Seasonal Dissolved Inorganic Carbon Dynamics in the East Frisian Wadden Sea: From Net Sink to Episodic Source

The intertidal system of the back-barrier area of Spiekeroog in the East Frisian Wadden Sea is biogeochemically dynamic and essential for understanding regional dissolved inorganic carbon (DIC) budgets. However, DIC fluxes in this region remain poorly quantified compared to total alkalinity (TA), limiting our understanding of coastal carbon cycling. From October 2021 to December 2022, a range of discrete and in-situ measurements were conducted. Using these data, a seasonal linear regression model was developed to estimate lateral DIC fluxes continuously and to investigate seasonal DIC source–sink dynamics.

Results indicate that the Wadden Sea acted as a net DIC sink to the adjacent North Sea, with an import of 0.711 ± 1.48 mol m⁻² d⁻¹ (equivalent to 3.58 Gmol yr⁻¹) in 2022. The strongest import rates occurred in winter 2021 and spring 2022, likely driven by sediment–water exchange, remineralization, and biological uptake. During summer, import rates were lower, although intensified photosynthetic activity and elevated TA continued to modulate DIC dynamics, promoting CO₂ uptake. In contrast, during autumn, the Wadden Sea episodically exported DIC to the North Sea, driven by enhanced remineralization of organic matter following the summer production peak, intensified sediment–water exchange, and physical processes such as wind-induced mixing and storm events.

Air-sea CO₂ exchange and submarine groundwater discharge (SGD) were integrated into the seasonal carbon budget, revealing significant internal retention and transformation of DIC within the system. The findings highlight the function of the Wadden Sea as a coastal carbon sink and demonstrate substantial seasonal variability. SGD also represents a major knowledge gap, emphasizing the need for integrated, high-resolution measurements and modelling to constrain regional carbon budgets and inform climate change mitigation strategies.