Variability of CO₂ and CH₄ in a coastal peatland rewetted with brackish water from the Baltic Sea derived from autonomous high-resolution measurements

Abstract. Rewetting peatlands is an important measure to reduce greenhouse gas (GHG) emissions from land use change. After rewetting, the areas can be highly heterogeneous in terms of GHG exchange and depend on water level, vegetation, previous use, and duration of rewetting. Here, we present a study of a coastal peatland that was rewetted by brackish water from the Baltic Sea and thus became part of the coastal shallow Baltic Sea water system through a permanent hydrological connection. Rewetting suppresses carbon dioxide (CO₂) emissions by preventing aerobic decomposition of organic matter. Conversely, the anoxic conditions in the soil lead to an increase in methane (CH₄) emissions, which counteracts CO₂ mitigation effects. Unlike to rewetting with freshwater, the effects of rewetting with brackish, sulfate-containing water are less studied, although positive effects are expected as sulfate-reducing bacteria may become established and might out-compete methane-producing archaea (methanogens) for substrates, resulting in lower CH₄ emissions. Both aspects, environmental heterogeneity and the brackish water column formation, require improved quantification techniques to assess local sinks and sources of atmospheric GHGs. We conducted nine weeks of autonomous and high-resolution, sensor-based bottom water measurements of marine physical and chemical variables at two locations in a permanently flooded peatland in summer 2021, and derived GHG fluxes. Results show considerable temporal fluctuations of CO₂ and CH₄, expressed as multi-day, diurnal and event-based variability and spatial differences for variables dominantly influenced by biological processes. The multi-day variability resulted in a pronounced magnitude of measured GHG partial pressures during the deployment ranging between 295.0–8937.8 µatm (CO₂) and 22.8–2681.3 µatm (correspond to 42.7–3568.6 nmol L⁻¹; CH₄), respectively. In addition, the variability of the GHGs, temperature, and oxygen was characterized by pronounced diurnal cycles, resulting e.g., in a mean daily variability of 4066.9 µatm for CO₂ and 1769.6 µatm for CH₄. The diurnal variability led to a pronounced discrepancy between the measurements during the day and at night as well as depending on the location, resulting in CO₂ and CH₄ fluxes that varied by a factor of 2.1–2.3 and 2.3–3.0, respectively. The rewetted peatland was further impacted by fast system changes (events) such as storm, precipitation and major water level changes, which impacted biogeochemical cycling and GHG partial pressures. The derived average GHG exchange amounted to 0.12 ± 0.16 g m⁻² h⁻¹ (CO₂) and 0.51 ± 0.56 mg m⁻² h⁻¹ (CH₄), respectively. These fluxes are high (CO₂) to low (CH₄) compared to studies from temperate peatlands rewetted with freshwater. Comparing these fluxes with the previous year (i.e., results from a reference study), the fluxes decreased by a factor of 1.9 and 2.6, respectively. This was potentially due to a progressive consumption of organic material, a suppression of CH₄ production, and aerobic and anaerobic oxidation of CH₄, indicating a positive evolution of the rewetted peatland into a site with moderate GHG emissions within the next years.