The followings are suggested from the published reports on the present conditions of sediment and benthos.
(1) Sediment
The sea areas where all the contents of T-P, T-N and TOC in sediment (upper 5 cm layer of piled-up bottom sludge) have been decreased from 1980s to 1990s are following 3 areas. They are the northern area from Subo Open Sea to Bisanseto Strait, the southern area with piled-up sludge in Hiuchi Open Sea, and almost all area of Hiroshima Bay.
The water and sediment in the Seto Inland Sea indicates the total environment of the Seto Inland Sea has not been improved. Signal of slight deterioration of the habitat environment is also observed from the reports on macro-benthos.
On the other hand, water pollution in the several individual open seas and bays has been stopped proceeding or improved judging from the correlation between reduction measures in pollutant loading and environment in water and sediment.
Osaka Bay: The discharged pollutant load has remarkably reduced. As the results, the sea area with anoxia condition at the bottom water layer tends to decrease. Remarkable improvement appears in the interior part of the bay.
Hiuchi Open Sea: Qualities of water and sediment have been improved as reduced the discharged T-P load. Without the control measures, Environment in the water and sediment would have been more vigorously deteriorated under the social structure conditions of increasing population, industrial production and so on,
(2) Benthos
Kii Channel: Numbers of species and individuals are large in the sea regions with fully exchanged water. They decrease in the sea regions where high content of sludge, COD, T-P and T-N are observed in the sediment.
Osaka Bay: Almost all the species of benthos observed in the interior part of the bay covered with sludge at the bottom are indicator species of the polluted sea region with organic matter. The eutrophication indicator species are observed in the high ratio in the northern area of the bay.
Harima Open Sea: Numbers of both species and individuals are large around Akashi Strait, Shodoshima Island and Naruto Strait.
Bisanseto Strait: Numbers of species and individuals are large in the sea regions with gravel and sand sediment.
Binngo Open Sea: Numbers of species and individual and diversity index value tend to decrease in the last 10 years.
Hiuchi Open Sea: Biota is poor and the no-biota-sites have been observed in the central area of this open sea.
Aki Open Sea: Distribution pattern of biota is homogeneous all over this open sea.
Hiroshima Bay: The bottom all over the bay is covered with sludge and organic-pollution indicator species are observed in high ratio.
Iyo Open Sea: Annelida is observed in high ratio in the coastal zone, while Arthropoda is observed in high ratio in the central part. The numbers of these species tend to decrease compared with the observation in 10 years ago all over the open sea.
Subou Open Sea: Number of species is large and eutrophication indicator species are observed in high ratio in the coastal zone in Yamaguchi Prefecture.
Beppu Bay: Number of species is large in the mouth of the bay. Biota is poor and no biota sites have also been observed in the interior part of the Bay.
Bungo Channel: Numbers of both species and individuals are small in Uwa Sea, the central zone of the northern area of the channel, and the coastal zone in Ooita Prefecture.
Hibiki Open Sea: Numbers of species and individuals are large in the northern part with sand sediment.
(3) Correlation between Environments in Water and Sediment
Because of the human activity and small tidal current, it is easy to accumulate pollutant in the sea regions with high content of sludge in sediment although the correlation among the qualities of water and sediment and the environment of aquatic animals and plants are not completely understood. With respects of water quality in such sea regions, the transparency is generally small and COD concentration in summer is especially high.