Seagrasses, marine angiosperms, provide numerous ecosystem services for coastal and estuarine environments. They are often called ‘ecosystem engineer’ as they can modify surrounding marine environments, creating their own habitats. Zostera marine (common name ‘eelgrass’) is one of the seagrass bed-forming species distributed widely in the northern hemisphere including the Korean Peninsula. Recently, however, there has been a drastic decline in the population size of Z. marina worldwide including Korea, primarily due to anthropogenic pressure such as climate change and habitat destruction. Although an understanding of the extent of intraspecific genetic diversity and genetic structure of ‘natural’ populations is a key factor in the ultimate success of conservation efforts, little attention has been paid to the population genetic structure of eelgrass populations in South Korea. In the present study, we first assessed the current population status of this species on the entire coasts of South Korea by estimating the levels of genetic diversity and genetic structure using eight microsatellite markers. Six hundred-twenty two specimens were collected from 21 geographic localities on the southern coast (6 populations), eastern coast (6), western coast (4) and Jeju Island’s coast (5), which is located approximately 155 km off the southernmost region of the mainland.
The levels of genetic diversity were found to be significantly lower for populations in Jeju Island than for those in the mainland’s coasts (p < 0.05). South Korean eelgrass populations were all strongly genetically divergent from one another, suggesting that limited contemporary gene flow has been occurring among populations. In particular, the degree of genetic differentiation for five populations in Jeju Island was much higher than for those in the mainland’s coasts, although Jeju Island’s populations analyzed are geographically more closely located each other (0.6−26.4 km). Both STRUCTURE and FCA (factorial correspondence analysis) analyses suggest that South Korean eelgrasses are most likely to form two genetic clusters (K = 2; Jeju Island and the mainland). Depleted genetic diversity, small effective population sizes (Ne) and limited population connectivity, particularly for populations in Jeju Island indicate that these island populations may be vulnerable to local extinction under changing environmental conditions, especially given that Jeju Island is one of the fastest warming regions around the world. Overall, our work will inform on conservation and restoration efforts, including transplantation for eelgrass populations at the southern tip of Korea for this ecologically important species.