Inorganic Carbon Fluxes in the Inner Elbe Estuary, Germany

Abstract

Estuaries are strong sources for CO2 to the atmosphere. At the same time, they are intense reactors for dissolved matter at the land–ocean interface. The temporal and spatial variations of inorganic carbon fluxes in the inner Elbe estuary were studied based on 18 transects and a budget was constructed. The freshwater part of the Elbe estuary, a potential hotspot for carbon transformation, was evaluated using GIS-derived surface areas, isotopic DIC data and modelled pCO2. Calculation of pCO2 with commonly used constants yield 20 % higher values if compared to thermodynamically modelled results, integrating the actual ionic composition of the sample. With these improved calculations, it could be confirmed that this part is important for carbon turnover. Highest pCO2 values and strongest CO2 efflux per square metre compared to the rest of the inner Elbe estuary were observed. It contributes 10 % of the CO2 fluxes to the atmosphere, and 82 % of seaward transport of dissolved CO2. Overall, the estuary is net heterotrophic and releases 13.2 × 109 mol CO2 year−1 to the atmosphere. About 20 % of the DIC exports from the estuary are attributed to atmospheric CO2 exchange, while 80 % are transported seawards in dissolved form. Additionally, the Elbe estuary is a source for DIC besides CO2. The flux to the coastal zone is increased by about 18 %. Main sources are marshes and sediments. These findings should be considered in North Sea budgets, which to date rely on simple two source mixing of freshwater and seawater to derive riverine DIC inputs. Findings from this study corroborate the anthropogenic impact on carbon fluxes and clearly confirm the necessity to examine the entire estuary including the freshwater part.