In this study the seasonal and spatial variation of dissolved silica (DSi) and biogenic silica (BSi) in tidal marshes in the inner Elbe estuary was investigated. Seasonal sampling was conducted at three sites in the dyke foreland-the stretch of land between dyke and estuary-of the Elbe estuary, Germany. To assess the potential DSi export from the dyke foreland geographic information systems were used to calculate the DSi flux. Mean annual seepage DSi concentrations increased along the salinity gradient from 270 mu mol L (-aEuro parts per thousand 1) to 380 mu mol L (-aEuro parts per thousand 1). BSi concentration of the soil showed the opposite trend decreasing from 15.8 to 4.8 mg g (-aEuro parts per thousand 1). Temporal variations of DSi concentrations were lowest at the freshwater site. At the brackish and saline site DSi concentrations increased about 2-fold from March to November from 200 to 500 and from 300 to 550 mu mol L (-aEuro parts per thousand 1), respectively. In March a diurnal signal of DSi uptake by diatoms could be observed at the saline sampling site, highlighting the role of sampling time and irradiance for the DSi flux estimate. DSi concentrations were reduced by 18.6 \% between sunrise and noon. The DSi export from the dyke foreland is significant and equals the riverine DSi input into the estuary during times of low DSi concentrations. Furthermore the marsh DSi fluxes surpass DSi fluxes from highly active weathering regions, as reported in the literature, which corroborates the importance of tidal marsh areas for the coastal silica cycle. Factors steering the seasonality of DSi export and differences between the sites (temperature, hydrology, salinity and plant uptake of DSi) are discussed.