Net community metabolism of a\n \n Posidonia oceanica\n \n meadow

Abstract

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, GPP [83 ± 16 mol O2 m −2 yr] > −CR [−60 ± 9 mol O2 m yr]), 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 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 yr distinctly stronger than the estimated net air-sea CO2 flux (−1 mol CO2 m yr). This suggests that CO2 input by vertical mixing and/or transport by horizontal advection also strongly contribute to the net atmospheric CO2 exchange. The balance at community scale between gross primary production (GPP) and community respiration (CR, negative flux) is net community production (NCP = GPP + CR) and reflects whether a community is net autotrophic (NCP > 0, GPP > CR) or net heterotrophic (NCP < 0, GPP < CR) (Supporting Information Fig. S1). This is an important metric to describe organic matter flows in marine communities, because a net autotrophic community is a net producer of organic matter and either stores or exports organic matter to adjacent communities, while a net heterotrophic community is a net consumer of organic matter that is sustained by inputs of organic matter from adjacent communities. In theory, a net autotrophic community should act as a sink of atmospheric CO2, and a net heterotrophic community should act as a source of CO2 to the atmosphere, although, in practice, this is rarely the case because CO2 in seawater will also depend on other biological processes such as calcification (CAL), as well on a variety of other processes such as horizontal (advection) and vertical (mixing) transport of CO2 (Borges et al. 2006). Among coastal systems, estuaries are in majority net heterotrophic, while mangroves, salt-marshes, coral reefs, macrophytedominated communities, and the rest of continental shelves are in majority net autotrophic (Gattuso et al. 1998). Marine phanerogams are macrophytes of terrestrial origin that spend their entire life cycle submerged in the ocean and form extensive meadows. In seagrass meadows, there are two main primary producer compartments: the seagrass itself and epiphytic algae, and a smaller compartment related to microphytobenthos on the sediment and on the seagrass shoots. Epiphytic algae contribute between 20% and 60% of total seagrass meadow primary production (Marbà et al. 2006). Regarding the fate of seagrass biomass, only 10% is lost through direct grazing (Cebrián and Duarte 1998) and the majority of the biomass loss is through shedding of leaves that contribute to other detritus (organic particles from the water column plus animal feces) collectively called “litter.” Seagrass litter is the main transfer pathway to secondary production through detritivory and to adjacent communities through physical transport (Cebrián and Duarte 1998). Posidonia oceanica is a seagrass endemic of the Mediterranean Sea that grows in mono-specific meadows that occasionally extend down to 40 m deep. These meadows cover a surface estimated between 12,000 and 50,000 km corresponding to 0.5% to 2.0% of the total surface of the Mediterranean Sea and 2.5% to 10.0% of its continental shelf (Telesca et al. 2015). The extent and density of P. oceanica meadows is declining in the Mediterranean Sea due to degradation of water quality, although the *Correspondence: [email protected] Additional Supporting Information may be found in the online version of this article.