Methane and Carbon Dioxide Fluxes in a Temperate Tidal Salt Marsh: Comparisons Between Plot and Ecosystem Measurements

Tidal wetlands are comprised of complex interdependent pathways where measurements of carbon exchange are often scale dependent. Common data collection methods (i.e., chambers and eddy covariance) are inherently constrained to different spatial and temporal scales which could generate biased information for applications of carbon accounting, identifying functional relationships and predicting future responses to climate change. Consequently, it is needed to systematically evaluate measurements derived from multiple approaches to identify differences and how techniques complement each other to reconcile interpretations. To accomplish this, we tested ecosystem‐scale eddy covariance with plot‐scale chamber measurements within a temperate salt marsh. We found good agreement (R2= 0.71–0.95) when comparing measurements of CH4emissions and CO2exchange but this agreement was dependent upon canopy phenology with discrepancies mainly arising during senescence and dormancy phenophases. The environmental drivers for CH4and CO2fluxes were mostly preserved across different measurement techniques, but the number of drivers increases while their individual strength decreases at the ecosystem scale. Empirical upscaling models parameterized with chamber measurements overestimated annual net ecosystem exchange (NEE; 108%) and gross primary production (GPP; 12%) while underestimating ecosystem respiration (Reco; 14%) and CH4emissions (69%) compared to eddy covariance measurements. Our results suggest that the environmental complexity of CH4and CO2fluxes in salt marshes may be underestimated by chamber‐based measurements, and highlights how different techniques are complementary while considering limitations at each level of measurement. Tidal wetlands are important landscape features which play a significant role in regional carbon cycling between land, ocean and the atmosphere. These ecosystems are very productive, with high rates of photosynthesis during the growing season, but they also emit methane and carbon dioxide which can offset carbon gains. Therefore, it is important to accurately measure and understand what factors regulate carbon cycling processes to better characterize how these systems function, calculate carbon budgets and help predict possible reposes to climate change. We compare several common measurement techniques including chamber and eddy covariance which incorporate information across different scales to evaluate the level of agreement and how these techniques can be combined to increase our understanding of ecosystem functionality. We found that techniques agree well when comparing time‐matched measurement windows but different plant development stages across the ecosystem likely impacted agreement during autumn and early winter. We found that primary factors regulating fluxes were similar across techniques but when considering data collected over more hours of the day with eddy covariance, additional factors were identified indicating measurements from chambers which are limited in both coverage across space and time may present an oversimplified view within tidal wetlands. Plot‐level and eddy covariance measurements had good overall agreement, but the strength of agreement was dependent upon plant phenologyThe relevance and influence of environmental drivers varied for all fluxes across different techniquesModels parameterized with plot‐level data overestimated ecosystem‐scale net ecosystem exchange but underestimated CO2and CH4emissions Plot‐level and eddy covariance measurements had good overall agreement, but the strength of agreement was dependent upon plant phenology The relevance and influence of environmental drivers varied for all fluxes across different techniques Models parameterized with plot‐level data overestimated ecosystem‐scale net ecosystem exchange but underestimated CO2and CH4emissions