Résumé
Seagrass meadows are among the most dominant vegetated ecosystems in temperate areas and contribute greatly to the carbon and oxygen cycles in coastal regions. However, their seasonal metabolism and how it varies along environmental gradients is rarely assessed and documented. Benthic chambers were used to measure carbon and oxygen fluxes during immersion in Zostera marina and Zostera noltii habitats and adjacent uncolonised sediments across part of their distribution range and at different times of year. Compared to bare sediments, seagrass increased both gross primary production through photosynthesis and oxygen consumption, probably by providing a complex habitat structure for the associated heterotrophic community. A seasonal trend was observed for all sites, with maximum benthic fluxes during warmer months (16.4 mmol h-1 m-2 for oxygen consumption and 28.4 mmol h-1 m-2 for dissolved inorganic carbon gross production). Using an extensive dataset combining oxygen and carbon flux assessments with the concomitant collection of habitat features (seagrass characteristics and environmental data), we showed that water temperature explained most of the variation in metabolism. We also provided quantitative estimates of Community Respiration Quotient (CRQ, between 0.63 and 1.49) and Community Photosynthetic Quotient (CPQ, between 0.88 and 1.43), two metrics characterizing benthic ecosystem functions, encompassing a wide range of environmental conditions and seagrass habitat features, making this study the first of such comprehensive and extensive coverage.