Air-sea gas exchange in a seagrass ecosystem

Abstract. Seagrass meadows are one of the most productive ecosystems in the world and could play a role in mitigating the increase of atmospheric CO₂ from human activities. Understanding their role in the global carbon cycle requires knowledge of air-sea CO₂ fluxes and hence knowledge of the gas transfer velocity. In this study, gas transfer velocity and its controlling processes were examined in a seagrass ecosystem in south Florida. Gas transfer velocity was determined using the ³He and SF₆ dual tracer technique in Florida Bay near Bob Allen Keys (25.02663° N, 80.68137° W) between 3 and 8 April 2015. The observed gas transfer velocity normalized for CO₂ in freshwater at 20 °C, k(600), was 4.8 ± 1.8 cm h^-1. The result gas transfer velocities were lower than previous experiments in the coastal and open oceans at the same wind speeds. Therefore, using published wind speed/gas exchange parameterizations would overpredict gas transfer velocities and CO₂ fluxes in this area. The deviation in k(600) from other settings was weakly correlated to tidal motion and air-sea temperature difference, implying that wind is the dominant factor driving gas exchange. The lower gas transfer velocity was most likely due to wave attenuation by seagrass and limited wind fetch in this area. A new wind speed/gas exchange parameterization is proposed (k(600)=0.125u₁₀²), which might be applicable to other seagrass ecosystems and wind fetch limited environments.