Molecular identification of major green tide-forming Ulva species and their spatiotemporal patterns on 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 and their spatiotemporal dynamics 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 tufA 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. A total of 11 Ulva species were identified from both regions, with Ulva ohnoi and U. australis being dominant on Jeju Island and U. australis and U. linza prevailed on the southern coasts. Our results provide essential genetic insights into major bloom-forming Ulva species and their spatiotemporal dynamics in order to support effective management of green tide events in Korean coastal ecosystems.