Productivity and carbon sequestration potential of seagrass ecosystems in the eastern Aegean Sea

Atmospheric CO₂ levels have been increasing at ever faster rates, fueled by anthropogenic activity. Natural ecosystems,which typically form net autotrophic habitats such as seagrass meadows, could be crucial to counteracting CO₂ emissions. Increased fragmentation of Posidonia oceanica seagrass meadows within the eastern Mediterranean basin, linked to increased sea surface temperature, places these meadows at high risk of loss. Annual metabolism estimates showed patchy shallow water P. oceanica within the eastern region of the Aegean Sea to be overall autotrophic. P. oceanica net apparent productivity was heterotrophic in Autumn and significantly less than Summer when autotrophic, influenced by relative changes in irradiance and seagrass aboveground biomass. Seagrass biometrics also acted as predictors of carbon sequestration spatially, demonstrating higher productivity in the meadow center compared to the meadow edge. Future forecasts of autochthonous carbon storage must consider seasonal changes in productivity, potentially alongside seasonal changes in irradiance and aboveground biomass. Ultimately shallow patchy P. oceanica meadow’s contribution to carbon sequestration should not be overlooked. The non-indigenous seagrass Halophila Stipulacea was first recorded in the Mediterranean, within the Aegean Sea. Its tropical origin may enable it to thrive given global climate change predictions for the Mediterranean. However, the H. stipulacea community was highly heterotrophic during Autumn. Utilising periods of increased irradiance in Summer may enable the plant to persist at this locality, but it seems to live near its limits for survival.The presence of an uncommon endosymbiotic phytomyxid is documented and its potential influence on H.stipulacea metabolism discussed. Overall, this work demonstrates shallow water P. oceanica meadows in the Aegean Sea are annually autotrophic and if able to persist will continue to remove atmospheric carbon. Knowing H.stipulacea is near its limits in terms of metabolic balance and survival, indicates Mediterranean autochthonous carbon sequestration may decrease should H. stipulacea increase in abundance simultaneous to knownP. oceanica regression.