be made in much fewer words. Suggest revising for clarity and brevity.

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General comments

This study describes the novel observation of the diatom Hemiaulus, including biological fixing symbionts, blooming in the mid-shelf of coastal Northeastern United States during the summer of 2019. No information is provided about the biological nitrogen fixation activity of the diatom-diazotroph association (DDA) bloom. Instead, the authors focus on understanding: 1) the effect of the bloom in the production and transfer of phytoplankton organic matter, and 2) the environmental conditions responsible for the bloom formation. They use for that a broad compilation of physical, chemical and biological data obtained on multiple cruises within the framework of the NES-LTER program, together with information from remote sensing. The authors conclude that the DDA bloom led to increases in the production and export of phytoplankton organic matter and the transfer efficiency from phytoplankton to microzooplankton, whereas the environmental drivers responsible for the bloom formation remain unresolved.

The study addresses an interesting topic, as despite their ubiquitous nature and significance, DDA remain understudied and poorly understood. However, I have several main concerns about this study which prevent me for recommendation for publication, at least in the current form.

Specific comments

1. Data do not clearly support conclusions. Based on the information collected in August 2019 the study shows how the DDA bloom was associated, in general, with higher total and large-sized fractionated chlorophyll, and increased net community production (Figure 1, 3, 4 and 8), compared to the inner and outer shelf. Then, the authors used climatological data (2018-2022) collected in the region (Table 2, Figures 5,6, 7 & 9) to conclude that the DDA bloom led to increases in net community production (NCP), gross oxygen production (GOP), net primary production (NPP), higher export efficiency, and higher trophic transfer efficiency from phytoplankton to microzooplankton. However, this conclusion is based on the comparison of particular variables and individual years, and it is not evident when all the information is combined, as an important interannual (or derived from the comparison of different years) variability exists. Authors should consider using different statistical analysis and/or graphical visualization (as for example summer and winter box-plots for the inner, mid and outer shelf) in order to compare results from summer 2019 to the other years, and to convince the reader about the implications of the DDA bloom for the ecosystem functioning. The observation that other years (i.e 2018) also shows increases in Net Community Production despite different community composition and low chlorophyll concentration, questions the conclusion that the DDA bloom significantly impact productivity rates in the region.
2. Comparison of variables involving different time-space scales and limitations. Several rates (NCP, GOP, NPP, growth rates) derived from different approaches are used to investigate the impact of the bloom in the production and transfer of organic matter. These approaches involve different temporal and spatial scales and methodological limitations. Therefore, the comparison of production rates derived from different approaches, specially when obtained in particular sampling dates (instead of the comparison of longer studies resolving for example seasonal scales) might be tricky. For example, is NPP derived from growth rates from dilution experiments comparable to NPP from 13C incorporation experiments? (Figure 7d). According to the methods section NCP rates computed in this study reflect the conditions over the past few days. Considering this relatively short period of time, could the NCP/GOP ratio be considered informative of the magnitude of carbon export efficiency? These facts could contribute to explain that the DDA bloom was characterized by high NCP but low phytoplankton growth rate, which despite the explanation included in the discussion section seems contradictory. These issues should be incorporated in the discussion.
3. Unresolved formation mechanism: Despite the discussion regarding the role of temperature and nutrients (based on phosphate and silicate concentrations, as nitrate was below the detection limit, which are not clearly different from the other years), the environmental conditions responsible for the bloom formation remain unresolved. The lack of information about nitrate concentrations and more important, nitrogen to phosphate ratios, clearly limits this task. The relevance of stratification is mentioned in the discussion, but no information is presented. The authors should consider including this variable in the analysis. They also mention that DDA bloom is associated with an ephemeral hydrographic feature which must require a different approach with higher temporal and spatial resolution. In this regard, animated versions of individual Chl a and SST images are NOT available at: http://science.whoi.edu/users/olga/outgoing/Aug_2019_chl/NEW_2019Hemiaulus/.
4. Messy structure and writing: I find the structure and writing a bit messy, the arguments very speculative and hard to follow, as some references are missing, or necessary information is not included. For example, it is impossible to understand the conclusion that the bloom was decaying, as the methods section does not mention that the same geographical locations were occupied more than once. Structure and writing need revision.

Additional comments

-I recommend starting the abstract by explaining the relevance of DDA, instead of general sentences about the relevance of productivity terms.

-Introduction should include a description of the previous knowledge regarding seasonal variability (instead of the winter/summer comparison) of phytoplankton biomass activity and composition in the region.

-Lines 38-40 The Margalef ´s mandala [Margalef, 1978] already mentioned the variety of life-strategies in diatoms.

-Incorporate the detailed description (time-space) of the cruises used in this study.

-Modify the structure of the results section following: hydrography, nutrients, and finally biological parameters.

-Line 115 Cruise data are different from those in Table 1.

-Table 1 Include cruise track for all cruises, and available information for each one.

-Line 127 Need to specify how depths of the mixed layer, euphotic zone, and Chl-a maximum were calculated (better to explain it here instead in line 259-261).

-Detailed information about the variables used in this study is missing, for example sampling depths. Remove the description of those variable which are not used in this study (i.e ammonium).

-Lines 264-265. This sentence needs a reference to justify the used Chl-a to carbon ratio. Revise similar sentences through the manuscript where several references are missing.

-Lines 363-364. Consider to incorporate Hemialus abundance data from previous years.

-Lines 381-392. Theses sentences indicate that the same sampling transect was occupied more than once. No information is provided.

-Lines 394-406. I do not believe that the pattern observed in 3 out 5 years support this statement. In addition, Figure S2 shows important variability in temperature between July and August for the different years, which compromises the comparison made in Table 2 (comparison between years based on samplings carried out in different months).

-Lines 580-582. Speculation. Low values of phosphate, similar to August 2019, were sampled in July 2021.  

-Lines 586-587. No clear how the stage of the bloom was inferred from Figure 6.

-Line 588. Recent studies suggest a unimodal relationship (instead inverse) between phytoplankton cell size and growth rate [Marañón, 2019].

-Figures: Incorporate marks for differentiating inner, mid and outer shelf.

-Figure 3. Is the depth range 50-100 correct?

-Table 1. Incorporate information for variables available for each cruise,

-Table 2. Longitudinal range used for the mid-shelf should be specified, number of data (n) for averages should be indicated.