Since 2015, troublesome masses of floating Sargassum horneri have been introduced via ocean currents and winds to the southwestern coastline of Korea, including Jeju Island. These massive mats have caused considerable damage to the aquaculture industry, tourism, and the marine ecosystem. Most previous studies of S. horneri have focused on cultivation, the development of gene markers, and photosynthetic activity, but few data on population dynamics are available. We investigated the population dynamics of native S. horneri off the southwestern coast of Korea with the aim of predicting the formation of golden tides. Populations at two sites had obligate annual life cycles. Thalli were recruited during the period September–November, grew during the period December–April, and senesced by July. This pattern reflected seasonal trends in water temperature. Specific growth rates and heights of the thalli at Munseom were significantly higher than those at Jindo. The greatest environmental difference between the two sites is probably the degree of exposure to wave action. Mortality density (thalli lost per unit area) in the Munseom population was highest during the period December–January (i.e., 2–3 months after recruitment) and in March. Most thalli in the Jindo population died off in July when water temperatures increased. The maximum average biomass of S. horneri thalli detaching from the substrata reached 1.6 kg fresh weight m–2 during January and March. Thus, large-scale drifting mats were formed by S. horneri detachment from the substrata. Despite the differences in space and environment between China and Korea, our findings will enable quantitative assessments of the overall floating Sargassum biomass in the East China and Yellow Seas.

Drifting and inundating brown seaweed Sargassum horneri biomass is called “golden tide”, as it resembles golden massive algal blooms like green tides. This phenomenon occurs globally and its serious ecological impacts on coastal ecosystems have recently begun to be paid attention to. In the present study, by sequencing whole organelle genomes of Korean indigenous S. horneri, we aimed to develop novel molecular markers that can be used for differentiating indigenous from nonindigenous individuals. To this end, we analyzed sequence polymorphisms in mitochondrial (mt) and chloroplast (cp) genomes of two Korean benthic samples in comparison to Chinese ones as a reference. We mapped mt genomes of 34,620~34,628 bp and cp genomes of 123,982~124,053 bp for the Korean samples. In comparative analyses, mtDNA cytochrome c oxidase subunit II (cox2) gene showed the highest number of single nucleotide polymorphisms (SNPs) between Korean and Chinese individuals. NADH dehydrogenase subunit 7 (Nad7)-proline tRNA (trnP) intergenic spacer (IGS) in the mt genome showed a 14 bp insertion or deletion (indel) mutation. For the cp genome, we found a total of 54 SNPs, but its overall evolution rate was approximately four-fold lower than the mt genome. Interestingly, analysis of Ka/Ks ratio in the cp genome revealed a signature of positive selection on several genes, although only negative selection prevalent in mt genome. The ‘candidate’ genetic markers that we found can be applied to discriminate between Korean indigenous and nonindigenous individuals. This study will assist in developing a molecular-based early detection method for effectively managing nonindigenous S. horneri in Korean waters.