Water Depth is a Proxy Variable for Benthic Silica Recycling in the Baltic Sea

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. Benthic release of DSi has a potentially large influence on the ecology of coastal marine areas, but even so, direct rate measurements of DSi mobilisation from bottom sediments have rarely been conducted in many such areas. For example, the spatial coverage of benthic DSi flux data from the inland brackish Baltic Sea in northern Europe is rather low. We report data from mostly in situ but also ex situ benthic DSi flux measurements in the Baltic Sea during 2001–2017. In total, 128 in situ and 59 ex situ benthic flux measurements were conducted at water depths ranging from 12–210 m in the basins Gulf of Bothnia and Baltic proper, with the aim of constructing a predictive model for benthic DSi fluxes in the Baltic Sea. Station-averaged DSi fluxes ranged from 0.6–9.4 mmol m^-2d^-1with the lowest values from sediments at intermediate water depths of ~60 m and considerably higher fluxes from shallower as well as from deeper areas. Thus, the data was fitted to a cubic model with water depth as a proxy variable for benthic DSi fluxes. The model was reasonably consistent with previously published DSi fluxes in the Baltic Sea. Using the model in combination with bathymetric and bottom substrate data for the Baltic Sea, the annual integrated benthic DSi release from mud and sand sediments was estimated to be 4323 thousand tonnes (kt) which is a factor of five larger than the annual riverine DSi input to this inland sea. Our results imply that the total integrated BSi deposition in the Baltic Sea amounts to 4933 kt yr^-1which is approximately 30% more than the integrated export BSi production by diatoms. It is suggested that the remainder of the depositing BSi (i.e., 1220 kt yr^-1) originates from allochthonous (riverine) sources. The burial efficiency of biogenic Si (BSi) was estimated to 12% in the Baltic Sea.