PS 53-200 - Effects of increasing water temperatures on survival and growth of the golden tides-forming alga, Sargassum horneri in Korea: implications for climate change

Background/Question/Methods

Rising seawater temperatures are changing the distribution and abundance of macroalgal species in the world but the magnitude of global warming varies regionally. Jeju Island is one of the fastest global warming regions in the world and harbors many large brown algae such as Sargassum and Ecklonia species, providing high levels of delivery of ecosystem services. Over the past 30 years, the distribution and abundance of native large brown algae are steadily decreasing along the coast of Jeju Island. Sargassum horneri have received considerable attention in Korea as this species is known to be the major golden-tide seaweed in the northwestern Pacific coasts including China, Japan and Korea. Since the 2010s, large-scale drifting S. horneri mats originating from the eastern coastal zones of China have caused considerable damage to the aquaculture industry, tourism, and the marine ecosystem in Korea. However, few studies have examined population dynamics of this species. The aim of this study was to investigate population dynamics of S. horneri and elucidate the relationship between the growth and survival of this species and water temperature. In the present study, we measured monthly growth parameters in S. horneri and environmental factors in Jeju Island, Korea from 2017-2021.

Results/Conclusions

Water column nutrients (NH₄⁺, NO₂^-+NO₃^- and PO₄^3-) concentrations were less than 2 μM. Water temperature showed a strong seasonal trend, ranging from 13.7 to 30.1°C. While annual mean water temperature was relatively constant during 2017-2021, monthly mean water temperature during February-March gradually increased over the years. The density and growth of S. horneri showed distinct annual life cycles and reflected seasonal and annual trends in water temperature. Sargassum horneri appeared from September to November, grew during the period December–April, and had disappeared from July to August. The growth rate showed the highest during February–April 2018 when water temperature descended below 15°C but it fell when water temperatures exceeded 16°C in May. However, the growth rate and recruit density dramatically decreased during 2019-2021 when the lowest monthly mean water temperatures exceeded 15°C. The recruit density and thallus height were decreased by more than 90% in 2021 compared to the levels in 2018. This suggests that water temperature can be considered a major factor controlling the growth and maturation of S. horneri. Thus, the magnitude and development of golden tides in the East China Sea may be affected by the variability of water temperature during the winter season.