Submarine Groundwater Discharge-Derived Carbon and Nitrogen Fluxes from a Rewetted Coastal Peatland

Abundant coastal peatlands along the Baltic Sea are characterized by high soil carbon, low topographic gradient, and low hydraulic conductivity, owing to decades of agricultural diking and drainage. Peatland rewetting in recent years can restore hydraulic gradients and will likely impact submarine groundwater discharge (SGD) and coastal biogeochemical processes. This study aims to quantify SGD and its associated dissolved organic and inorganic carbon (DOC, DIC) and total dissolved nitrogen (TDN) fluxes from a rewetted coastal peatland. We deployed six seepage meters to determine SGD and DOC, DIC, and TDN fluxes along two transects perpendicular to the shore, on June-July 2021. Our preliminary mean SGD flux was found to be 1.14±0.55 cm d-1 (range: -1.34 to 2.28 cm d-1). The integrated shoreline flux average of 0.25±0.10 L min-1 m-1 translates to 1.23±0.50 m3 d-1 SGD flux for a 5 x 25-m section of the study site. Concentrations of DOC (6.43±0.22 mg C L-1) and DIC (40.58±2.34 mg C L-1) were significantly higher in SGD than ambient seawater (DOC: 3.93±0.26 mg C L-1; DIC: 17.42±0.10 mg C L-1) but lower than groundwater (DOC: 10.72 ± 1.07 mg C L-1; DIC: 105.57±1.69 mg C L-1). Moreover, SGD TDN concentration (2.87±0.30 mg N L-1) is significantly higher than seawater (0.28±0.06 mg N L-1) and twice higher than groundwater (1.33±0.05 mg N L-1). Carbon and nitrogen flux estimates from SGD along the narrow coastal peatland section are 11±5 kg C-DOC d-1, 72±33 kg C-DIC d-1, and 5±3 kg N-TDN d-1. These high solute concentrations and flux estimates from a rewetted coastal peatland indicate their potentially large, SGD-derived inputs to the Baltic Sea, which may be vital to local biogeochemical processes and coastal marine ecosystems. However, our findings should be interpreted with caution since field sampling was conducted where discrete seepage sites are located.