Intertidal Regions Regulate Seasonal Coastal Carbonate System Dynamics in the East Frisian Wadden Sea

Abstract. Seasonal and regional changes in carbon dynamics in the Wadden Sea, the world's largest intertidal sand and mud flats system, were analyzed to quantify the influence of biogeochemical processes (CaCO3 dissolution and formation, photosynthesis, respiration) on the carbonate system at the land-sea interface. With a focus on the East Frisian Wadden Sea and the highly turbid Ems River estuary, we successfully implemented the proxy of the difference between total alkalinity (TA) and dissolved inorganic carbon (DIC) ([TA-DIC]), as well as the calculated ΔTAexcess, ΔDICexcess and ΔTAP to identify how ongoing biogeochemical processes regulate the carbonate system dynamics and the land-sea interface. In spring, a phytoplankton bloom with high biological activity was indicated by (a) supersaturated oxygen (up to 180 in % saturation), (b) elevated chlorophyll a (up to 151.7 µg L-1) and (c) low pCO2 (as low as 141.3 µatm). As a result, nitrate (NO3-, 19.29 ± 18.11 µmol kg-1) and DIC (159.4 ± 125.4 µmol kg- 1) decreased, whereas TA slightly increased (9.1 ± 29.2 µmol kg-1) in the intertidal regions from March 2022 to May, most likely through nitrate assimilation. The regression analysis of the differences in NO3− concentrations (ΔNO3−) against the differences in DIC (ΔDIC) between March and May 2022 yielded a slope of 6.90 which is close to the Redfield ratio of 6.625 for the C:N ratio of freshly produced phytoplankton biomass. In summer, high seasonal TA values (up to 2400 µmol kg-1) in the Western part of the East Frisian Wadden Sea, along with positive ΔTAexcess at 73.3 % of all stations, indicated production of TA during this season in the intertidal regions, complemented the DIC dynamics. The increase of TA enhances the coastal ocean’s ability to absorb and store CO2 through buffering, chemical equilibrium, biological calcification and the carbonate pump, and suggests that the intertidal regions can be a source of total alkalinity to the coastal regions during the warm productive seasons. The study highlights the complex relationships of these factors, emphasizing the need for a comprehensive understanding of regional and seasonal variations to better assess the role of coastal systems in carbon cycling, storage and climate regulation.