Seagrass meadows have a substantial capacity to store carbon, but their cover loss may pose a significant risk to that capacity and increase carbon emissions from underlying sediments. Here we aimed to determine if seagrass cover loss could affect carbon flow in a Posidonia oceanica meadow. The study was conducted at Pefki Bay, Greece (36^o 04.08 N, 28^o 02.99 E), which hosts a meadow intermixed with dead matte, representing 46 % and 6 % of the sea bottom, respectively, between 5 and 30 m of water depth. We estimated organic carbon (C_org) stocks in 1 m cores, accumulation rates through sediment dating by ²¹⁰Pb and fluxes of methane (CH₄) following a 24-h cycle using in situ benthic chambers, in living and adjacent dead P. oceanica mattes. Based on the ²¹⁰Pbxs inventories, seagrass loss was estimated to have occurred approximately 32 years before our sampling (i.e., 2021). C_org stock in top meter was significantly lower at P. oceanica than dead matte (7.5 ± 2.4 kg m^-2 and 10.2 ± 0.4 kg m^-2, respectively), while CH₄ efflux showed the reverse pattern (172.2 ± 162.1 and 88.3 ± 81.1 μg m^-2 h^-1, respectively). After seagrass loss, the accumulation rate was similar between P. oceanica and dead matte (24.4 ± 1.8 g m^-2 y^-1 and 25.8 ± 13.2 g m^-2 y^-1, respectively), but the abundance of mud increased at the latter. Overall, CH4 efflux was very low (0.07 ± 0.06 and 0.03 ± 0.03 g CO₂equivalent m^-2 y^-1 at P. oceanica and dead matte, respectively) compared to the rate of carbon accumulation at both mattes, suggesting that cover loss does not necessarily relate to considerable CH₄ emissions.

Primary Presenter: Eugenia Apostolaki, Hellenic Centre for Marine Research (eapost@hcmr.gr)

Authors:

Eugenia Apostolaki, Hellenic Centre for Marine Research  (eapost@hcmr.gr)

Briac Monnier, Université de Corse Pasquale Paoli  (briac.monnier@gmail.com)

Pavlos Efthymiadis, Hellenic Center for Marine Research  (p.efthymiadis@hcmr.gr)

Catalina Garcia Escudero, Hellenic Center for Marine Research  (c.garcia@hcmr.gr)

Salvatrice Vizzini, Universita di Palermo  (salvatrice.vizzini@unipa.it)

Helen Kaberi, Hellenic Center for Marine Research  (ekaberi@hcmr.gr)

Georgios Hatiris, Hellenic Center for Marine Research  (gahatiris@hcmr.gr)

Vasilios Kapsimalis, Hellenic Center for Marine Research  (kapsim@hcmr.gr)

Evangelia Krasakopoulou, University of the Aegean  (ekras@aegean.gr)

Louisa Giannoudi, University of the Aegean  (lgiannoudi@hcmr.gr)

Elli Pitta, University of the Aegean  (ellip@hcmr.gr)

Ioannis Hatzianestis, Hellenic Centre for Marine Research  (jhat@hcmr.gr)

Christina Zeri, Hellenic Centre for Marine Research  (chris@hcmr.gr)

Oscar Serrano, Centre of Advanced Studies of Blanes   (oserrano@ceab.csic.es)