Marsh-atmosphere exchange of CO2 in a New England salt marsh

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We study marsh-atmosphere exchange of carbon dioxide in a high marsh dominated salt marsh (PIE LTER) using the eddy covariance technique since 2012. Tidal inundation at the site occurs only during bi-weekly spring tides, during which we observe a reduction in NEE fluxes during day and night. Seasonal canopy development is monitored by NDVI measurements. NDVI decreases during inundation temporarily, when the marsh canopy is submerged. We estimate NEE, GPP and Reco using the PIRLE model, which requires PAR radiation, temperature and NDVI as control variables. Two time series of NDVI, including and excluding effects of tidal inundation, allow us to model fluxes during inundation and corresponding fluxes under air exposed conditions. The effect of the flux reduction is small (2-4% of total flux) at our site, but likely higher for marshes at a lower elevation. In 2012, which was an exceptional warm year in the entire region, we observe an early start of net uptake, but higher respiration than in 2013 and 2014 due to higher temperatures in August. This results in the lowest NEE during the study period (-248.9gCm2). 2013 shows the highest seasonal net uptake (-322.6gCm2),which is linked to higher rainfall and temperature in July than the other two years. Mean sea level is stable during the three seasons, highlighting the influence of climatic factors on marsh productivity which is important for marsh resilience facing an acceleration of sea level rise. For a complete carbon budget we supplement these atmospheric fluxes with measurements of tidal carbon fluxes and long-term burial rates.