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

Abstract

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 µM, with highest values in winter and lowest in summer. For the brackish and freshwater section of the river Δ 13 C DIC values ranged from −7.5 to −17.5‰, 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 µatm) 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 (>100 mg chlorophyll a [Chl a] L −1 ) coincided with lower pCO 2 and CO 2 waterߚair efflux and less negative Δ 13 C 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 −2 yr −1 ) 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 Δ 13 C DIC relative to the main river system.