Variability of CO2 and CH4 in a Coastal Peatland Rewetted with Brackish Water from the Baltic Sea Derived from Autonomous High-resolution Measurements

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, for example, on water level, vegetation, temperature, 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. Environmental heterogeneity and the brackish water column formation require improved quantification techniques to assess local sinks and sources of atmospheric GHGs. We conducted 9 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, the second year after rewetting. For the study, we used newly developed multi-sensor platforms (landers) customized for this operation. 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. Episodic and diurnal drivers are identified and discussed based on Spearman correlation analysis. The multi-day variability resulted in a pronounced variability 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, for example, in a mean daily variability of 4066.9 µatm for CO2 and 1769.6 µatm for CH4. Depending on the location, the diurnal variability led to pronounced differences between the measurements during the day and night, so the CO2 and CH4 fluxes varied by a factor of 2.1-2.3 and 2.3-3.0, respectively, with higher fluxes occurring over daytime. 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.