Impact of the Wadden Sea-North Sea interactions on the sea water carbonate system

Despite the small area that shelf seas occupy, these regions play a significant role in the global carbon cycle. Through the so-called continental shelf pump mechanism they support more than 20% of the global marine uptake of atmospheric CO₂. However, these regions are highly dynamic and the complex interactions within them make it difficult to identify and estimate the contribution of the different processes involved and changes therein. The North Sea, a shelf sea situated on the north-western European shelf is an iconic example of such a shelf sea, making it ideal to study these processes, connections and their variability. The North Sea is connected with the tidal basin of the Wadden Sea via channels that allow not only exchange of water but also dissolved and suspended materials. Through these connections, the Wadden Sea supplies large amount of dissolved inorganic carbon (DIC) but also total alkalinity (TA) generated from processes such as organic matter reduction and as a conduit from riverine inputs. In this study, pH, DIC and TA were monitored on discrete samples allowing the highest possible accuracy and precision at the NIOZ monitoring platform at the edge of the Marsdiep channel, through which most of the water exchange occurs between Wadden Sea and the North Sea. The flow direction reverses with the tides, so both North Sea and Wadden Sea waters are dominant at different points through a tidal cycle. The platform is also equipped with salinity, temperature and pH sensors, which measure every 10 minutes. Here, we use these data to assess the impact of the Wadden Sea on the North Sea, quantifying the TA and DIC supply to the North Sea and the role of the Wadden Sea in regional CO₂ uptake. Through cross calibrating these parameters we aim to unravel the relative impacts of the different processes involved in mixing in a tidal inlet. The permanent observatory moreover allows us to investigate the drivers of variability in the TA and DIC exchange from diurnal to seasonal and even multi-annual timescales.