Mechanisms controlling the recycling of nutrient silicon in freshwater sediments: An experimental study of the interactions between silicon, iron and phosphorus

Dissolved silicon (DSi) is an essential nutrient for numerous terrestrial and aquatic organisms. In freshwater systems, including streams, rivers and lakes, an important class of siliceous algae are diatoms. Human activities, including land use changes, nitrogen (N) and phosphorus (P) enrichment, and hydrological alterations, have caused a decrease of DSi availability relative to N and P. In turn, these changes in macronutrient availability may contribute to shifts in phytoplankton communities that increase the likelihood of nuisance and harmful algal blooms. Internal loading of nutrient silicon (Si) from bottom sediments is one key process regulating the availability of DSi in the overlying water column. The magnitude and timing of internal DSi loading in freshwater bodies are controlled by biogeochemical reactions in sediments whose mechanisms and kinetics remain to be fully understood. In this thesis, I use controlled laboratory experiments to unravel the roles of different reaction pathways in the immobilization and release of DSi in freshwater sediments. Starting with initially very simple synthetic reaction systems, I progressively include additional components, specifically iron (Fe) and P, in order to mimic more realistic biogeochemical reaction networks, and ultimately, perform experiments with real freshwater sediments. After an introduction of the Si biogeochemical cycle, a review of the literature, and an outline of my thesis (Chapter 1), I present a study on the dissolution kinetics of amorphous silica (ASi) as a function of pH and salinity across the ranges typically found in natural freshwater (Chapter 2). The surface properties of ASi of various synthetic and natural sources are characterized with potentiometric titrations whose results are interpreted with the help of a constant capacitance model. Next the dissolution kinetics of ASi are measured in batch experiments, and the observed dissolution kinetics of ASi are fitted to a surface reaction model. The results confirm the previously reported non-linear relationship between the dissolution rate of ASi and the degree of undersaturation, implying that at least two dissolution rate constants are needed to describe the dissolution kinetics at high (typically, >0.4) and low (typically,