We report on net ecosystem production (NEP) and key environmental controls on net ecosystem exchange (NEE) of carbon dioxide (CO₂) between a mangrove forest and the atmosphere in the coastal Florida Everglades. An eddy covariance system deployed above the canopy was used to determine NEE during January 2004 through August 2005. Maximum daytime NEE ranged from −20 to −25 μmol (CO₂) m⁻² s⁻¹ between March and May. Respiration (R_d) was highly variable (2.81 ± 2.41 μmol (CO₂) m⁻² s⁻¹), reaching peak values during the summer wet season. During the winter dry season, forest CO₂ assimilation increased with the proportion of diffuse solar irradiance in response to greater radiative transfer in the forest canopy. Surface water salinity and tidal activity were also important controls on NEE. Daily light use efficiency was reduced at high (>34 parts per thousand (ppt)) compared to low (<17 ppt) salinity by 46%. Tidal inundation lowered daytime R_d by ∼0.9 μmol (CO₂) m⁻² s⁻¹ and nighttime R_d by ∼0.5 μmol (CO₂) m⁻² s⁻¹. The forest was a sink for atmospheric CO₂, with an annual NEP of 1170 ± 127 g C m⁻² during 2004. This unusually high NEP was attributed to year‐round productivity and low ecosystem respiration which reached a maximum of only 3 g C m⁻² d⁻¹. Tidal export of dissolved inorganic carbon derived from belowground respiration likely lowered the estimates of mangrove forest respiration. These results suggest that carbon balance in mangrove coastal systems will change in response to variable salinity and inundation patterns, possibly resulting from secular sea level rise and climate change.