Net community metabolism of a Posidonia oceanica meadow

We report a 12-yr data-set (August 2006 - October 2018) of nearly-continuous estimates (n=3275) of gross primary production (GPP), community respiration (CR), and net community production (NCP) in a Posidonia oceanica seagrass meadow, computed from O2 measurements on a mooring at 10 m bottom depth in the Bay of Revellata (Corsica). Both NCP and CR were correlated to GPP and followed the leaf biomass seasonal cycle. The meadow was net autotrophic (NCP of 23±8 mol O2 m-2 yr-1, GPP (83±16 mol O2 m-2 yr-1) > -CR (-60±9 mol O2 m-2 yr-1)), in agreement with oxygen over-saturation (104% at annual scale, 101% in winter and 109% in summer). Calcification (CAL) and CaCO3 dissolution (DIS) rates were evaluated from dissolved inorganic carbon measurements in benthic chamber incubations (August 2006-2009). The meadow was found to be a net sink of CaCO3 (DIS>CAL) at an annual rate of 7 mol CaCO3 m-2 yr-1 that matched estimates of CaCO3 deposition on the meadow by sedimentation from the water column. CAL from epiphyte coralline algae was correlated to GPP, but CAL:GPP ratio (0.1) was lower than reported for coralline algae in cultures (0.6) due to the additional contribution of Posidonia to GPP. Both NCP and net DIS contributed to an annual CO2 sink of -30 mol CO2 m-2 yr-1 distinctly stronger than the estimated net air-sea CO2 flux (-1 mol CO2 m-2 yr-1). This suggests that CO2 input by vertical mixing and/or transport by horizontal advection also strongly contribute to the net atmospheric CO2 exchange.