Abstract: Massive green tides caused by macroalgae Ulva prolifera are a periodic event in the South Yellow Sea of China in the past 15 years. As U. proliferagrows rapidly producing a large amount of organic biomass, it also releases substantial amounts of dissolved organic carbon (DOC) into the ambient seawater. However, the bio-availability of the released DOC, which is important for its fate, is poorly understood. During a massive macroalgal bloom in June 2016, three stations located in the U. prolifera bloom zone and three stations located the nonU. prolifera bloom zone were respectively selected to enrich the DOC of surface seawater and conduct a long-term (300 days) DOC degradation experiment in the laboratory. It revealed that in 60 days the DOC concentration of seawater enriched at different stations decreased rapidly associated with a sharp increasing microbial abundance, indicating the rapid consumption of labile DOC (LDOC). The remaining DOC was found to be resistant to microbial utilization and did not show any significant reduction during the experiment, suggesting that this DOC was recalcitrant DOC (RDOC) with strong stability. After 300 days of incubation, the RDOC accounted for 46% of the DOC in the U. prolifera bloom seawater, which was significantly higher than that in the nonU. proliferabloom zone (36%). Moreover, the labile protein-like components of fluorescent dissolved organic matter (FDOM) were rapidly consumed by the microorganisms and the recalcitrant humus-like component gradually accumulated, suggesting that LDOC was gradually transformed into RDOC in the process of degradation. Hence, we infer that the outbreak of U. prolifera green tides not only increases the concentration of DOC in seawater in a short-term, but also contributes to the long-term accumulation of RDOC in the coastal seawater.