Dissolved inorganic carbon in a highly polluted estuary (the Scheldt)

During 34 months (1996-1999), we studied the dissolved inorganic carbon (DI C) system of the highly polluted Scheldt River and upper estuary. DIC range d between 3,300 and 7,100 muM, with highest values in winter and lowest in summer. For the brackish and freshwater section of the river delta C-13(DIC ) values ranged from -7.5 to -17.5 parts per thousand, the most negative si gnals were during winter and the least negative during summer. In all seaso ns, surface waters were significantly supersaturated in CO2 with respect to the atmosphere (pCO(2) ranged from 2,200 to a maximum of 15,500 mu atm) in dicating that the system is always heterotrophic. Biological processes (res piration and carbon fixation.) and CO2 evasion to the atmosphere affected t he isotopic composition and magnitude of the inorganic carbon pool. In spri ng and summer 1997 and 1998, large phytoplankton biomasses (> 100 mug chlor ophyll a [Chl a] L-1) coincided with lower pCO(2) and CO2 water-air efflux and less negative delta C-13(DIC) values, indicating considerable CO2 drawd own by phytoplankton. Mass balance calculations indicated that organic carb on to DIC conversion exceeded CO2 consumption year round, (net organic carb on conversion ranged from 410 to 520 a C m(-2) yr(-1)) emphasizing the effe ct 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 pC O(2), and depletion in C-13(DIC) relative to the main river system.