From net sink to episodic source: Seasonal dissolved inorganic carbon budget in an intertidal basin

Abstract

The back‐barrier basin of Spiekeroog in the East Frisian Wadden Sea is a biogeochemically dynamic intertidal system that plays a key role in regulating coastal carbon fluxes and regional dissolved inorganic carbon (DIC) budgets. Compared to total alkalinity, DIC fluxes in this region remain poorly quantified, limiting a comprehensive understanding of coastal carbon cycling. From October 2021 to December 2022, we combined discrete and in situ measurements, which allowed to model lateral DIC fluxes continuously and to investigate seasonal DIC sink/source dynamics. On average, based on 134 measurement days distributed across all seasons in 2022, the back‐barrier Spiekeroog basin acted as a net DIC sink to the adjacent Wadden Sea, with a mean daily flux of 0.18 ± 1.48 mol m ⁻²  d ⁻¹ . Integrated over the area of the back‐barrier basin, this corresponds to a total annual DIC uptake of 4.22 ± 1.85 Gmol yr ⁻¹ in 2022. Summer also exhibited net DIC import, albeit at lower rates, reflecting ongoing photosynthetic activity. In contrast, autumn was characterized by episodic DIC export, linked to sediment–water exchange, remineralization, and physical drivers such as storm‐induced mixing. Additionally, a seasonal carbon budget was established by integrating air–sea CO ₂ exchange and submarine groundwater discharge (SGD) estimates from the literature. This highlighted significant internal retention and transformation of DIC within the system. Submarine groundwater discharge was identified as a major knowledge gap, underscoring the need for integrated high‐resolution measurements and modeling to constrain regional DIC budgets and guide climate change mitigation strategies.