Carbon stocks in intertidal Ulva blooms and adjacent sediments in Korean tidal flats

Seaweeds have recently attracted increasing attention as potential contributors to coastal blue carbon cycles due to their high productivity and rapid biomass accumulation. Among them, opportunistic Ulva blooms frequently occur in intertidal tidal flats and may influence coastal carbon dynamics through biomass production, organic matter deposition, and interactions with adjacent sediments. However, their contribution to coastal carbon storage remains poorly understood. In this study, we quantified carbon stocks associated with intertidal Ulva blooms and adjacent sediments in six tidal flats in Korea. We measured photosynthetic efficiency and tissue carbon content of bloom-forming Ulva species, along with sediment organic carbon content across sediment depths. The carbon stock of Ulva biomass ranged from 0.02 to 4.2 Mg CO2 ha-1, while sediment carbon stocks ranged from 23 to 732 Mg CO2 ha-1, indicating that Ulva blooms may contribute to coastal carbon cycling together with adjacent sediment carbon pools within intertidal ecosystems. Seasonal and regional differences in bloom-forming species resulted in variations in physiological performance, biomass persistence, and carbon storage among sites. In Jeju Island, where Ulva blooms occur throughout the year, relatively high biomass-associated carbon storage and elevated sediment organic carbon contents were observed. In contrast, west coast tidal flats exhibited lower sediment organic carbon concentrations but still stored substantial amounts of carbon due to their large spatial extent. Although Ulva biomass itself represents a transient carbon pool due to rapid decomposition and turnover, bloom-derived organic matter may contribute to coastal carbon cycling through sediment deposition and organic matter transfer pathways. These findings provide baseline data for evaluating the role of macroalgal blooms and associated sediments in coastal carbon storage and highlight the need to integrate such ecosystems into future blue carbon assessments.