National Research Institute of Fisheries and Environment of Inland Sea, Hatsukaichi, Hiroshima, 2-17-5, Japan
The environmental conditions of bottom sediments were estimated from the changes of the distribution and species composition of macrobenthos and meiobenthos in Hiuchi-Nada and Bingo-Nada five times from May 2002 to January 2003. The organic content of bottom sediments was as high as 20 years ago in eastern Hiuchi-Nada and Bingo-Nada. The dissolved oxygen of bottom water in summer has fallen to 2.4 mg/L in part of eastern Hiuchi-Nada and Bingo-Nada. The acid volatile sulfide of the bottom sediments was low at all stations except one. The abundance of macrobenthos decreased in September, but the species diversity of macrobenthos was not related to the dissolved oxygen at bottom water of all stations. Sigambra tentaculata (Polychaeta) occurred at all stations. The nematode/copepod ratio, a pollution indicator, was low at less than 100 during the investigation period at almost all stations. It is suggested that the environment of the bottom sediment has improved in Hiuchi-Nada and Bingo-Nada in comparison with the bottom conditions of 20 years ago.