Dissolved inorganic carbon in a highly polluted estuary (the Scheldt

During 34 months (1996–1999), 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 mM, with highest values in winter and lowest in summer. For the brackish and freshwater section of the river d13CDIC values ranged from 27.5 to 217.5‰, the most negative signals were during winter and the least negative during summer. In all seasons, surface waters were significantly supersaturated in CO2 with respect to the atmosphere (pCO2 ranged from 2,200 to a maximum of 15,500 matm) indicating that the system is always heterotrophic. Biological processes (respiration and carbon fixation) and CO2 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 (.100 mg chlorophyll a [Chl a] L21) coincided with lower pCO2 and CO2 water–air efflux and less negative d13CDIC values, indicating considerable CO2 drawdown by phytoplankton. Mass balance calculations indicated that organic carbon to DIC conversion exceeded CO2 consumption year round, (net organic carbon conversion ranged from 410 to 520 g C m22 yr21) 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 pCO2, and depletion in 13CDIC relative to the main river system