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 analysed to quantify the influence of biogeochemical processes on the carbonate system at the land-sea interface. With a focus on the East Frisian Wadden Sea (EFWS), we successfully used the difference between total alkalinity (TA) and dissolved inorganic carbon (DIC) ([TA-DIC]), 1TAexcess, 1DICexcess (deviations beyond conservative mixing) and 1TAP (alkalinity production due to primary production) to quantify local biogeochemical influences on carbonate system dynamics. In spring, a phytoplankton bloom with high biological activity, 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), resulted in decrease in nitrate (NO3−, 19.29 ± 18.11 µmol kg−1) and DIC (159.4 ± 125.4 µmol kg−1), and a slight increase in TA (9.1 ± 29.2 µmol kg−1). The regression analysis of the differences between March and May 2022 in NO3− concentrations (1NO3−) against the differences in DIC (1DIC) yielded a slope of 6.90, matching the Redfield C: N ratio, and suggesting that uptake of nitrate by primary producers increased total alkalinity during the spring bloom. In summer, we assume that organic matter remineralization, along with CaCO3 dissolution in sediments, enhances TA production in the coastal and nearshore regions of the Western EFWS (concentrations up to 2400 µmol kg−1). In the Eastern EFWS, enhanced CaCO3 formation may consume TA ([TA-DIC] < 200 µmol kg−1), but the region still acts as a net source of TA, likely due to sedimentary processes such as organic matter decomposition, which follow the time of increased biological activity during the spring bloom. The increase of TA enhances the coastal ocean’s ability to absorb and store CO2 through buffering and suggests that the EFWS can be a source of TA to the coastal regions during the warm productive seasons. This study highlights the complex relationships between 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 and storage, as well as climate regulation.