Experimental Carbon Emissions From Degraded Mediterranean Seagrass (Posidonia oceanica) Meadows Under Current and Future Summer Temperatures

Abstract

Posidonia oceanica beds are recognized to be an essential carbon sink in the Mediterranean Sea. However, when seagrass cover disappears, seagrass sediments may become net carbon sources contributing to the CO2 atmospheric increase. Determining CO2 efflux from degraded seagrass beds is required to understand potential emissions from this Mediterranean ecosystem. This study presents one of the very first experimental CO2 efflux estimates from P. oceanica degraded seagrass sediments under future warming and hydrodynamic scenarios. Sediments were sampled from a P. oceanica seagrass meadow in Pollença, a sheltered bay on Mallorca Island. Sediment samples were kept in mesocosms for 98 days at five different temperatures (26, 27.5, 29, 30.5, and 32) and two treatments (agitation/repose). CO2 effluxes were higher in the agitated treatments (0.75 μM m−2 s−1) than in repose (0.51 μM m−2 s−1). CO2 efflux clearly increased with temperature in repose treatments, while treatments under agitation presented larger variation. Our results indicate that exposure to hydrodynamics plays a key role in the remineralization of organic carbon (Corg) under climate change conditions and that denuded seagrass meadows have the potential to emit in a single summer season (3 months) the amount of carbon (C) that a healthy meadow sequestrates during an entire year. This study provides brand new information required to quantify the dynamics of Corg stock loss and CO2 emissions in degraded seagrass meadows.