Reproductive traits of floating Ulva prolifera sporophytes and gametophytes and their contribution to the Yellow Sea green tide.

Since 2007, the Yellow Sea green tide has become a recurring ecological phenomenon with a profound impact on coastal landscapes and marine ecosystems. Floating Ulva prolifera, the primary species driving the Yellow Sea green tide, exhibits morphologically indistinguishable sporophyte and gametophyte stages. This study is the first to systematically explore the growth and reproductive characteristics of U. prolifera sporophytes and gametophytes. The growth rate of gametophytes was significantly higher than that of sporophytes, while sporophytes demonstrated higher spore release capability, showing no significant correlation with temperature. Analysis of 756 floating U. prolifera specimens collected from the Yellow Sea from 2022 to 2024 revealed that the dominant type varied annually: mixed sporophytes and gametophytes in 2022, a predominance of gametophytes in 2023, and sporophytes in 2024. However, the dominant type remained consistent across different survey periods within each year. Given that U. prolifera exhibited a higher probability of in-situ germination in laboratory cultures than in the field, we inferred that floating U. prolifera might employ different reproductive strategies: primarily asexual reproduction in the laboratory and vegetative reproduction in the wild. Through statistical analysis of annual algal types and green tide parameters, we found that the dominant algal type showed a weak correlation with green tide parameters, indicating comparable contributions of sporophytes and gametophytes to green tide formation without significant differences. This study provides new insights on the differences between stages in isomorphic haploid-diploid species analysis and the biological mechanisms behind green tide formation, offering important theoretical foundation for prediction and prevention research.