The role of freshwater marshes in estuarine silica cycling (Scheldt estuary)

The amount of Si, delivered through estuaries, is essential in influencing the occurrence of eutrophication events in the coastal zone. The role of tidal marshes fringing estuaries in Si processing had however never been investigated. This thesis was aimed at uncovering freshwater marshes' role in estuarine silica cycling.; Tidal exchange experiments showed that freshwater tidal marshes act as recycling surfaces for dissolved Si (DSi) from biogenic silica (BSi). Freshwater marshes import BSi along with suspended matter in the flood water, while DSi is exported at ebb tide. The relative export of DSi is highest during summer when DSi concentrations in the inundating water are lowest. Similar patterns were observed in a saltwater marsh. Marshes thus act as buffers in the estuarine silica cycle: most DSi is recycled when the demand of DSi by diatom communities is highest.; A seasonal quantification of BSi in freshwater marsh sediment and vegetation showed that sediment is the largest stock of BSi in the marsh. The amount of BSi in sediment is tenfold the amount of BSi fixed in vegetation. Modelling revealed that young freshwater marshes, with lower elevation, are less efficient BSi recyclers compared to older freshwater marshes with an elevation around mean high water. Decomposition experiments showed that vegetation fixed BSi is recycled more rapidly compared to sediment BSi.; Secondary production of economically valuable fish and invertebrates in coastal waters has been observed to be positively related to total surface of nearby tidal marsh habitats. Our data show that the ability of tidal marshes to enhance internal recycling of biogenic Si to dissolved Si might partially explain this relationship, through the stimulation of diatom growth and consequent efficient transfer of diatom C into the foodweb. Extrapolations suggest that restoration of tidal marsh habitat along estuaries is an option to improve the availability of DSi in nearby coastal waters: enhanced DSi recycling from the recreated tidal habitats might reduce the frequency and duration of periods with significant DSi limitation of diatom growth.