Increasing sea surface temperature due to global climate change is a major threat to the growth and survival of many temperate seagrasses, while cold-water upwelling can potentially ameliorate high-temperature stress on temperate seagrass meadows during growing seasons. Seagrasses in the coasts of Korea are increasingly influenced by rising water temperatures and water temperature anomalies in recent decades due to climate change, but the impacts of such events on reproductive phenology of seagrasses remain poorly understood. We investigated the reproductive phenology and sexual reproduction of Zostera marina in four seagrass meadows along the coasts of Korea. Two study sites were located on the south coast where water temperature shows significant warming trends, while two other study sites were located on the east coast where coastal cold-water upwelling is prevalent. Water temperature exhibited stronger seasonality with cooler winter and warmer summer in the south coast sites than in the east coast sites. Cold-water upwelling events occurred frequently along the east coast of Korea, leading to anomalously cool water temperatures during the growing season. Z. marina flowering was prolonged and/or precocious at the east coast sites relative to the south coast sites. The mean flowering period was 5-7.5 months at the east coast sites and 3.5 months at the south coast sites. Moreover, reproductive shoot density and biomass, as well as the corresponding flowering frequency and reproductive efforts were much higher (2-3 times) at the east coast sites than at the south coast sites. Consequently, the potential seed production was higher in the east coast sites than in the south coast sites. Overall, high summer water temperature appeared to reduce sexual reproduction of Z. marina in the south coast sites, while regional cold-water upwelling on the east coast changed seasonal temperature patterns and decreased summer temperature highs, consequently provided a relatively long period of optimal water temperature for production of reproductive shoots. Regional water temperature variability has an important role in shifting the flowering responses of seagrass, which could affect the long-term stability and maintenance of seagrass meadows via the regulation of the sexual-asexual reproductive trade-off.
