Trace metal geochemistry of organic carbon-rich watercourses draining the NW German coast Melanie Beck * , Olaf Dellwig 1 , Sibylle Fischer, Bernhard Schnetger, Hans-Jürgen Brumsack Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University, Carl-von-Ossietzky-Str. 9-11, D-26129 Oldenburg, Germany article info Article history: Received 6 September 2011 Accepted 15 March 2012 Available online 29 March 2012 Keywords: dissolved and particulate trace metals organic-carbon rich watercourses tidal flats North Sea coast Wadden Sea NW Germany abstract Numerous small watercourses are draining the hinterland of the NW German coast. The waters mainly originate from marsh and fen areas and have yellow to deep brownish color. During their flow path, the waters exhibit gradients in salinity (0.2e3), pH (6.2e8.8), particulate organic carbon (5e25%), and iron oxides (7e12%), which alter the concentrations of most dissolved and particulate trace metals. For example, dissolved Fe is rapidly removed from solution at increasing salinities by flocculation, whereas dissolved U is removed in the very low-salinity zone by Fe- and organic-rich colloids. The waters at the flood-gate of Neuharlingersiel, where a composite sample of the entire study area is collected before the freshwater is discharged into the marine-dominated tidal flat area in front of the mainland dike, have the following average trace metal concentrations: dissolved Fe 11 mM, Mn 4 mM, Mo 10 mM, U 1.8 mM, V 75 mM and particulate Fe 7 mg kg 1 , Mn 1200 mg kg 1 , Mo 2 mg kg 1 , U 1.7 mg kg 1 , V 140 mg kg 1 . After passing the flood-gate most Mo- and U-salinity distributions fit well to the conservative mixing line that connects the seawater of the Wadden Sea to the low-salinity river water entering the tidal flat area. In contrast, dissolved Fe, Mn, and V are removed from solution, especially at intermediate salinities, which may be due to mixing of freshwater with seawater depleted in these elements, flocculation and/or scavenging by particulate matter. In general, processes similar to those in estuarine systems alter the geochemical signatures on transects from land to sea. Compared to average river water, the watercourses are enriched in dissolved and particulate Fe, Mn, Mo, U, and V. Due to the fast removal of dissolved Fe in the salinity gradient and the high concentrations of Mo and U in seawater, the watercourses only form a source for Mn, V, terrestrial organic carbon, and particulate Fe for the Wadden Sea. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Rivers are carriers of a wide variety of chemical signatures to the oceans. The effect of the transported compounds on the chemistry of the coastal oceans depends on the quantity of dissolved and particulate compounds carried by rivers as well as on the fate of these compounds in the estuarine mixing zone. Estuaries are zones where seawater mixes with freshwater and through which trace constituents must pass when being transported seaward. In the estuarine mixing zone, river-transported compounds are subjected to a variety of physical, chemical, and biological transformation processes, including transfer reactions between the dissolved and particulate phase. For example, some dissolved trace metals such as Fe are generally removed from solution when freshwater encounters saltwater in the natural environment (Eckert and Sholkovitz, 1976; Sholkovitz, 1976; Boyle et al., 1977; Sarin and Church, 1994). In contrast, other dissolved trace metals such as Mn either behave conservatively, are removed from solution, or are added to estuarine waters by pore waters or release from suspended particulate matter (Sholkovitz and Copland, 1981; Eastman and Church, 1984; Ouddane et al., 1997; Klinkhammer and McManus, 2001). The behavior of constituents in the estuarine mixing zone is governed by processes such as adsorption, desorption, complex- ation, and flocculation, which are induced by variables like salinity, pH, organic carbon, and turbidity (Sholkovitz and Copland, 1981; Gustafsson et al., 2000; Andersson et al., 2001; Lyven et al., 2003). Due to the large variety of processes, globally valid reaction schemes can hardly be defined. Therefore, it is essential to describe each estuarine system separately and to distinguish between globally relevant estuaries and those of more local importance. Along the coastline of the southern North Sea, several rivers (e.g. Scheldt, Rhine, Ems, Weser, Elbe) transport dissolved and particulate compounds to the coastal ocean (Eisma, 1975; Duinker et al., 1979). * Corresponding author. E-mail address: m.beck@icbm.de (M. Beck). 1 Present address: Leibniz Institute for Baltic Sea Research (IOW), Seestraße 15, 18119 Rostock, Germany. Contents lists available at SciVerse ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2012.03.025 Estuarine, Coastal and Shelf Science 104-105 (2012) 66e79