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Carbon Budget: Research Questions

The observation of the fast-changing carbonate chemistry in the oceans is of paramount importance since the oceans adsorb CO2 from the atmosphere in large part. This leads to an acidification of sea water with far-reaching consequences to the marine ecosystem. However, the role of the shelf seas and coastal regions with much higher biological activity as a source or sink of CO2 is still not well quantified. In the case of the North Sea this area has been discussed to be a site of efficient pumping of carbon dioxide from the atmosphere to the North Atlantic Ocean. The variability in the carbonate system in this region, however, necessitates high-frequency measurements of these parameters. Therefore, measurements of pCO2, pH and alkalinity have been carried out with automated sensors on a FerryBox.

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Results: CO2 concentrations

The picture shows pCO2 concentrations across the North Sea between Norway and Belgium in the years 2013 to 2017. It can be seen that pCO2-deficits (blue) occur in spring when CO2 is taken up by growing algae (CO2-sink). In late summer, when algae blooms break down, CO2 is released (CO2-source). The results can be used in climate models to assess the role of shelf seas in the carbon budget.

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Time series of pCO2 data from 2012 to 2017 (all transects Halden, NO - Zeebrugge, BE). pCO2 data in the central North Sea depicts undersaturation (<400 µAtm) in spring and early summer due to algae blooms and slightly oversaturation in late summer and autumn due to remineralization processes releasing carbon dioxide again. Left panel: Map of the route of the cargo vessel Lysbris.

Identification of a Seasonal Cycle

The graph depicts the seasonal alkalinity of the North Sea in 2017. It reveals that the highest alkalinity occurs in the German Bight from June to October. This is probably due to alkalinity production in the summer months in the adjacent Wadden Sea and tidal transport into the North Sea. During this time, the oceanic buffer capacity and the uptake of atmospheric CO2 is increased. The measurements allowed to identify a seasonal cycle, decoupled from salinity and present along the entire transect in the shallow southern North Sea. This seasonal cycle was more pronounced near the Wadden Sea. In addition, a steep alkalinity gradient between the near-shore and adjacent coastal regions was observed, likely driven by total alkalinity inputs from the Wadden Sea. This high total alkalinity in the southeastern North Sea, could significantly affect the local carbonate buffer, and thus the regional capacity for pCO2 sequestration.

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