Controls on Seasonal Atmosphere‐Ecosystem Carbon Dioxide Exchanges in a Temperate Salt Marsh

Salt marshes play a vital role in the biogeochemistry of coastal zones, yet the biophysical controls on CO₂ exchange with the atmosphere, or net ecosystem exchange (NEE, positive upwards) remain poorly quantified. We investigated a Spartina alterniflora monoculture salt marsh on the eastern shore of Virginia, United States, by estimating half-hourly NEE from March 2016 to February 2017 using the eddy-covariance method. Maximum marsh-atmosphere CO₂ exchanges occurred during June and July when hourly averaged NEE values reached -10.0 ± 2.5 μmol CO₂ m^-2 s^-1 (mean ±1 standard deviation). During the most productive time of the year, a tidal inundation of 0.7 m reduced daytime CO₂ assimilation and nighttime CO₂ release to the atmosphere by 5.0 ± 1.2 μmol CO₂ m^-2 s^-1 and 3.0 ± 0.7 μmol CO₂ m^-2 s^-1, respectively. Diffuse photosynthetically active radiation (PAR) conditions promoted quantum use efficiencies (α) of the ecosystem that were approximately three times greater than under direct PAR conditions (α_Cloudy = 0.012 ± 0.004 versus α_Clear = 0.004 ± 0.001 mol CO₂ per (mol photons)). Under diffuse light, NEE increased more rapidly with PAR and photo-saturation occurred at higher PAR levels compared to clear-sky conditions. On average, under the influence of diffuse light, the assimilation of CO₂ increased by 30% relative to equivalent PAR levels under direct sunlight. During March 2016 to February 2017 the marsh exchanged -269.1 ± 9.1 g of carbon per m² with the atmosphere. The findings demonstrate that tides and light quality are key regulators of carbon cycling in tidal marshes. These factors should be incorporated into models of tidal marsh biogeochemistry, particularly as both are undergoing rapid changes due to sea level rise and atmospheric warming.