Salt marshes are vulnerable ecosystems since they are found in locations often preferred for urban development. They produce a great amount of biomass, and also are of great interest because of the ecosystem services they provide from which carbon storage stands out. This work aims to study the energy and mass exchanged by a salt marsh located in southeastern Buenos Aires province (Argentina) and to characterize its net ecosystem production. A field campaign was carried out from February 2014 to March 2015. Sensible heat, water vapor (H2O) and carbon dioxide (CO2) fluxes were measured with eddy covariance technique at 6 m height over a Spartina densiflora canopy. Fifty five percent of the data were lost and 15% more were discarded due to low turbulence conditions. This gaps were filled with a combination of techniques (look-up tables and mean diurnal variations), which allowed the estimation of monthly mean net ecosystem exchange, gross primary production and ecosystem respiration. As in other marshes, the latent heat flux consumed more than 55% of the available energy of the system, reaching up to 85% after a large flood event. This flux systematically exceeds the values of sensible heat measured throughout the study period. Unlike other environments, this southern salt marsh behaved as a CO2 sink throughout the year. The net ecosystem production from March 2014 to February 2015 was approximately −10.5 t of CO2 ha−1 yr−1 which is greater than reported results for other wetlands. These preliminary results for a southern S. densiflora salt marsh are encouraging, although new field studies are under way to confirm their accuracy.