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.