Mechanisms for the Expansion of Hypoxic Water in the Inner Areas of Ariake Bay during Summer

Abstract

In the inner areas of Ariake Bay since 2001, we have observed / hypoxic water under the halocline of the stratified water in high temperature conditions during June and July after the rainy season. This hypoxic water has expanded throughout the inner areas of the bay, and the DO of the hypoxic water has decreased further. Such hypoxic water is caused by an increase in organic discharge to the seafloor, due to the occurrence of large-scale red tides. Since the organic matter content of the sediment tends to continue to increase in the inner areas of the bay, the hypoxic water will develop further in the near future. Nevertheless, the large scale of the red tides was not caused by an increase in nutrient loading from the land of the coastal areas of the bay. The red tides and hypoxic water commonly occurred when the water was stratified by a decrease in salinity in the surface layer. In the inner areas of the bay, large-scale red tides have occurred in autumn (October to November) since 1998. It should be noted that the gates of the dike were closed in Isahaya Bay, the inner part of Ariake Bay, in 1997. It is very likely that the closure of the gates of the dike / decreased the tidal currents in the inner areas of Ariake Bay, strengthened the stratification of the water, enlarged the scale of the red tides which occurred in the surface layer of the stratified water, increased the organic loading to the seafloor, and thus caused the hypoxic water during summer.