Tuft Dynamics and the Reproductive Phenology of Zostera caespitosa on the Southern Coast of Korea

The aim of study is to determine which environmental factors could influence the biological traits of Z. caespitosa, a unique tuft-forming seagrass. This study examined the dynamics of tufts and the growth of Z. caespitosa, along with the environmental factors. The reproductive traits were also examined to estimate the potential importance of sexual reproduction in population persistence. The density of tufts remained constant, and no new tufts produced through seedling recruitment were observed throughout the sampling period. On the other hand, the tuft size and growth exhibited clear seasonal manners and strong correlations with the water temperature, indicating that water temperature regulates the tuft dynamics and growth. The optimal growth temperature for Z. caespitosa at the study site was approximately ~22.5 °C during early summer, with growth severely inhibited during periods of high-water temperatures. Z. caespitosa was characterized by a low flowering percentage and fewer inflorescences, resulting in extremely low potential seed production. Z. caespitosa maintained its populations through clonal tuft growth with low sexual reproduction and restricted growth at high water temperatures. Hence, this seagrass species may be vulnerable to disturbances, exhibiting low resilience and facing a high risk of becoming a threatened species in coastal waters.