Recolonization Dynamics of Warm Affinity Halophila nipponica in a Temperate Seagrass Meadow With Zostera marina

Because Halophila nipponica has only recently been reported in the temperate coastal waters of the northwestern Pacific, the recolonization dynamics of this species have not yet been investigated in temperate seagrass meadows. H. nipponica typically occurs in monoculture or in mixed meadows with Zostera marina, the most abundant seagrass species in this region. In this study, un-bordered and bordered gaps (0.5 × 0.5 m) were created in a mixed seagrass meadow of H. nipponica and Z. marina at Namhae Island on the southern coast of Korea to compare recolonization dynamics of the two species. Un-bordered gaps were marked using only a steel stake at each corner, while the margins of the bordered gaps were blocked to a sediment depth of approximately 20 cm using stainless steel blades to prevent penetration of seagrass rhizomes. Shoot densities of Z. marina and H. nipponica were measured in the gaps and in natural reference plots to estimate percent recolonized. In the bordered gaps, a few Z. marina seedlings and H. nipponica fragments were observed during winter, but no shoots of either species survived to the end of experiment. In the un-bordered gaps, the density of H. nipponica increased rapidly, with approximately 60% recovery after 2 months and reaching 85% after 10 months through only asexual reproduction via clonal growth. By contrast, recolonization of Z. marina was much slower than that of H. nipponica, with only approximately 25% recovery after 10 months through vegetative growth and recruitment of a few seedlings. Thus, small fast-growing H. nipponica rapidly recolonized, compared to relatively large slow-growing Z. marina in the small-size gaps. Asexual reproduction was the principle reproductive mechanism for the recolonization of both Z. marina and H. nipponica at the study site. According to our results, gaps created naturally and anthropogenically in mixed seagrass meadows may be primarily recolonized by H. nipponica rather than Z. marina, leading to a change in the seagrass ecosystem structure in Korean coastal waters.