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 O-2 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 O-2 m(-2) yr(-1), GPP [83 +/- 16 mol O-2 m(-2) yr(-1)] > -CR [-60 +/- 9 mol O-2 m(-2) yr(-1)]), in agreement with oxygen oversaturation (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.

Automated summary

The study presents a 12-year dataset of gross primary production, community respiration, and net community production in a Posidonia oceanica seagrass meadow in the Bay of Revellata. The meadow was found to be a net autotrophic community, acting as a net sink of CaCO3. Both net community production and net DIS significantly contribute to an annual CO2 sink, indicating other factors beyond net air-sea CO2 flux play a role in atmospheric CO2 exchange.