The response of seagrass (Posidonia oceanica) meadow metabolism to CO₂ levels and hydrodynamic exchange determined with aquatic eddy covariance

Abstract. We investigated light, water velocity, and CO₂ as drivers of primary production in Mediterranean seagrass (Posidonia oceanica) meadows and neighboring bare sands using the aquatic eddy covariance technique. Study locations included an open-water meadow and a nearshore meadow, the nearshore meadow being exposed to greater hydrodynamic exchange. A third meadow was located at a CO₂ vent. We found that, despite the oligotrophic environment, the meadows had a remarkably high metabolic activity, up to 20 times higher than the surrounding sands. They were strongly autotrophic, with net production half of gross primary production. Thus, P. oceanica meadows are oases of productivity in an unproductive environment. Secondly, we found that turbulent oxygen fluxes above the meadow can be significantly higher in the afternoon than in the morning at the same light levels. This hysteresis can be explained by the replenishment of nighttime-depleted oxygen within the meadow during the morning. Oxygen depletion and replenishment within the meadow do not contribute to turbulent O₂ flux. The hysteresis disappeared when fluxes were corrected for the O₂ storage within the meadow and, consequently, accurate metabolic rate measurements require measurements of meadow oxygen content. We further argue that oxygen-depleted waters in the meadow provide a source of CO₂ and inorganic nutrients for fixation, especially in the morning. Contrary to expectation, meadow metabolic activity at the CO₂ vent was lower than at the other sites, with negligible net primary production.