A general description of the Scheldt estuary, including the hydrology, the sediment transport, the productivity and the biodegradation with respect to their influence on the trace metal behaviour in the Scheldt estuary, is given.The river basin can be divided in several sections according to their morphological, hydrodynamical and sedimentary properties. The zone from km 78 to 55, which corresponds roughly with the salinity zone from 2 to 10 psu, is the zone of high turbidity, high sedimentation and of oxygen depletion, especially in the summer period. That area is called the geochemical filter because the solid/dissolved distribution of the trace metals is controlled by redox, adsorption/desorption, complexation and precipitation/coprecipitation processes. The sedimentation rate in that area is estimated at 280 Mkg y ,1 .In the downstream estuary the phytoplankton activity increases due to the restoration of oxygen and to the much lower turbidity values. That area is called the biological filter because incorporation of trace metals by the plankton communities lowers the trace metal concentrations during the productivity period, while transformation of metal species, especially observed with mercury, occurs during that period too.

The measurement of stable isotope ratios on dissolved organic carbon (DOC) has long posed analytical problems and limited the use of this powerful tracer in biogeochemical studies in aquatic systems. Here, we provide a detailed description of a successful coupling of a custom-modified total organic carbon analyzer (Thermo HiPerTOC) to an isotope-ratio mass spectrometer (IRMS). The method is based on the wet oxidation of up tõ 20 mL aqueous sample in a closed reactor, whereby complete oxidation is ensured by a combination of sodium persulfate addition, heating, and UV irradiation. The produced CO 2 is carried over a water trap and purification column in a stream of He and introduced into the IRMS via a conventional open-split interface, enabling both quantification and δ AcknowledgmentsFunding for the development of this setup was provided by the Fund for Scientific Research (FWO-Vlaanderen, contracts 1.5.070.05 and G.0118.02) and by the Research Council of the Vrije Universiteit Brussel, which we gratefully acknowledge. S.B. is a postdoctoral researcher at the FWO-Vlaanderen. David P. Gillikin, Elizabeth Minor (associate editor), and two anonymous referees provided useful and constructive suggestions to an earlier version of this manuscript.

Abstract. Total mercury (Hg) and methylmercury (MeHg) concentrations were determined in sediments and in the polychaete worm Nereis diversicotor at 13 stations of a brackish water intertidal nmdflat of the Scheldt estuary. Hg and MeHg concentrations in sediments ranged from 144 to 1192 ng g-~ dw and from 0.8 to 6 ng g-~ dw, respectively. Both Hg and MeHg concentrations increased with an increase of organic matter (OM) content and fine grain fraction. In contrast, Hg accumulation hy N. diversicolor was significantly (p < 0.05) higher at stations with sandy sediments (mean value: 125 ng g-i dw) than at stations with muddy sediments (mean value, 80 ng g-~), probably because Hg availability for bioaccumulation at muddy stations was reduced by high OM content of the muddy sediments. MeHg accounted for an average of 0.7% of the total Hg in sediments and 18% of the total Hg in N. divelwicolor. Seasonal variations significantly affected Hg concentrations in sediments and MeHg in N. divezwicotor. Total Hg concentrations in sediments were significantly (p < 0.05) higher in autumn and winter than in spring and summer whereas MeHg concentrations were lowest in winter compared to the other seasons. On the other hand, total Hg concentrations in the worms were lowest in spring whereas MeHg concentrations were significantly (p < 0.01) higher in spring and summer than in autumn and winter.

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