2015 Volume 10 Issue 4 Pages 187-201
In the inner part of Ariake Bay, located on the west coast of Kyushu, in western Japan, red tides have occurred with more frequency since the second half of the 1990s. Hypoxic waters have occurred during the summer months since the 2000s, despite the fact that nutrient loading from the land to the bay has not increased over the last five decades. We monitored water conditions at nine stations in the inner part of the bay, conducted benthic environmental surveys, and quantitative samplings of macro-benthic communities at the innermost four stations between 2002 and 2008. Each summer, the water was well-stratified due to the development of a halocline and a thermocline. The DO of the water below the pycnocline fell to hypoxic conditions. At the innermost three stations in the bay, the mud content and organic matter content of the sediment increased significantly, and the carbon stable isotope ratios of the organic matter contained in the sediment ranged between −21.3±0.5‰ and −20.7±0.5‰ of δ13C. These facts indicated that the organic matter was derived photosynthetically from marine phytoplankton. The increase in the mud content of the sediment indicates a deceleration in the tidal current. This may be a key event that induces a series of environmental changes and disturbances, including the stratification of the water, the more frequent occurrence of red tides, the progress of the organic enrichment of the sediment, and the occurrence of hypoxic water during the summer.