Full aquatic carbonate system measurements at the land-ocean interface in the German Bight

The land-sea interface is characterized by rapid changes in thermodynamics constants and is influenced by local metabolic processes, watershed characteristics, tidal forcing and anthropogenic pressures. As a result, the aquatic carbonate system parameters are also rapidly changing along salinity gradients. In order to understand this variability and correctly represent it in biogeochemical models, detailed observations must be regularly obtained, and in the SEA-ReCap Project we do this in marginal seas such as the Black Sea or the North Sea. A research cruise was organised in autumn 2024 on the RV Heincke to sample the German exclusive economic zone of the North Sea and three major estuaries draining into the German Bight. Surface seawater pCO₂ and pH were measured continuously using a FerryBox system installed on board. Total alkalinity was measured every 15 minutes with an automated sensor, while dissolved inorganic carbon was measured on board from discrete water samples. With all four carbonate system parameters directly observed, the land-ocean continuum can be well characterized. For example, all the dissolved inorganic carbon concentrations converged towards the open North Sea end-member of 2877 ± 4 µmol kg^-1, but while the Elbe River estuary measurements decreased with decreasing salinity, to below 1800 µmol kg^-1, the Ems and Weser measurements increased with decreasing salinity, exceeding 3100 µmol kg^-1 at the most upstream locations sampled. The Elbe freshwater end-member, upstream of our last sampled location, is characterised by high primary production and low dissolved inorganic carbon, while the sampled area of Hamburg and further downstream is a site of remineralisation and dissolved inorganic carbon accumulation. The Ems and Weser Estuaries, on the other hand, are sources of dissolved inorganic carbon and alkalinity through denitrification and fluid mud anaerobic processes, leading to the high concentration freshwater end-members. Additionally, the seawater pCO₂ in the open North Sea was 461 ± 4 µatm, while at the lowest salinities sampled in each estuary, the values exceeded 2000 µatm, indicating the dominance of heterotrophy. These results emphasise the large heterogeneity that can arise over small spatial scales and the sometimes contrasting patterns of the carbonate system in nearby estuaries. This has implications for the strength of the buffering capacity of the German Bight and the role of the coastal ocean as a carbon source or sink.