the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Dec 2025
Worms or storms? Distinguishing bioturbation from physical mixing using multiple tracers
Abstract. Sediment transport and seabed composition can both be influenced by bioturbation and hydrodynamically driven sediment mixing and deposition. In a dynamic intertidal environment, it is challenging to distinguish the relative contribution of both processes. We aim to unravel their relative importance by combining several tracers, each having its own specific timescale and target particle size. We combined (1) 210Pb that quantifies long-term (years–decades) mixing of fine sediment fractions with (2) Chlorophyll-a and (3) luminophores that both quantify short-term mixing of fine sediment fractions (days–weeks), and (4) multi-grain quartz and single-grain feldspar luminescence dating, which use the bleaching of sand grains’ inherent luminescence signal by light to assess mixing of sand and thereby quantifies long-term mixing. Single grain feldspar luminescence is here for the first time applied in the intertidal environment. We compare results for a sandy and a muddy intertidal flat at the island of Texel (Wadden Sea, the Netherlands), each with their own characteristic benthic community. Recent bioturbation became apparent from Chlorophyll-a and luminophore profiles: particles were rapidly reworked to a depth of decimetres. 210Pb also suggested mixing and non-local exchange of particles by bioturbation. Luminescence age distributions suggest that quartz luminescence signals were fully reset upon recent deposition, while bioturbation enhanced resetting of feldspar luminescence signals. Coarse- and fine-grained tracer profiles show the differential behaviour and reworking of the mud and sand fraction within the sediment matrix: as expected with particle-selective bioturbation, mud is preferentially bioturbated and infiltrates passively, while sand grains have a higher ability to conserve layering. Single-grain feldspar luminescence is a promising technique to demonstrate the long-term reworking of sand grains, however, in young and dynamic environments, a combination of tracers remains necessary to inform on the origin of mixing.
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Status: open (until 09 Feb 2026)
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RC1: 'Comment on egusphere-2025-6029', Anonymous Referee #1, 26 Jan 2026
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AC1: 'Reply on RC1', Tjitske Kooistra, 03 Feb 2026
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We thank Referee 1 for the thorough and constructive review, which helps us to improve the manuscript.
We acknowledge that the title and introduction cause some unclarity regarding the main aim of the paper. However, we do believe that the aim can be two-fold: testing the application of a new method while answering questions on the relative contributions of biological and physical processes. To clarify this dual objective, we will critically revise the abstract and introduction. Furthermore, we plan to slightly rephrase the title to “Worms and storms: shedding light on bioturbation and physical mixing by combining multiple tracers”, in order to clarify that we look at the relative importance of both, not one over the other.
Although deciphering the ecological implications was not among the main aims of this study, we could strengthen the ecosystem significance part in the discussion (after L580), for instance, by adding on the implications for redistribution of food particles, oxygen and nutrients into deeper sediment layers. However, given that this aspect was not the main focus of the study and in the interest of conciseness, we would prefer to keep this discussion succinct.
Lastly, concerns were expressed on the generalisability of the interactions between physics and biology. We wish to not overgeneralise these interactions, as they are usually context-dependent: they differ between system, between sites and over time as the biotic community changes. We will mention this limitation more explicitly in both the introduction and the site description. Furthermore, we will emphasize that while the relevant biophysical interactions are site-specific, the methodological framework itself is more broadly applicable, provided that the limitations discussed in the manuscript and illustrated in Fig. 7 are considered.
Regarding the minor comments, we will implement the suggested textual revisions, figure readability modifications, provide additional clarifications, and include the requested references. We will explicitly acknowledge the limitations of a small number of long cores (n=2) for quantifying biogenic effects on short time scales, and clarify that the aim was to capture longer-term processes with these tracers, which provide and integrated net outcome over a longer period.
Kind regards,
Tjitske Kooistra and co-authors
Citation: https://doi.org/10.5194/egusphere-2025-6029-AC1
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AC1: 'Reply on RC1', Tjitske Kooistra, 03 Feb 2026
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RC2: 'Comment on egusphere-2025-6029', Anonymous Referee #2, 26 Jan 2026
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**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.
Citation: https://doi.org/10.5194/egusphere-2025-6029-RC2 -
AC2: 'Reply on RC2', Tjitske Kooistra, 03 Feb 2026
reply
We wish to thank Referee 2 for the positive review of our manuscript and the detailed comments.
We will make textual modifications and add the requested references. Furthermore, we will clarify the limitations of the methods applied, where asked for. Specifically, we will stress need for more replications, especially when using short-term tracers, to enable quantification of bioturbation in a spatially heterogeneous environment. We will also metion that aligning sampling protocols can increase the comparability between tracers, especially when the aim is to quantify mixing. Lastly, we will modify figures 2 and 3 to improve readability.
Kind regards,
Tjitske Kooistra and co-authors
Citation: https://doi.org/10.5194/egusphere-2025-6029-AC2 -
RC3: 'Reply on AC2', Anonymous Referee #2, 03 Feb 2026
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I am personally satisfied with the authors' responses to the comments made by the two reviewers.
Citation: https://doi.org/10.5194/egusphere-2025-6029-RC3
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RC3: 'Reply on AC2', Anonymous Referee #2, 03 Feb 2026
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AC2: 'Reply on RC2', Tjitske Kooistra, 03 Feb 2026
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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.