Modelling the Influence from Biota and Organic Matter on the Transport Dynamics of Microplastics in the Water Column and Bottom Sediments in the Oslo Fjord

Round 1

Reviewer 1 Report

First of all, I want to thank the authors for the significant work performed to produce this paper. The research proposal is clear, the objectives too, and the effort done to build the complete model is obviously considerable, with substantial attention paid to the details. In addition, the paper is well-written, clear, and I had the chance to learn a few things in the process. So congratulations for the good job.

However, I have a few comments, most of them minor. Full list is provided in the commented PDF version of the manuscript I have submitted to the editorial, but I will sum up here the main aspects that, in my view, requires review.

The most important one for me, is the fact that authors present the results and discussion of the paper as if finding seasonality in the sinking processes of the microplastics, as per their connection to ingestion by zooplankton and primary production, was something never observed or thought by the community, which is not accurate. There is a good number of publications that already link sinking rates to zooplankton ingestion or biofouling. A few examples of such works:

Cole, M., Lindeque, P. K., Fileman, E., Clark, J., Lewis, C., Halsband, C., & Galloway, T. S. (2016). Microplastics alter the properties and sinking rates of zooplankton faecal pellets. Environmental science & technology, 50(6), 3239-3246. (cited by the authors in the manuscript).

Tekman, M. B., Wekerle, C., Lorenz, C., Primpke, S., Hasemann, C., Gerdts, G., & Bergmann, M. (2020). Tying up loose ends of microplastic pollution in the Arctic: distribution from the sea surface through the water column to deep-sea sediments at the HAUSGARTEN Observatory. Environmental science & technology, 54(7), 4079-4090.

Coyle, R., Hardiman, G., & O’Driscoll, K. (2020). Microplastics in the marine environment: A review of their sources, distribution processes, uptake and exchange in ecosystems. Case Studies in Chemical and Environmental Engineering, 2, 100010.

Kvale, K. F., Friederike Prowe, A. E., & Oschlies, A. (2020). A critical examination of the role of marine snow and zooplankton fecal pellets in removing ocean surface microplastic. Frontiers in Marine Science, 6, 808.

Kvale, K., Prowe, A. E. F., Chien, C. T., Landolfi, A., & Oschlies, A. (2020). The global biological microplastic particle sink. Scientific reports, 10(1), 1-12.

Porter, A., Lyons, B. P., Galloway, T. S., & Lewis, C. (2018). Role of marine snows in microplastic fate and bioavailability. Environmental science & technology, 52(12), 7111-7119.

Lima, A. R. A., Barletta, M., & Costa, M. F. (2015). Seasonal distribution and interactions between plankton and microplastics in a tropical estuary. Estuarine, Coastal and Shelf Science, 165, 213-225.

To cite some of the works.

It is true these works do not refer to the seasonality effects, as they focus in the interaction between zooplankton ingestion and sinking rates of the marine snow, microplastics and/or faecal pellets, or well mention the role of biofouling, but they already stablish such link between the two elements: zooplankton/primary production, and microplastics sink rates. Therefore, any phenomena affecting zooplankton densities in water column, will also impact MPs sink rates. In this case, increase of primary production which leads to growth of zooplankton density, is the link explaining the seasonality. Zooplankton seasonality is well know, and therefore, this can be hardly considered a demonstration or discovery, rather than an exercise to confirm by models what can be expected from our current understanding of such links.

For me, the novelty of the described work is, precisely, having developed a more complete theoretical numerical model able to consider such aspects, and not as much its results in terms of seasonality behaviour (which are expected and, in fact, propagated by construction of the model to the results).

The second aspect that authors pass-by with no mention is the role of vertical seawater stratification in the experiment. This is a really important factor, which tends to be coupled also with primary production and zooplankton densities, as result of the formation of a seasonal thermocline due to changes in sun light radiation intensities and air temperature, plus the changes due to the inputs of freshwater from ice/snow melting, which is also seasonal and drags too stratification.

There are some studies revealing already the relevance of such stratifications and the vertical distribution of microplastics, like:

Zobkov, M. B., Esiukova, E. E., Zyubin, A. Y., & Samusev, I. G. (2019). Microplastic content variation in water column: The observations employing a novel sampling tool in stratified Baltic Sea. Marine pollution bulletin, 138, 193-205.

Stratification will have a major role in the sinking rates, especially for those MP particles with buoyancy is close to seawater (whereas due to its natural density or as resulting of the ballasting). This is not discussed, and in fact, the results pointing towards a homogeneous distribution of neutral buoyancy MPs seem opposed to this element. This may happen only if the model uses a homogeneous water column, which is a strong assumption and should be made very clear as such.

The third aspect of concern I have is that model is designed as 2D model, as a vertical transect (i.e. only in axis x and z), but they use only horizontal water velocities in the simulated dynamics (line 239 of the manuscript). They include, however, vertical turbulent coefficients, which essentially is the turbulent term determining vertical fluxes between depth layers, but without vertical forcing due to vertical velocity. I have a bit of a problem with this, because vertical velocity can traditionally be neglected in many models, due to its comparably much smaller contribution to fluxes vs the horizontal fluxes. However, as this model tackles transport of cuasi-neutral suspended particles, small vertical components may have a more then serious impact in the residence time of such particles in the water column. Whereas this can be considered an assumption/simplification of the model, it is important to highlight this as a specific limitation that may yield very different results.

And as a final general comment, I also consider that the authors shall be more explicit about the validity of the results: in my opinion, they are interesting from an analytical point of view, to better understand known processes, but not as much to quantify them. Due to the many limitations of the model (understandable, as is of a high-complex nature), results have a qualitative interest, but not quantitative. I know that reading the paper sends this message, but it is not clear enough in the conclusions.

I want to thank again the authors for their effort put in this paper.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper is very important and interesting. I am sure it will be a important element of microplastic monitoring and for deepening the understanding of natural processes such as biological pomp and their role in the acceleration of MP burying and reduction in the water column.

Author Response

Thank you very much for your positive feedback!

Reviewer 3 Report

Authors presented of modeling the influence from biota and organic matter on the transport dynamics of microplastics in the water column and bottom sediments in the Oslo Fjord. The topic is very interesting and will be of the use to the scientific community in the field. Additionally, the results are clearly presented and the conclusion are supported by the results. Therefore, I can recommend the manuscript for publication in it`s present form.

Author Response

Thank you very much for your positive feedback!

Reviewer 4 Report

Comments on the manuscript with the title: Modelling the influence from biota and organic matter on the transport Dynamics of microplastics in the water column and bottom sediments in the Oslo Fjord. In this study, the authors of the manuscript present the results of the evolution of the injection of different types of MP and at different water depths in the water column of a fjord. For their predictions they use a model that considers the interaction between MP particles and the ecosystem. The model considers the interaction between MP and phytoplankton, zooplankton, biofouling. It also considers MP with different buoyancies. The aim of the study is interesting and the introduction is well set. However, I have some concerns that I recommend should be considered before its acceptance. 1. The model is explained in high detail, too much from my point of view. I would suggest to move all the details for an appendix section. I do not mean to remove them, just move them to another section. In the current form it is difficult to see the main parameters that are going to be presented in the results. I would also suggest including more references on how these equations are based. 2. In contrast to the high detail of the model, the results are poorly explained. The results in Figures 7-12 should be better detailed. These are the main results of the manuscript. Figure captions should be detailed, with a recall on how is means each variable presented (MP-free, MP-biof, MP-het…..). 3. In these figures there are some results that I found intriguing. In Figure 10, MP-free is different from that in Figure 11 and Figure 12, as I would expect. These figures present the annual evolution of MP. However, I cannot see any difference between the results of MP-free between figures 8 and 9. Why? Did I miss something? Why the annual evolution shows difference and the evolution along years does not? 4. Actually, Figures 8 and 9 look like the same. Shouldn’t be different? I would expect differences between positive and negative buoyant MP particles. 5. In the captions of these figures I would suggest to include the explanation of the detailed figure below each on the figure panels. I mean the plot that has the vertical axis scale between -5 and 5 m. Could authors explain something else about the results presented in these small figures? Otherwise I do not see the point in presenting them. 6. In the discussion of the results authors present a table (table 3) where they compare the results of their model with those obtained by other authors in the different compartments of the water column. In the upper layer the results obtained by the model seem to describe the observations by others. In deep layers (30 m depth) authors present the model results but it seems unable to do a comparison due to a lack of data in such compartment. However, in the sediment compartment, the model seems to over predict the amount of MP. Can authors suggest any reason for this discrepancy? Authors should comment such difference and they could give some cause of these mismatch.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

I think that authors have improved the manuscript based on the comments I first made. They decided to maintain all the information on the model in the main body of the manuscript. From my point of view this might confuse the readers and I would still suggest that since it is a long section it would fit better in an appendix section. Beside of this, I think that the results might be valuable of being published.

Author Response

Thank you for your positive feedback!

We followed your suggestion and moved the OxyDep and BioPlast parameterizations to supplementary materials. We have duplicated the main system of equations (1) in the appendix to avoid confusing and saved the numeration. We also renumbered the rest of the formulas and tables since the previous ones were moved from the main manuscript.