REASONS FOR THE ABSENCE OF GREEN TIDES IN INNER TOKYO BAY IN 2017-2018

Abstract

 The Yatsu Higata tidal flat located in the inner part of Tokyo Bay is one of the most significant tidal flats remaining in Japan. The tidal flat was registered as a Ramsar site in 1993 because of the numerous migratory birds that visit the wetland to breed and feed. However, in recent years, the occurrence of green tides due to the extraordinary growth of Ulva spp. has become problematic and the green tides have severely impacted the ecology of the area. However, the green tides disappeared suddenly in 2017 and no green tides were observed in 2018. In this study, the cause of the disappearance of green tides in the Yatsu Higata tidal flat was examined based on field observation data collected from 2003 to 2018. The findings revealed that, while the water quality remained nutrient levels in the tidal flat remained high, Ulva spp. died due to a lack of light caused by the red and green tides in 2017. In 2018, higher than normal water temperatures were observed for an extended period in Tokyo Bay, and these elevated temperatures contributed to the death of Ulva spp..