| 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.
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)