Unraveling the major Ulva species driving local green tides through molecular analyses: spatiotemporal patterns along the Korean coast

Green tides – massive proliferations of green macroalgae ( Ulva spp.) – have increasingly occurred worldwide in recent years, driven by accelerating climate change and anthropogenic nutrient inputs. These blooms disrupt coastal ecosystems, leading to biodiversity loss and economic damage. In Korea, green tides have persisted on Jeju Island since the 2000s, and have also been sporadically reported on the southern mainland coasts. However, the specific Ulva species responsible for these blooms remain largely unknown. Here, we investigated Ulva community structure and relative frequencies from 46 sites (966 specimens) on Jeju Island and the southern coasts, using chloroplast tuf A gene-based phylogenetic analysis, complemented by additional nuclear 5s rDNA marker. We found considerable differences in Ulva community composition between Jeju Island and the southern coasts, along with pronounced seasonal variation. On Jeju Island, nine Ulva species were found, with Ulva ohnoi and Ulva australis dominant, whereas 10 species were observed with U. australis and Ulva linza prevailed on the southern coasts. The presence of nonindigenous Ulva species highlights the need for continuous monitoring to track their spread and biomass growth. Our results provide essential genetic insights to support effective management of green tide events in Korean coastal ecosystems.