Benthic fauna of the inner part of Ariake Bay: long-term changes in several ecological parameters

We investigated the benthic fauna of the inner part of Ariake Bay in August 2006. We set 45 sampling stations including at 28 mud and 17 sand bottoms in subtidal areas. In each station, sediment samples were taken once with a Smith-McIntyre grab. The sediment samples were sieved with a 1 mm mesh, and the residue was fixed with 10% formalin solution. In the laboratory, benthic organisms were sorted, identified and weighed by their wet weight. The data obtained were compared to the samples collected in 1989. In both mud and sand bottoms, benthic fauna in 2006 was dominated by bivalves and polychaetes as in 1989. However, the abundance of polychaetes, bivalves, ophiuroidea, and the other taxa except crustaceans significantly or nearly significantly decreased from that in 1989 irrespective of bottom types, leading to an overall decrease in macrobenthos. The biomass of mud bottoms tended to decrease from that in 1989, though did not differ in the sand bottom. The distribution of bivalves dominating in 1989 also drastically decreased. The cause of the drastic decrease in macrobenthos may represent the consequence of the frequent occurrence of hypoxia in summer and/or change in sediment grain size, both of which have recently occurred in the inner part of Ariake Bay.