Seagrass bed in Kagoshima Bay: The distribution status in 2021 and long-term decline

Abstract

In 2021, the current distribution status of seagrass beds in Kagoshima Bay, Kagoshima, Japan was investigated at 77 sites including four sites located outside the bay. At each site, we measured the seagrass bed area by the naked eye using scuba, snorkeling, and coastal walking and recorded the coverage for quad-rats (50 cm long and wide), which were randomly placed inside the seagrass bed. In addition, we measured the water quality including temperature, salinity, and nitrogen levels at the study sites or in the laboratory. The results showed that five species of seagrass were confirmed from the 77 sites, including Zostera marina, Zostera japonica, Halophila nipponica, Halophila major, and Ruppia maritima. Of these species, the first three were found in Kagoshima Bay. However, the total area of seagrass beds in Kagoshima Bay drastically declined to 4.39 ha excluding 2.07 ha from outside of Kagoshima Bay, which was only 11.6% of the area observed in 2006 and 2.4% of the area observed in 1978. This decline is a serious concern for the conservation of seagrass beds in this region, particularly since Z. marina in Kagoshima Bay represents the southern distributional limit of this species in Japan.