the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Feb 2026
The integrated benthic silicate flux in the Baltic Sea suggests a major land-derived reactive silicon source
Abstract. Coastal marine environments are hot spots in the global marine silicon (Si) cycle. Dissolved silicate (DSi) is an essential macronutrient for diatoms, which often dominate primary productivity in temporal coastal seas and constitute a key food source for grazers. Even though benthic release of DSi influences the ecology of coastal marine areas, direct rate measurements of DSi mobilisation remain scarce. The Baltic Sea is no exception, and the spatial coverage of benthic DSi flux data is low and limited largely only to regional reports. We report data from 305 individual measurements (mostly in situ) of benthic DSi fluxes conducted in different basins and sediment types of the Baltic Sea during 2001–2021. Using the benthic DSi flux data in combination with literature values, representative average fluxes for various sediment types in the major basins of the Baltic Sea were determined. An areal extrapolation using Geographical Information System (GIS) tools suggests an integrated annual benthic release of 8520 metric kilotonnes (kt) of DSi for the entire Baltic Sea. This benthic release of DSi is about ten times higher than the reported riverine transport of DSi to the Baltic Sea. Furthermore, this benthic load, together with the reported annual burial rate, is more than three times higher than the autochthonous export production of biogenic silica out of the photic zone. The integrated benthic DSi release being substantially larger than that cycled by diatoms may explain the trend of the increasing DSi standing stock in most of the Baltic Sea basins which has been observed since the 1990s. Overall, other major sources of reactive Si (estimated to be 6390 kt yr-1) to the sediment are suggested to exist, such as deposition of river and groundwater derived reactive (dissolvable) particulate amorphous and/or lithogenic Si. Our results strongly suggest that the biogeochemical Baltic Si cycle is more heavily influenced by reactive Si of terrestrial origin than previously known.
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Status: open (until 30 Mar 2026)
- RC1: 'Comment on egusphere-2026-707', Anonymous Referee #1, 08 Mar 2026 reply
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RC2: 'Comment on egusphere-2026-707', Anonymous Referee #2, 17 Mar 2026
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Ekeroth et al complied 305 individual benthic DSi measurements in the Baltic Sea over the past two decades. This is a very much appreciated effort to refine today’s Baltic Si budget. Given this dataset, it is clear that benthic BSi and LSi dissolution is properly the most important process supplying DSi to the Baltic Sea. However, it is a pity that the authors work hard to address the missing Si pool derived from land that they cannot provide any direct data/evidence from this study in terms of the mass balance (such as Fig 6), while there are some potential findings that requires more attention get lost in the discussion. Below are major suggestions.
- It is great to see so many in situ DSi flux measurements across the Baltic. Meanwhile, these measurements also show large spatial heterogeneity that cannot be simply explained by seabed types. E.g., in Figure 2a, for the mud-muddy sand seabed, the DSi flux varies between 1-10 mmol/m2/day. Is this a natural variability or this is something to do with in situ technique from different years or the sampling locations (river mouth vs fjords vs shallow open sediments vs deep basins etc.) or with temporal periods (the peak of eutrophication to the post-eutrophication period). Anyway, I think there is a large potential to dig from this dataset, which is worthy more efforts and in turn improve the certainty of benthic DSi estimates.
- I see the point that the total calculated benthic Dsi flux is much larger than BSi export from water column, but another uncertainty here is the accumulative effects of sedimentary BSi due to eutrophication. There was a clear increasing trend of BSi contents in the Baltic sediments before, so this older Bsi pool could continue dissolving as part of this benthic flux. This adds more uncertainty to estimates of LSi dissolution in this study.
- There is no doubt LSi in seafloor is an important player, but some factors should be considered. (1) the rate of LSi dissolution and authigenic clay formation, usually these are much slower than BSi process. if the in situ measurements are running from hours to days, how much LSi dissolution signal it is really captured is a question. (2) Si consumption by authigenic clay formation may can also be considered in the overal Si sink.
- Overall, both the introduction (almost one page) and discussion sections (4.1) start with the important role of riverine inputs, giving a feeling of a study about Si delivery by rivers, but the methods and results exclusively leans on benthic flux. I think it would be much better to focus more on discussing benthic flux variability that this study has solid evidence, and speculate to a lesser extent regarding the mass balance of Si in the Baltic, in which riverine and groundwater inputs and LSi dissolution, two major sources, are largely unconstrained.
Citation: https://doi.org/10.5194/egusphere-2026-707-RC2
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The integrated benthic silicate flux in the Baltic Sea suggests a major land-derived reactive silicon source, Zenodo [data set] N. Ekeroth et al. https://doi.org/10.5281/zenodo.18484732
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- 1
For marine biogeochemists the Baltic Sea is a world ocean fascinating part. Indeed, this semi-enclosed sea is not easily renewed by North Sea episodical inputs, and thus is submitted to perturbations generated from the land which indirectly impacts the evolution of the dioxygen content of the deep reservoir and of the nutrient cycles of the Baltic ecosystem as a whole.
In this study the authors implemented an ambitious program to estimate of the benthic silicic acid flux at the sediment-water interface for the different types of sediments that composed the Baltic Sea bottom (sand, mud-muddy sand, rock and boulder, mixed types). Using data acquired from benthic landers in the many sub-basins of the Baltic Sea and from ex situ measurements, thanks to GIS tools the authors spatially extrapolate from local to Baltic scale. They report silicic acid benthic fluxes ranging between 0.3 to 9.0 mol-Si m-2 d-1 with a mean of 3.7 mmol-Si m-2 d-1, which is consistent with analogous systems over the world. Then, extrapolating to annual scale the authors calculated a total flux of 8520 kt-Si yr-1 (304.3 Gmol-Si yr-1). This part of the article is without problems (however see comments below).
Difficulties rise when the authors try to built an equilibrated budget of Si at Baltic Sea scale, integrating different fluxes and processus that are presently not well known, and/or not constrained.
1-Playing the authors’game, assuming steady state, to get a balanced budget of Si for the Baltic Sea, I build the below figure, inspired from Tréguer et al. (2021).
Figure 1 : A possible scenario for a steady state Si cycle in the Baltic sea
This budget is built assuming that the benthic flux is generated by the amount of biogenic silica deposited in sediments that escapes long term accumulation (21.8 Gmol-Si yr-1).
For an annual benthic flux of 76.1 Gmol-Si y-1, the mean daily benthic flux is 0.56 mmol-Si m-2 d-1, which actually is in the range of the authors’measured fluxes (0.3-9.3 mmol m-2 d-1) but more than six times below the mean calculated by the authors. Assuming spatial extrapolation through GIS is correct, could the authors’ annual benthic flux be over estimated due extrapolation of fluxes measured during a short period of time?
If not, could the export of biogenic silica to depth be underestimated ? Could the gross production biogenic silica in the surface layer be underestimated ?
2-It is clear that more data dealing with the biogenic matter fluxes and/or of the silicic acid fluxes (Si release from particulate matter transported by rivers, direct dissolution of lithogenic silica of sediments,…) are needed to build a realistic budget of Si for the Baltic sea. The authors’ speculative Figure 6, a conceptual mass balance in the Baltic Sea, is not helpful given the uncertainties as regards the rSi sources which, according to the authors totalize 326 Gmol-Si.
3-My recommendation would be that the authors try to built a « realistic » Si budget analogous to the above figure 1, expliciting their hypothesis regarding presently unknown fluxes.
Minor points :
-dSi and not DSi (which is ambiguous for a chemist)
-bSi and not BSi (idem)
-aSi and not ASi (idem)