Co-occurrence and potential chemical competition of phosphorus and silicon in lake sediment

Various chemical approaches were used to characterize P and Si in the surface sediment of Lake Vesijärvi, Southern Finland. The dynamic equilibrium between solid and solution P in aerobic and anaerobic sediment was investigated by means of desorption-sorption isotherms. The sediment material was rich in hydrated Al and Fe oxides and, accordingly, had a high P sorption capacity and a very low P concentration (5 μg l −1) in the interstitial water under aerobic conditions. Approximately 25–30% of total P and almost 90% of total Si were bound in mineral lattices, i.e. in a stable form not participating in biological and chemical transformations. Organic P comprised about 20–25% of total P and was mainly in practically insoluble form not extractable by dilute base or acid. Phosphorus participating in sorption-desorption reactions was considered to originate from reserves bound on oxide surfaces. When the oxide bound P reserves were determined by fractionation analysis, a large amount of Si (corresponding to about 30% of biogenic reserves) was also dissolved. Anaerobiosis increased distinctly the occurrence of both elements in the interstitial water. These dissolution patterns suggest that P and Si are bound to the same components and, therefore, compete with each other for the sorption sites. This chemical competition, in turn, may be of importance in the nutrient exchange between solid and solution phase in sediment.