Kooistra et al Worms or storms? Distinguishing bioturbation from physical mixing using multiple tracers for EGU Biogeochemistry

This is an interesting paper that addresses an important question concerning the importance of physical and biological processes in sediment mixing.  I think the Introduction of the paper does not match the title very well.  This initially mis-led me.  Firstly, except for the extremes of physical activity the issue is about the importance of both physics and biology not one over the other.  Secondly, the paper needs to be clearer on prioritising the importance of physical and biological processes to sediment ecosystem processes or the methods used to make the measurements.  Both are important but the text jumps around, which does not help with the clarity of the message.  The end of the Introduction states that the goal of the study is to “unravel the relative contributions of bioturbation and physical dynamics to sediment” and to do this appropriately a range of tracers that work over different times scales are used.  But the Introduction then jumps to more methodological questions “(i) can single-grain luminescence be meaningfully applied as a bioturbation tracer in such dynamic environments? (ii) how do short- and long-term tracers aid in distinguishing bioturbation from physical mixing?”   The ecosystem significance is the results are returned to at the end of the Discussion (c L580) but this is not well developed.

The central hypothesis of the paper is stated as “if biotic mixing dominates, we will observe tracer-dependent mixing and, depending on the benthic community, diffusive or advective transport”. While the paper is based on a field study in mud and sand habitats – the assumptions of how physics and biology interact in mixing sediment is not framed in a generalisable way.  Storms (extreme events) are not the only source of sediment transport.  In many intertidal flat environments, there is an important role for locally generated wind waves in mixing sediment and generating ripples.  This can involve an interaction between tides, waves and biological sediment stabilisation or destabilisation. The description of processes around L 65 is too narrow – it may well represent the specific of this study site  but think how this is framed.  This interaction of physical and biological phenomena does not lead to the assumption of event driven sediment horizons vs deep mixing by animals (around L425). 

Minor points:

Abstract: “Luminescence age distributions suggest that quartz luminescence signals were fully reset upon recent deposition, while bioturbation enhanced resetting of feldspar luminescence signals”  this is not a very clear statement for the Abstract.

“sedimentary habitats by so-called ‘bioturbation” – omit so-called.

The paper is methodologically appropriate in terms of the tracers, albeit I have not experience with one set of these.  However, the sediment long cores with an n=2 will underestimate biogenic effects in heterogeneous sediments.  This needs to be acknowledged as a limitation and used in the interpretation of the results particularly around the relative importance of sediment mixing and episodic deposition.   

“de Boer et al., in review”  do not cite unless in press/published

Please explain how the sand bleaching rates are linked to levels of radiation – are these a constant or do they vary with latitude or air pollution?

Fig 2 -depth scale on the long cores is not visible

Around L210: “The downcore distribution of 210Pb in the mud fraction (<63 μm) of the sediment was determined indirectly  through alpha spectrometry measurement of its grandchild isotope 210Po.  Spell out Polonium-210 on first use.  There is no reference cited for this method. 

“For cosmic dose rate calculation (Prescott et al., 1994), we assumed gradual burial of the samples to the present depth. Why given intro text where you introduce episodic events. 

“Macrozoobenthic community biomass and abundance were higher on the muddy site …”  acknowledge that this does not include the Arenicola.

**General comments**

The ms by Kooistra et al. presents an interesting work that aims studying both physical and biological particles mixing in intertidal environment. The in situ work carried out in qualitative comparison between different experimental short-term and natural long-term tracers, including a tracer (luminescence signal) that, to my knowledge, has never been used in such a multi-tracer approach in the marine environment.

The experimental protocol used in such natural conditions and had its limitations. They were mentioned by the authors, for example the relating to the size of the core samplers used for Chl-a and luminophores, which were insufficient to capture the biological heterogeneity. Adressed also by the authors in their comparison of tracers, the specific characteristics that need to be taken into. For example, the luminescence signal can be reset, unlike other signals.

Based on the combination of all the information provided by the various tracers, the authors proposed scenarios for the history of deposits in the two studied sandy and muddy sites. But also a smart decision tree guiding the choice of different tracers to study the transport capacity of intertidal environmental particles, whether the movements generated by abiotic or biotic processes are of interest. It also takes into account the grain size distribution of the site under study. A very interesting tool for geochemists and/or biologists interested in sediment transport.

I recommend this manuscript for publication after minor corrections.

**Specific comments**

L77: perhaps write "(per)turbation" instead, in order to recall the etymology and meaning of the word bioturbation?

Table 1, it is indicated an Investigation timescale of 15 days - 3 years for Chl-a. Theoretically I agree, but practically I think it should be rather considered as a non-conservative short time tracer. Also this will depend if experimental pulse protocols (like with luminophores) or natural measurements are implemented.

L140: "Moreover, biotic mixing may occur during low tide…". Is this a general fact whatever the species involved? Any references to support this statement?

Figure 2. One cannot read the depth indications on Fig 2d. Increase the font size and darken the characters, or indicate a general scale?

L175: indicate the range of depths

L178: I guess the surface area covered by the spade was the same as that covered by the hand corer?

L258: why not have applied the same type of protocol (i.e. vertical slicing, homogenisation then measurement) for the luminophores than for the Chl-a? You could have obtained results for both tracers within the same cores.

Paragraphs 2.7 and 2.8 from L264: How many sediment cores were collected each time for the different tracers? Looks like three to me for luminophores from the text (but two from Fig. 5c). On the other hand, the information I cannot find out for Chl-a. Linked to this, there is no variation in the data presented in fig. 5. (line 350). Using a small 3.6 cm diameter tube in a 30 cm x 30 cm square gave you many opportunities for replication to catch the biological heterogeneity. A point you mentioned in line 543. Enhancing replication could be suggested as a solution here.

Fig. 3. Distinguish the colours of the 250-500 and 500-1000 micron fractions a little better.

L341: Even though Scrobicularia plana is able to bury deeper than Cerastoderma edule, 18 cm and 20 cm deep is extremely deep (if not impossible to reach) for those two bivalve species and more especially small individuals (as captured in 3.6 cm diam core).  I suggest nuancing this explanation and perhaps also suggesting the hypothesis that you may have captured a burrow opening into which the tracers entered?

L346, more replication could have helped resolve this variability issue in such a heterogeneous environment.

Paragraph 4.3 the "community part" from L469: Here, you establish some indirect connexions between some tracers distributions and the community present overall (i.e. not determined from the specific tracer cores). On the other hand, for the luminophores cores, a direct relationship with specific species found in the core is proposed. To be consistent with my previous comment on the role of bivalves in tracers transfer in one luminophore core, I suggest having the sentence "…as evidenced by luminophores that had moved rapidly to the living depth of this bivalve (Fig. A2)." presented as a hypothesis rather than a fact.

L578: "…the sand fraction is not intensively reworked". This could be different under the influence of other benthic bioturbating communities.