Biogeochemistry of Silicon in Soils and Sediments

This research project is focused on the characterization and reactions of silicon (Si) in soil, sediment and water. Silicon is one of the major nutrients that dictates the magnitude of the primary production and, especially, the composition of the phytoplankton community. For the diatoms (Bacillariophyceae), Si is a macronutrient which is incorporated into their protective shells, whereas other phytoplankton groups require Si at only micronutrient levels. A decrease in the external supply of Si and in the Si:P and Si:N ratios in several estuarine areas of the world, including the Baltic Sea, has taken place during the last decades. Such a shift in the availability of Si can drastically influence the composition of the phytoplankton community and contribute to increases in cyanobacterial blooms. The changes in the external supply of Si (e.g. processes in the drainage area) and in the internal recycling of the element need to be clarified.
There is also increasing evidence that the cycles of Si and P are linked. The concentrations of dissolved Si and P in pelagial areas of the Baltic Sea are positively correlated, and it has been shown that large pulses of Si may release P adsorbed to sediment particles through competitive ligand exchange. This process may increase the internal loading of P from sediment while immobilizing Si, leading to a further decrease in Si available for primary production.
The aim of this project is to provide new information on insufficiently known internal and external processes which affect the short-term availability of Si for autotrophic uptake in water ecosystems. The main emphasis is placed on chemical and biological processes which affect the (non-photosynthetical) binding and release of Si to or from particulate material along the river-lake-estuary continuum: soil, suspended material, and sediment.