Metabolic rates in a pristine Posidonia oceanica meadow in the Balearic Islands and potential export of oxygen to the water column.
Supervisors: Iris E. Hendriks, Susana Flecha, IMEDEA
Seagrass meadows are among the most productive ecosystems in the world and provide a series of ecosystem services that include oxygenation of the water column, carbon sequestration, prevention of coastal erosion, stabilisation of sediments, use as spawning and nursery area and export of nutrients and organic carbon. Seagrasses have shown great potential as mitigators of rising anthropogenic CO2 and thus increasing temperatures through CO2 capture, which is one of the main goals of the Paris Agreement on Climate Change. On the other hand, seagrasses worldwide are under threat due to a number of anthropogenic stressors, putting all the services they offer at risk. One of the most widely studied seagrass species is Posidonia oceanica, an endemic angiosperm found in the Mediterranean Sea. Several studies have shown its remarkable role as marine habitat in shallow waters and its extraordinary capacity as a carbon sequestrator, the latter through calculations of the metabolic rates of the plant; gross primary production (GPP), respiration (R) and net community production (NCP), via mass balance from dissolved oxygen (DO) measurements as well as capture of allochthonous particles. For this work, DO was regularly measured in a P. oceanica meadow located in shallow, undisturbed area in the Balearic Islands (Cabrera National Park) during different periods between October 2019 and October 2021. Measurements were obtained via three different deployment types; water column sensors, multiparametric sensors installed in the benthos and closed benthic chambers. DO values by water column sensors and multiparametric sensors were introduced into a model along with in situ values of temperature, salinity and wind speed to calculate metabolic rates. Furthermore, air-sea oxygen fluxes were calculated from air-sea gas exchange coefficient (k), calculated for each wind speed measurement using a parametrisation found in the literature, selected after comparison with other published parametrisations. A few modifications were made to an existing model to adapt its applicability to the specific ocean and atmospheric conditions in our study site. Overall, our calculations indicated the meadow is a net autotroph ecosystem with strong seasonal variations, implying a net O2 production throughout the year. The calculated metabolic rates resulted in higher NCP values from benthic multiparametric sensors (217.0 ± 226.4 mmol O2 m-2 day-1) than from sensors in the water column (61.0 ± 51.0 mmol O2 m-2 day-1), suggesting a stronger O2 production in the benthic compartment closer to the meadow. Metabolic rate calculations from benthic chambers were carried out by measuring DO changes during the day and night. These also resulted in a net autotroph condition of the meadow, although production was lower than that obtained for open systems with 22.9 ± 37.1 mmol O2 m-2 day-1, and no seasonal differences were found. Oxygen export from the benthic chamber NCP values resulted in 360,160 g and 28,312 g O2 for May and July, respectively, contributing to the DO values measured in the water column.