Benthic Diatoms on Sheltered Coastal Soft Bottoms (Baltic Sea)—Seasonal Community Production and Respiration

Round 1

Reviewer 1 Report

Biological activity of microphytobenthos was studied at two stations in a shallow area of the Baltic Sea coast. The authors test the hypothesis that in the areas of the littoral shelterted from wind and waves, fluctuations in biomass and primary production are associated with the change of seasons and confirm it. It should be noted that the contribution of microphytobenthos to gross primary production in Baltic Sea is being studied quite actively for quite a bit of time and amount of knowledge on this topic is accumulated. Therefore, that the conclusion of the manuscript «benthic diatoms, although often ignored until now, represent a key component in primary production of these coastal habitats», I suggest to rephrase it in a way to emphasize the data obtained.

2. «Diatom community» is mentioned both in the text and in the title of the MS. However, it is described only as the black box having photosynthetic activity and respiration. Still even a brief description of the species composition (at least the dominant species) together with the images of communities («sand grains inhabited dense communities of rather small epipsamic diatoms (< 10μm) as documented by scanning electron microscopy»). Information of this kind may be interesting to readers, especially since the activity of the diatom community notably depends on its species composition. Without these evidences I suggest using the term «microalgae community or «benthic organisms». It is especially important because authors note in the Discussion, «At Boiensdorf the heterotrophic activities were enhanced as this site is sheltered and enriched in organic matter content (Table 1) which probably stimulated microbial and other biological activities, thereby supporting a rich fauna…»
3. The major novelty of the paper is the application of planar oxygen optodes for measurements of oxygen turnover ex situ in the intact sediment cores. This method was introduced in 2010 and tested under Arctic field conditions at depths ranging from 5 to 30 m. Here it’s used at soft bottoms in brackish conditions of Baltic Sea at 2-49 cm. It is interesting if this method was used in other studies during the decade passed since it’s development?

4. Please comment how the results can be used or interpreted from the perspective of future research.

Small comments

Line 22. An abstract is hardly a place for methodology

Line 237. Water temperature, not salinity

Lines 466-476. text from guidelines for authors?

Author Response

Dear referee,

many thanks for your friendly and helpful comments. We like to address your open points:

1. although microphytobenthic primary production is widely studied in coastal regions all over the world, for the Baltic Sea still few data exist. In addition, the few Baltic Sea studies represent often environmental snapshots, are rather old and had been carried out using different methodological approaches (¹⁴C vs. O₂). Consequently, direct comparison of the limited data is difficult. The latest studies (Ask et al. 2016, Kuriyama et al. 2021) nicely indicate an aquatic primary production share of 23-31% originating from benthic diatoms, which is still not considered in any biogeochemical model of the Baltic Sea. Our data are in agreement with those on Ask and Kuriyama, and hence we like to stick to our conclusion.
2. Thanks for this important comment (also mentioned by referee 2). We added to the discussion another paragraph on the microphytobenthic community composition (diatoms and other microalgae; cell numbers per cm-2, data on biodiversity).
3. Oxygen optodes with different geometrical design are now standard approaches for most oxygen measurements in the lab and in the field. They widely replaced the former intensively used so-called electro-chemical Clark electrode. Optodes have many technical and practical advantages over the Clark electrode, and hence are used for numerous applications in all fields of marine biology, microbiology, ecology etc. (see webpage of company Presens, Germany). Our ex-situ approach with the planar optodes was used in other studies too (e.g. Kuriyama et al. 2021).
4. As mentioned in the conclusion, we think that benthic diatoms in all coastal regions of the Baltic Sea are ecologically important, and hence more data on their primary production are required in space and time. Such information should be included into the current biogeochemical Baltic Sea models, which concerning carbon input are mainly based on phytoplankton. However, phytoplankton is missing in autumn and winter, while benthic diatoms are present all around the year with reduced, but still measurable activities in the cool/dark season.
5. we addressed the minor comments, for example, by taking methodological aspects out of the abstract

Reviewer 2 Report

In my view, this is a straightforward and clear-cut study that is adequately put out in this well-written manuscript. I am sure that the data presented in the study will be a valuable contribution that will be used in the future as as a comparative dataset in microhytobenthos productivity studies in the Baltic Sea and elsewhere. The research question that the authors asked and the hypothesis that they evaluated (i.e. higher productivity of the epipsammic community in the sheltered lagoon in comparison to previously published exposed locations) was adequately clarified by the presented data and it was well discussed in the text.
 
My critique of various parts of the text and my comments are listed below. I believe that the authors will be able to prepare an acceptable revision that might then be published.
 
1) l. 14-15: "Benthic diatom communities dominate sheltered shallow inner coastal waters of the atidal southern Baltic Sea, however, their photosynthetic oxygen production and respiratory oxygen consumption is rarely evaluated."
The sentences in this part of the text should preferrably be kept as short as possible. Thus I duggest to split this sentence into two. [... Sea. However, ...]
 
2) Right after this introductory sentence(s) I am missing a very brief substantiation of the study. Why is it important to study the oxygen budget of the benthis diatom community in this habitat? At this point, this should be very briefly indicated.
 
3) l. 27: "confirm our expectations"
Which expectations in particular? The only ones mentioned are those relateds to seasonal variation; so, did you mean that? If yes, then, please specify this explicitly in the text.
 
4) l. 28-29: "These data clearly indicate that benthic diatoms, although often ignored until now, represent a key component in primary production..."
How do the results indicate this? Is it, for example, because the measured values show high productivity of this community with macroalgal vegetation, or with similar microcommunities in other habitats? If yes, then it should be briefly mentioned, as well.
 
5) In general, the "Abstract" should be as exciting as possible to attract the readers further into the paper. Thus, besides the core data values you have to substantiate your statements and conclusions to illustrate the importance of your study in this research field.
 
6) l. 37-38: " <5 cm at Bay of Gdansk, Poland"
Any reference?
 
7) l. 51: "and thus are suppliers" < "and thus they are suppliers"
 
8) l. 57-58: "Microphytobenthic communities typically consist of a highly diverse set of diatom species..."
This statement is too general to be precise. Microphytobenthos may, of course, be composed of multiple protist and cyanobacteria groups. Besides diatoms, the zygnematophycean green microalgae or the coccoid Cyanbacteria come to mind. Thus, you either might narrow this to epipsammic benthic communities, or to expand the satement to include other groups, as well.
 
9) l. 71: "function is much less studied" < "function has been much less studied"
 
10) l. 102: "The water depth was not exceeding 49 cm..."
In the Abstract it has been mentioned that depth varied from 2 to 49 cm. Is it some kind of a long-term measurement covering the seasonal variations, or is it just an observation made during the sampling?
 
11) l. 197-198: "Selected sediment particles with epipsamic diatoms were visualized using a field emis-197 sion scanning electron microscopy (SEM)..."
This is apparently the part of the study when you looked into the actual composition and taxonomic structure of the studied community. So, it would be useful to know if it was entirely composed of diatoms or if there were any other microalgae (or cyanobacteria) present, too. What about the identification of the diatoms (and possibly other groups) in the samples?
 
12) Table 1 is messed. For example, the temperature values are listed in a column entitled "Water Salinity (°C)" Well... This needs to be carefully checked and corrected accordingly.
 
13) l. 286: "Benthic diatom primary production and respiration"
Of course, that title is correct provided that the community really consisted solely of diatoms.
 
14) l. 294: "because of technical problems" < "due to technical problems"

Author Response

Dear reviewer,

many thanks for your friendly and constructive comments, which are grately appreciated. We like to address your open points as follows:

1. we changed this long sentence to 2 separate sentences.
2. we introduced a short substantiation of our study.
3. we specified our expectations.
4. we clarified this statement in comparison to other studies.
5. we tried to improve our abstract by polishing the text.
6. we provided a reference as requested.
7. we improved the sentence as suggested.
8. we introduced a paragraph into the discussion on microphytobenthic community composition (see also referee 1)
9. we improved this sentence as requested.
10. we provided additional information to justify this statement.
11. please see point 8. we provided more information on the diatom structure at the study site.
12. sorry for table 1 which was really a mess - we fixed the columns to the right position.
13. benthic diatoms always dominated to >80% the samples, and hence we prefer to stick to the title.
14. we improved this sentence as requested.