Biogenic silica in tidal freshwater marsh sediments and vegetation (Schelde estuary, Belgium)

To date, estuarine ecosystem research has mostly neglected silica cycling in freshwater intertidal marshes. However, tidal marshes can store large amounts of biogenic silica (BSi) in vegetation and sediment. BSi content of the typical freshwater marsh plants Phragmites australis, Impatiens glandulifera, Urtica dioica, Epilobium hirsutum and Salix sp. was analysed year-round. All herbaceous species accumulated silica in their tissue during their life cycle. Of the live plants, P. australis contained the most BSi (accumulating from 6 to 55 mg g(-1)). Dead shoots of P. australis had the highest BSi content (up to 72.2 mg g(-1)). U. dioica (<11.1 mg g(-1)), I. glandulifera (<1.1 mg g(-1)), E. hirsutum (<1.2 mg g(-1)) and Salix sp. (<1.9 mg g(-1)) had a much lower BSi content. Except for R australis rhizomes (<15 mg g(-1)) underground biomass contained low amounts of BSi (<6 mg g(-1)). Sediment BSi content decreased from the surface (9 to 10 mg g(-1)) to deeper layers (5 to 7 mg g(-1)), There was seasonal variation in sediment BSi. Dissolved Si in porewater was highest in summer (ca. 600 mu M) and lowest in winter (ca. 400 mu M). P. australis vegetation (aboveground and roots) contained up to 126 g m(-2) BSi, while the upper 30 cm of sediment accumulated up to 1500 g m(-2), making sediment the largest BSi reservoir in the marsh. We conclude that P. australis wetlands could be an essential, but unrecognised, sink for BSi in the biogeochemical cycling of Si.