Seagrass meadows are blue carbon sinks, yet their capacity to mitigate climate change is threatened by the increasing frequency of marine heatwaves (MHWs). Their net climate benefit depends on accurate greenhouse gas (GHG) accounting, particularly for methane (CH4), which has a global warming potential 28-34 times higher than CO2 and can partially offset carbon sequestration. This study assessed the response of the Mediterranean seagrass Cymodocea nodosa to a simulated winter warming event, evaluating fluxes of carbon dioxide (CO₂) and methane (CH₄) as well as community metabolism. The presence of vegetation was the primary driver of CH4 emissions, with correlation between biomass and CH4 fluxes. Warming significantly enhanced CO2 retention in vegetated cores. Despite these shifts, the system showed full recovery of all metabolic rates and fluxes once temperatures returned to baseline levels, and CH4 emissions offset less than 2% of net CO2 uptake across the warming treatment. These findings suggest that C. nodosa can temporarily strengthen its carbon sink function during winter warming. Future research is needed with increased replication, combining metabolic measurements and assessment of cumulative heatwave impacts on long-term carbon dynamics.
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