Primary Research Paper Freshwater marshes as dissolved silica recyclers in an estuarine environment (Schelde estuary, Belgium) Eric Struyf 1, *, Stefan Van Damme 1 , Britta Gribsholt 2 & Patrick Meire 1 1 University of Antwerp, Department of Biology, Environmental Management Research Group, Universiteitsplein 1C, B-2610 Wilrijk, Belgium 2 Netherlands Institute of Ecology (NIOO-KNAW), Center for Estuarine and Marine Ecology Korringaweg 7, 4401 NT Yerseke, The Netherlands (*Author for correspondence: Tel.: 003238202278, Fax: 003238202271, E-mail: eric.struyf@ua.ac.be) Key words: estuary, dissolved silica recycling, biogenic silica, eutrophication, freshwater marshes Abstract Compared to knowledge about N and P processing in the aquatic continuum of lakes, wetlands and estuaries, knowledge concerning transport and cycling of Si is only fragmentary. Furthermore, Si research in estuaries has mainly been focused on subtidal benthic sediments and uptake and recycling by diatom communities. The biogeochemical cycling of Si in tidal wetlands, which can contain large amounts of Si, has thus far been neglected. We have conducted several whole ecosystem Si mass-balances on a freshwater marsh located in the Schelde estuary (6 tidal cycles, 2 with BSi included). Our measurements show that the freshwater marsh acts as an important source of dissolved Si to the main river (1–18% more export than import, on average 0.114 g m )2 ). This export is compensated by import of amorphous silica into the marsh (19–55% more import than export). The marsh was shown to act as silica recycler, resupplying biologically available dissolved Si to the estuarine ecosystem. Extrapolations show that during summer and spring months, when dissolved silica is depleted due to diatom growth, almost half of the total dissolved silica load in the main river channel could result from marsh recycling. Introduction In recent years, the transformation, retention and transport of nutrients through the vast continuum of rivers, lakes, wetlands and estuaries to coastal waters was the subject of many large research projects including the LOI- CZ-Programme (Land-Ocean-Interactions-in-the- Coastal-Zone-Programme), a core project of the International Geosphere–Biosphere Programme (IGBP), (Kondratyev & Pozdnyakov, 1996; Billen et al., 2001; Cave et al., 2003; Humborg et al., 2003). Compared to our knowledge concerning N and P processing, transport and cycling of dissolved silica in the aquatic contin- uum is significantly less known (Conley et al., 2000). In contrast to N and P, with large human inputs, anthropogenic input of Si to estuarine systems is negligible. The amount of dissolved silica that eventually reaches coastal waters through estuaries is, however, essential in influ- encing the occurrence of eutrophication prob- lems in the coastal zone. High anthropogenic inputs of N and P can eventually induce dis- solved silica limitation of diatoms and sub- sequent succession of a phytoplankton community dominated by diatoms to a non- diatom phytoplankton community (Schelske et al., 1983; Smayda, 1990; Lancelot, 1995; Smayda, 1997; Billen, 2001). Hydrobiologia (2005) 540:69–77 Ó Springer 2005 P. Meire & S. van Damme (eds), Ecological Structures and Functions in the Scheldt Estuary: from Past to Future DOI 10.1007/s10750-004-7104-0