Carbon dynamics in the freshwater part of the Elbe estuary, Germany: Implications of improving water quality

Abstract

The Elbe estuary is strongly affected by anthropogenic activities throughout its catchment. It was characterised by high pollution in the 1980s and before, which improved after 1989. This study analyses the effect of the water quality changes on the carbon dynamics during summer season. A focus on the tidal freshwater area emphasises the processes not studied previously. Two zones were distinguished: the oxygen minimum zone (OMZ) and the pre-OMZ, which borders on the non-tidal river. It is hypothesised that water quality improvement during the last 20 years affected the mineralisation efficiency of organic carbon in the OMZ leading to a strong filtering function before water enters the maximum turbidity zone. The analysis of 23 years of water quality data revealed that decreasing water pollution increased particulate organic carbon concentrations (POC, from 10% of the total organic carbon (TOC) pool to 30%). At the same time, dissolved organic carbon (DOC) concentrations decreased (from 90 to 70% of TOC respectively) and its quality changed. These changes were followed by an enhanced removal of POC in the OMZ. During the 1980s, POC passed through this zone virtually unprocessed while DOC was remineralised. With improving water quality, up to 50% of the POC entering the estuary was removed in the OMZ, well before the maximum turbidity zone, which is often considered the most important zone for POC remineralisation in estuaries. This removal by respiration had a reducing effect on OMZ oxygen levels. After a minor recovery during the 1990s (increase to 90% oxygen saturation), a deterioration was notable with saturations decreasing to around 70%. In contrast, the oxygen situation in the pre-OMZ improved over the whole study period (above 100% saturation after 1996) during summer season. The analysis also revealed changes in the dissolved inorganic carbon system. Due to environmental improvements, mean pH values rose from 7.2 in both zones to around 8.5 in the pre-OMZ and to 7.9 in the OMZ. Partial pressures of CO 2 (pCO 2) decreased accordingly from maxima of nearly 7000 μatm (OMZ) during the 1980s. Recent summer values of the pre-OMZ were below the respective atmospheric concentrations. The OMZ, although showing declining values with time, still had pCO 2 values up to 2500 μatm, emphasising the importance of the freshwater part as a significant CO 2 source. © 2012 Elsevier Ltd.