Sediment Condition and Macrobenthic Community in the Nearshore Area of Dokai Bay

Abstract

In Dokai Bay, effluents from factories which were located along the bay, and the sewage from Kitakyushu City (population ; approximately one million) had resulted in serious pollution of the water and bottom sediment of the bay since 1930. However, in the past two decades, the conditions of the water and bottom sediment of the bay have been much improved, owing to recovery projects of the environmental conditions of the bay. In this study, the physico-chemical conditions of the bottom sediment and the faunal aspects of benthic community in the nearshore area of the bay are described to evaluate the present stage of improvement of the bottom environment in this area. Results obtained in July, 1989 revealed that the nearshore area of the bay is clearly divided into two subareas in terms of the bottom environment in summer. One is mud or muddy sand bottom with the reduced condition due to organic pollution, and the other is sand bottom with the less organically polluted and oxygenated condition. Although a gradient of environmental conditions from the bay mouth to the innermost part of the bay was not obvious in the bottom of the nearshore area, the abundance and the biomass of benthic community markedly decreased at the innermost part of the bay. In the mud or muddy sand bottom, Prionospio pulchra (polychaete), Neanthes succinea (polychaete) and Nebalia bipes (Nebaliacea) predominated. In the sand bottom, both Capitella capitata (polychaete) and Corophium spp. (amphipod) were dominant. However, from autumn (or winter) to spring of the next year, the dominant species of the sand bottom in the last summer occurred with high densities in the mud bottom at this area. Seasonal change of bottom environment with the reduced and/or oxygenated condition in the sediment may influence the distributions of benthic organisms in the nearshore area of the bay.