Impacts of Oxygen-Deficient Water on the Macrobenthic Fauna of Dokai Bay and on Adjacent Intertidal Flats, in Kitakyushu, Japan

Dokai Bay is surrounded by the city of Kitakyushu, Japan. We assessed the chemical conditions of its waters and sediments, and carried out quantitative samplings of the macrobenthic fauna of the bay and on adjacent intertidal flats from 1992 to 1993. Large amounts of organic matter were deposited on the floor of the bay and intertidal flats due to eutrophication, and resulted in the occurrence of oxygen-deficient water from the bottom to the middle layer during the summer. On the tidal flats, the sediments were kept oxygenated and accessible to benthic animals throughout the year. However, the macrobenthos declined markedly in summer. The oxygen-deficient water of the bay seemed to temporarily upwell onto the intertidal flats causing a catastrophic environmental disturbance. Following this, only a few polychaete species survived, in extremely low densities.