<p> Electronic supplementary material for Silicon Isotope composition of dissoved silicon in surface waters of the Elbe Estuary and its tidal marshes</p><p>Andreas Weiss1*, Christina De La Rocha2, Thorben Amann1, Jens Hartmann1</p><p>1 KlimaCampus, Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany</p><p>2 CNRS UMR 6539, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Technôpole Brest-Iroise, Place Nicholas Copernic, 29280 Plouzané, France</p><p>Biogeochemistry Introduction This file contains five auxiliary figures: 1) the mass fractionation lines of all samples showing the quality of the data 2) the mean daily discharge of the Elbe river measured at the gauging station Neu Darchau 3) a photo indicating the width of the seepage streamlet of the tidal creek in the freshwater marsh after a flooding of the marsh and after a non-flooding high tide 4) nutrient and salinity data from the October cruise 5) the cumulative discharge of the tributaries of the Elbe Estuary, the approximate water residence time in the main estuarine channel and the dissolved silica concentrations observed during the December 2011 cruise. The discharge data was extracted from the monitoring database of the “Flußgebietsgemeinschaft Elbe” [ FGG Elbe , 2012]. The water residence time of the estuary was calculated according to Bergemann et al. [1996]. The last figure visualizes the difference between the seepage discharge after an intense flooding that occurred at the freshwater marsh. Figure 1: Values of δ29Si versus δ30Si for estuarine (black circles) and tidal marsh (grey triangles) samples. Error bars represent 1σ standard deviation. The solid line δ30Si = 1.93(δ29Si) assumes equilibrium mass fractionation, the dashed line δ30Si = 1.96(δ29Si) assumes kinetic mass fractionation. Figure 2: Daily mean discharge at the gauging station Neu Darchau (km 536) from September 2011 to December 2011. Vertical grey lines indicate sampling days. Figure 3: Width of the seepage streamlet after intense flooding events occurred prior and on the sampling day. For comparison the black lines indicate the approximate width of the streamlet on a sampling day without prior flooding (ca. 30 cm). Figure 4: Dissolved silica, nitrate, dissolved phosphorous, and salinity data from the October 2011 cruise taken at the same time as the samples for silicon isotopes. </p><p>Figure 5: Cumulative discharge of tributaries along the estuary in relationship to DSi concentrations and water residence times in the estuary. The cumulative discharge was calculated using the long term annual mean discharge of the individual tributaries feeding into the estuary. The water residence time is based on the Elbe discharge at Neu Darchau and the empirical relationship a*(discharge2) + b*(discharge) + c, which was derived from Figure 3 in Bergemann et al. [1996]. References</p><p>Bergemann, M., G. Blöcker, H. Harms, M. Kerner, R. Meyer-Nehls, W. Petersen, and F. Schroeder (1996), Der Sauerstoffgehalt der Tideelbe, Die Küste, 58, 200-261. FGG Elbe (2012), Datenbank der Flussgebietsgemeinschaft Elbe, edited by F. Elbe.</p>