Blue carbon ecosystems, including seagrass meadow, mangrove and salt marsh, are recognised as critical carbon reservoirs for mitigating climate change. Among these, seagrass meadows are the least understood systems, yet they contribute to over 10 % of the annual blue carbon sequestration despite their limited spatial coverage. Studies on carbon cycling in seagrass meadow ecosystems have grown, but most have focused on seagrass biomass or adjacent sediments, with little attention to dissolved organic carbon (DOC) in the water column. The refractory DOC (RDOC) holds large potential for long-term carbon sequestration, yet its dynamics in seagrass-dominated systems remain poorly studied. The current view is that DOC released from seagrass is influenced by various factors such as water temperature and light intensity. Given the diurnal fluctuations in these environmental factors, diurnal variations in seagrass-derived RDOC production are also likely. In this study, conducted as part of the Blue Carbon Ecosystems Assessment, Restoration and Accounting (BLUE-CARE) project, we performed in-situ benthic chamber experiments in a typical subtropical seagrass meadow ecosystem in Shandong, China, to evaluate these diurnal changes. Preliminary results indicate a consistent net production of DOC across daytime and nighttime periods. Subsequent microbial degradation experiments indicate that most of this DOC is resistant to further degradation, with RDOC accumulation preferentially occurring during nighttime. These findings reveal a novel diurnal cycle in RDOC production by seagrass.
 

Seagrass meadow ecosystem,Dissolved organic carbon,Carbon sink,Refractory dissolved organic carbon