Nutrient concentrations in the North Sea and adjacent estuaries are the end-result of basin-wide input, retention, mobilization and transport of N, P and Si. Traditionally, eutrophication has been approached as a problem of increased human inputs of N and P. In contrast, dissolved Si concentrations have mostly been considered as not anthropogenically influenced. Transfer of dissolved Si (DSi) to rivers has usually been considered to result from a pure geochemical process, involving only direct chemical weathering of soil minerals.