On the relationship between aquatic CO2 concentration and ecosystem fluxes in some of the world’s key wetland types

To understand patterns in CO₂ partial pressure (P_CO2) over time in wetlands’ surface water and porewater, we examined the relationship between P_CO2 and land–atmosphere flux of CO₂ at the ecosystem scale at 22 Northern Hemisphere wetland sites synthesized through an open call. Sites spanned 6 major wetland types (tidal, alpine, fen, bog, marsh, and prairie pothole/karst), 7 Köppen climates, and 16 different years. Ecosystem respiration (R_eco) and gross primary production (GPP), components of vertical CO₂ flux, were compared to P_CO2, a component of lateral CO₂ flux, to determine if photosynthetic rates and soil respiration consistently influence wetland surface and porewater CO₂ concentrations across wetlands. Similar to drivers of primary productivity at the ecosystem scale, P_CO2 was strongly positively correlated with air temperature (T_air) at most sites. Monthly average P_CO2 tended to peak towards the middle of the year and was more strongly related to R_eco than GPP. Our results suggest R_eco may be related to biologically driven P_CO2 in wetlands, but the relationship is site-specific and could be an artifact of differently timed seasonal cycles or other factors. Higher levels of discharge do not consistently alter the relationship between R_eco and temperature normalized P_CO2. This work synthesizes relevant data and identifies key knowledge gaps in drivers of wetland respiration.