Reply on RC1

Reviewer comment: This is valuable work on an under-studied benthic ecosystem. The manuscript presents oxygen fluxes over macrophyte (F. vesiculosus) detritus that accumulated in a topographical depression in shallow waters of the Baltic Sea. The manuscript is well-written and concise. The methods are clearly presented and appropriate to the goals. The primary findings are 1) that hypoxia occurs frequently in the depression and periodically in overlying water, and 2) that there is substantial detrital photosynthesis. These findings support the broader implications that 1) benthic hypoxia in shallow waters of the Baltic Sea is underestimated, and 2) the retention and export of F. vesiculosus carbon from coastal zones is likely greater than previously estimated. I enjoyed this work and have minor suggestions for its improvement. My primary criticism is that the manuscript does not provide more context for metabolism of the detritus, nor for its contribution to the occurrence of shallow water hypoxia in the Baltic Sea. On the first point, it is surprising that detrital gross primary production was so close to respiration, particularly in May and June. The authors begin the Discussion by comparing GPP to that of attached F. vesiculosus canopies (line 333), but it would be helpful to provide more detail. I recommend that the authors add a figure that shows how the metabolism of these detrital canopies differs from that of attached F. vesiculosus canopies.

Reviewer comment: On the second point, I recommend that the authors provide more perspective on the occurrence of shallow-water hypoxia in the Baltic Sea, and how F. vesiculosus detritus may contribute to it. Where else is shallow-water hypoxia observed? Does it naturally occur elsewhere (apart from areas of high anthropogenic impact)? Based on prior work, can one estimate how much detritus is exported from attached F. vesiculosus per year? Given this export and your results, what area of the topographical depressions in shallow water of the Baltic could behave as you have observed here? I acknowledge that there are complicating factors which may prohibit the authors from estimating this. For example, much of F. vesiculosus detritus decomposes in the intertidal zone and therefore would not contribute to oxygen uptake in shallow depressions. Nevertheless, it would be valuable to include a discussion of knowns and unknowns.
Author Response: Q1: Where else is shallow-water hypoxia observed? Information on shallow-water hypoxia is generally scarce, but we have some numbers that we will include in the revision. Our key reference is the study by Virtanen et al. (2019) for the northern Baltic Sea (Gulf of Finland and Archipelago Sea). This region has a total seabed area of 12435 km2 and a shallow-water area (0-5 m depth) of 2211 km2. Based on their model, the total area prone to hypoxia is 1351 km2 (all depths) and 16.5 km2 for shallow areas < 5 m depth. Of the 461 monitoring stations in this area of the Baltic Sea that registered hypoxia, only 11 were in waters < 5 m depth. These are likely underestimates since the O2 measurements driving the models are done 1m above the seafloor. We will state this explicitly in the revision.
Q2: Does it naturally occur elsewhere (apart from areas of high anthropogenic impact)? Yes, O2 deplete conditions and even sulfidic conditions are often observed in association with macrophyte detritus, even in remote and pristine environments such as the high Arctic (Glud et al. 2004, cited in L32). We will state this explicitly in the revision.

Q3: Based on prior work, can one estimate how much detritus is exported from attached F. vesiculosus per year? Given this export and your results, what area of the topographical depressions in shallow water of the Baltic could behave as you have observed here?
In a previous study we estimated that F. vesiculosus export ⁓0.3 kg C m-2 yr-1 (Attard et al. 2019b). Given that habitat distribution models for the area indicate a dominance of F. vesiculosus in shallow waters < 5 m depth (Virtanen et al. 2018), we have reason to believe that other topographical depressions accumulate macrophyte detritus and would likely function in a similar manner to our study site. We will state this explicitly in the revision.

Minor points
Reviewer comment: line 87 -two citations are used. One is relevant to the first half of the sentence, the other is relevant to the second half. I recommend separating the citations to denote the portions of the sentence that they are relevant to.