Studies of the Cross Section of Water in the Innermost Areas of Ariake Bay with the Recent Occurrence of Hypoxic Water and Red Tide

• Published in: Oceanography in Japan Vol. 12; no. 3; pp. 291 - 305
• Main Authors: Tsutsumi, Hiroaki, Okamura, Emiko, Ogawa, Mitsuyo, Takahashi, Tohru, Yamaguchi, Hitomi, Montani, Shigeru, Kohashi, Naoko, Adachi, Takahiro, Komatsu, Toshimitsu
• Format: Journal Article
• Language: Japanese
• Published: The Oceanographic Society of Japan 05.05.2003
• Subjects: Ariake Bay; hypoxic water; red tide
• ISSN: 0916-8362; 2186-3105
• DOI: 10.5928/kaiyou.12.291

In the innermost areas of Ariake Bay, on the west coast of Kyushu, hypoxia of the bottom has occurred during the summer since 1997 and red tides have occurred more frequently since the 1990s. We monitored water conditions in Ariake Bay monthly from August 2001 to February 2002 (I.e. vertical profiles of salinity, temperature, DO and chlorophyll a, and nutrient concentrations in the surface layers). The results of our studies indicate that both the hypoxic water in the summer and the large scale of red tides of diatoms in the autumn, occurred after heavy rain fall. The processes appear to be as follows: 1. Occurrence of hypoxia of the bottom water during the summer: Heavy rain during the rainy season results in an inflow of a large amount of fresh water from rivers causing stratification of the water column to develop. Lower salinity and higher temperature at the surface from the heavy rain and strong solar radiation during the summer then causes hypoxia due to a lack of mixing. 2. Large scale diatom blooms in the autumn: Heavy autumn rains cause the inflow of a large amounts of fresh water from the rivers which contain a high concentration of nutrients. The stabilization of the surface layer of water with low salinity and high nutrient concentration results in the diatom blooms. We further suspect that the water in the innermost areas of Ariake Bay has become more stagnant than before due to the construction of a dike in Isahava Bay, which is located in the inner bay of Ariake Bay. The restriction of water exchange in the innermost areas of Ariake Bay may have caused these two phenomena.