Atmospheric Effects on Carbon Cycling in a Coastal Salt Marsh

The amount of carbon an ecosystem can sequester has become very relevant in recent years as climate change impacts the Earth. Coastal wetlands, being among the most productive ecosystems in the world, are very significant in this realm of study. Two techniques to characterize ecosystem productivity were explored for a salt marsh on the eastern shore of Virginia: aerial net primary productivity (ANPP) measurements based on biomass harvests, and gross primary productivity (GPP) derived from an eddy covariance flux tower. The responses of the net ecosystem exchange (NEE) to characteristics of different air masses - specific humidity, air temperature and wind speed - were explored.

The site is influenced by both maritime and continental air masses. The moist, warm and windy conditions are associated with maritime air while drier air, cooler air masses and slower wind speeds characterize continental air. The ecosystem assimilates the most carbon when moist, warm and windy conditions prevail. Moreover, the maritime air is more conducive to increased ecosystem productivity, with maximum NEE occurring when specific humidity reaches 23 g/kg, air temperature is 303 K, and wind speed is 8.5 m/s.

Components of the surface energy balance can also influence the amount of carbon assimilated by the ecosystem. During certain times of the day the surface energy budget does not reach closure due to the influences of land and sea breezes. The greatest disparity among the energy fluxes occurs from 7:30 to 9:00 am; this is a period when the wind direction shifts from around 210° (indicating land breezes) to approximately 185° (sea breezes) as the ground heats up. This finding suggests that additional energy is being advected into the marsh-atmosphere system in the form of warmer, moister air thus reducing the correlation between net radiation and the sum of sensible and latent heat flux densities.