During 34 months (1996)(1997)(1998)(1999), we studied the dissolved inorganic carbon (DIC) system of the highly polluted Scheldt River and upper estuary. DIC ranged between 3,300 and 7,100 M, with highest values in winter and lowest in summer. For the brackish and freshwater section of the river ␦ 13 C DIC values ranged from Ϫ7.5 to Ϫ17.5‰, the most negative signals were during winter and the least negative during summer. In all seasons, surface waters were significantly supersaturated in CO 2 with respect to the atmosphere (pCO 2 ranged from 2,200 to a maximum of 15,500 atm) indicating that the system is always heterotrophic. Biological processes (respiration and carbon fixation) and CO 2 evasion to the atmosphere affected the isotopic composition and magnitude of the inorganic carbon pool. In spring and summer 1997 and 1998, large phytoplankton biomasses (Ͼ100 g chlorophyll a [Chl a] L Ϫ1 ) coincided with lower pCO 2 and CO 2 water-air efflux and less negative ␦
13C DIC values, indicating considerable CO 2 drawdown by phytoplankton. Mass balance calculations indicated that organic carbon to DIC conversion exceeded CO 2 consumption year round, (net organic carbon conversion ranged from 410 to 520 g C m Ϫ2 yr Ϫ1 ) emphasizing the effect of bacterial respiration. An intermediate river section receiving water from the main tributary (Rupel), which carries wastewater from the densely populated Brussels region, consistently showed decreased DIC, increased pCO 2 , and depletion in 13 C DIC relative to the main river system.Studies of the carbonate chemistry in some major rivers, such as the Amazon, Yangtze, and Rhine, showed that pCO 2 in river water is 10 to 15 times higher than in the atmosphere (Kempe et al. 1991). Such high values, typically reached in the downstream sections, suggest that rivers and estuaries not only transport carbon from land to the ocean, but also actively degas CO 2 into the atmosphere (Frankignoulle et al. 1998). In a recent review, Cole and Caraco (2001) report that this is the case for a majority of large rivers, which are accordingly categorized as net heterotrophic systems.European estuaries are subject to intense anthropogenic disturbance. This is especially the case for the Scheldt, which receives an organic matter load estimated at 100 ϫ 10 9 g C yr Ϫ1 (Wollast 1988). Because of the hydrodynamic conditions imposed by the strong tidal regime and the relatively low water discharge, this organic matter accumulates 1 Corresponding author (fdehairs@vub.ac.be).
AcknowledgmentsThis research was conducted in the framework of OMES (Onderzoek Milieu-Effecten Sigmaplan), coordinated by P. Meire, and supported by the regional government of Flanders. We are grateful to E. Keppens and K. Van den Driessche for assistance during mass spectrometer measurements and to M. Tackx for the use of unpublished chlorophyll a data. We also thank M. Frankignoulle, J. Middelburg, and P. Herman for useful discussions concerning pCO 2 and mass balance calculations and Lei Chou, Farid Elghal, and Roland Wo...
