The authors thank referee #1 for his/her interest in our study and for the detailed leads he/she provided to improve the manuscript. Here below the reply to referee #1’s comments.

The following points should be addressed by the authors:

• Can you give some information about the grain size distribution of the sediments you used?

Following referee #1’s advice, we performed additional grains size analyses on the original sediment used in the experiment by means of a laser diffraction particle analyser (Malvern Mastersizer 3000). The analyses revealed a unimodal grain size distribution with a mode of 6 µm (medium silt). The sediment is not well sorted, showing a D₅₀ of about 10 µm and a D₉₀ of 47 µm. The proportion is of 93% of silt and clay and 7% of sand. This complementary data will be available in the final version of the manuscript lines 152-154.

• Concerning total foraminiferal abundances (Fig. 5), I find it difficult to define one out of two counts as outlier point. What about T4 control, here you also have a very high variability between both replicates. I would recommend including all data points in your analysis.
• Do you also treat this T0 Control replicate as an outlier point when comparing the abundances of the two main species?

We agree that the term “outlier” for the point showing high variability in total foraminiferal abundances was not adequate.   We had indeed included these points in our analyses, so we had not considered them to be true outliers from a statistical point of view. Therefore, we have removed the term "outliers" in the final version of the manuscript.

Some minor points:

The brackets concerning the references do not always seem to be on the correct position. Especially in sentences like in line 71. I suggest adapting it consistently in the manuscript, e.g. in line 71 always put only the year in brackets: “…model from Jorissen et al. (1995)….” Done

Line 43: Please correct: “… flooding (Extence et al.,…………2015), glacier……..” Done

Line 54: Please correct: “… considered as one of ….” Done

Line 89-94: These sentences need language improvements. We agree these sentences were not clear enough and we have rewritten them (see lines 90-95) of revised manuscript:

“However, in natural environments, sediment supply organic matter input and oxygen availability often covary and synergically affect microhabitats distribution. Therefore, experimental studies are the only way to deconvolve the impact of each parameter is to test its effect in a controlled setting where environmental variable can be artificially reduced. We designed an experimental set-up to test, in microcosms, the effect of different patterns of fine-grained sediment deposits on benthic foraminiferal communities, without having the coupled effect of organic matter and oxygen variations.”

Line 102: slikke? -            

Muddy tidal flats are also called mud flats or slikke (name of Dutch origin). Following the referee #1’s advice, and to avoid confusion we refer now to “mudflat” in the whole text.

Line 164: The grid was removed after placing the foraminiferal samples, correct?

Line 164 : This grid was only used to have a guide during the introduction of foraminifera in the aquaria,  to ensure a distribution of individuals as homogeneous as possible. It was inserted above the water column and removed just after the foraminifera were inserted.

Fig. 6 and 7: The numbering is twisted. Right, we corrected it.

Fig. 6 (should be 7): Species names must be written in italics. Done

Line 389: I would cancel the following sentence: “We estimated a maximum speed of 1.4 mm.h-1 for  A. confertitesta  and for H. germanica." This information is more confusing here than helpful, because both species have different velocities. We agree, we cancelled the sentence.

Line 467: How fast would these species reach a size of 125 µm after reproduction?

In previous laboratory works, growth time of Ammonia tepida from the juvenile to the adult stage was found to be 3 months (Stouff et al., 1999) and more than 134 days in temperatures close to our experiment ones (Bradshaw, 1957). Temperature have been proved to be a key factor affecting foraminiferal growth and chamber formation. With low temperatures, metabolisms of individuals slow down hence, growth takes more time. It is therefore unlikely to observed a growth of population through reproduction or inserted propagules or juveniles. Concerning Haynesina germanica, the growth rates are much less known but as the species did not increase in abundance during the experiment, we suppose that we did not obtain reproductions in the microcosms.

Line 481: The following reference deals with H. germanica food preferences and may be helpful for this discussion here: Wukovits et al., 2021. Phytodetrital quality (C:N ratio) and temperature changes affect C and N cycling of the intertidal mixotrophic foraminifer Haynesina germanica. Aquatic Biology, 30, 119-132. https://doi.org/10.3354/ab00746

The cited study is very interesting for our discussion. We added a sentence to add the information about the ability of H. germanica to switch to lower quality feeding material if needed, but involving negative effects on its physiology; see lines 490-491

Line 526: Please add: “…. (Fig. 8b)…” Done

Line 539: The following sentence is confusing: “…Indeed, Koho et al. (2018) reported changes is Ammonia confertitesta (genetic type identified by Holzmann and Pawlowski (2000) and renamed Ammonia confertitesta  Zheng 1978, by Hayward et al. (2021) ultrastructure as a stress response to oxygen depletion.

The sentence has been rewritten ; see lines 549-552

“Koho et al. (2018) reported changes in Ammonia confertitesta ultrastructure as a stress response to oxygen depletion (NB : In Koho et al., (2018)  Ammonia confertitesta was mentioned as Ammonia sp. T6, one of the phylotypes of Ammonia distinguished by molecular identification (Holzmann and Pawlowski, 2000), and renamed Ammonia confertitesta by Hayward et al. (2021)). »

Line  545: Pseudopodia can sometimes be very long and maybe reach already oxygenated layers and provide the foram with oxygen before the body will arrive there.

We agree that the pseudopodia can be very long, even if the distance between the ancient surface and the new one after deposit in the OHV treatment is close to 3 cm, which means 100-fold the size of the adult specimens we had. However, in our setting the migration of most of the specimens toward the C-depleted new sediment surface rather indicates that oxygen limitation at 2.7 cm depth can be a driver for upward migration. Otherwise, we would have observed rather a concentration of the specimens in the ancient surface layer, much richer in organic matter.

Table 1: You should also add the name you are using in this study for Ammonia: “… Ammonia tepida /confertitesta Done

The authors appreciate the referee #2’s effort in understanding the reasons of their choice to perform this microcosm experiment and thank referee #2 for recognising the quality of the experimental design. Here below our reply to referee #2’s main comments.

I recognized that this article is kept high level techniques as microcosm experimental studies.  Each part is well designed and well managed.  But I cannot catch why the authors carried out such a time-consuming work for getting response of benthic foraminifers’ behaviors against flood events in connection to vigorous storms mainly due to climate change by global warming at some part of Europe.  What is the main question they would like to solve, ?   Why do they plan to carry out such a big microcosm experiment?   I think that small experimental works by previous foraminifera majoring authors can solve questions that you were raised.  Why do you need to construct such a big experimental system?   I think that we need big questions as motivation of research.  But, I cannot find any clear vision behind this experimental work.

As explained in the introduction (lines 77-97), our study focusses on the effect of abrupt sediment deposition on the physical stability of various benthic environments, not only in settings subjected to flood events and storms (Line 42-46). Because in natural environments subject to sediment supply, organic matter fluxes and oxygen content in the sediment covary, it remains difficult to understand how these factors constrain the vertical distribution, biodiversity, and density of benthic fauna. Therefore, we believed that an experimental approach was needed to determine only the effects of sediment deposition, whether small or large, frequent or not, over a short time scale, all other environmental drivers remaining stable. In our experiment, we simulated different kind of sediment supply (volume and frequency), controlling the geochemical stability of the system, and we use foraminiferal species which are largely known to tolerate culture conditions (Bradshaw, 1957; Stouff et al., 1999; Denoyelle et al., 2012; Geslin et al., 2014; Nardelli et al., 2014).

As suggested by referee #2, works to prove that foraminiferal species can respond quickly to sediment deposition and are able to migrate back to the surface are not worth the effort. However, our results show that the two species considered in our microcosm respond very differently in terms of abundances all along the experimental time and have different reaction to the two treatments (i.e., OHV, FLV). This means that even if the ability of macro and megafauna to crawl to the surface after a deposit is already demonstrated (as suggested by referee #2, for nematodes or ostracodes), this is potentially not true for all the sediment flux patterns, and certainly not well quantified for meiofauna like foraminifera.

The results of our experiments will contribute to the understanding of benthic ecology in Arctic fjords that are strongly influenced by increasing seasonal sediment discharges originating from melting tidewater glaciers (cf. introduction line 43-44). In Svalbard fjords, we study within a large research project BEGIN (funded by INSU CNRS and IPEV, see acknowledgments in the revised manuscript), recurrent sediment discharge is expected to rapidly increase due to global warming and Arctic amplification. The ultimate goal of our research is to determine how the glacier melting has accelerated in the recent past (i.e. historical times before satellite images), as foraminifera build protective tests that remain in the sedimentary archives as fossils, and thus record former environmental changes.

Does benthic foraminifera adequate to monitor responses of benthic organisms against vigorous depositional changes at coastal environments ?  I feel that metazoan meiofauna, either nematodes or ostracodes, and/or megabenthos such as molluscs or crastaceans are quickly recovering from the deposition of fine muddy sediments.  Because, they are big size in general and are easily crawling up from quickly deposited sediment layer.

As early as the 1960s, benthic foraminifera were used as proxy to describe the marine environments (see the review in Murray, 2006 and Shönfeld et al., 2012). During the last decades, bio-monitoring with foraminifera has developed, and they have been widely used as bioindicators to survey different marine settings (Hess and Jorissen, 2009: Goineau et al., 2012; Bolliet et al., 2014; Dessandier et al., 2016; Duros et al., 2017),  especially coastal ones (e.g., Frontalini & Coccioni, 2007; Frontalini et al., 2009; Laut et al., 2011; Bouchet et al., 2012, 2018; Martins et al., 2013, 2015, 2016a, 2016b; Barras et al., 2014; Nesbitt et al., 2015, Alve et al., 2016; ; Belart et al., 2018; Jorissen et al., 2018, 2022 ; Fontanier et al., 2020). This is explained in more details in the introduction of the paper (lines 65 and after).

As shortly explained in our introduction (line 61-64), and in more details in the review paper of Shönfeld et al., (2012), foraminifera present several advantages as bioindicators of coastal environments in comparison to the more commonly used macrofaunal organisms.

A final point in favor of foraminifera is that they are the most used bioindicators for past sedimentation events recorded in long sediment cores, because of their tests preserved in the sediment as fossils. For these reasons, the information about their ability to respond to physical instability, especially in coastal environments, is of particular interest.

Please connect each experimental components with each other for making discussion networks.  Otherwise, it is very difficult to analyze and discuss results and then to move to hypothetical conclusions after such a big experiment.

We are baffled by this comment because the structure of the discussions (1. Geochemistry, 2. foraminiferal abundances, 3. Vertical migration) follows exactly the same presentation as the results section (1. Geochemistry, 2. Foraminiferal abundances, 3. Vertical migration). So that it is very difficult to us to understand, on the base of this unique sentence, what exactly we should do to structure the manuscript in a more understandable way.