In an estuary there is a clear gradient of water salinity from the seaward to the upstream area due to river water inflow. However, the salinity profile of the water changes over time as the result of tidal action, meteorological influences and wind, etc. Estuarine organisms must tolerate such strong and variable environmental stresses. In general, the diversity of aquatic organisms tends to be limited in estuaries due to the relatively severe environmental conditions. However, information concerning the distribution of aquatic organisms in estuaries is limited. To this date, few estuaries have been intensively researched in this regard in Japan. In 1993, we carried out benthic sampling to describe seasonal changes in the distribution of macrobenthic animals and physicochemical conditions of the water and sediments in the tidal areas of the Murasaki River, Kitakyushu, Japan. A clear faunal replacement was observed in the dominant species of macrobenthic animals along the gradient of salinity of the bottom water. At the most seaward end of the river mouth, the polychaetes Cossura coasta and Lumbrineris longiforia and a bivalve, Theora fragilis, occurred densely. Upstream, a polychaete, Capitella sp., an amphipod, Grandidierella japonica, and the bivalves Musculista senhousia and Ruditapes philippinarum were the dominant macrobenthic animals. Furthest upstream, in the brackish areas of the tidal river, two brackish-water organisms occurred, a gastropod, Clithon retropictus, and a polychaete, Neanthes japonica. The distribution patterns and abundance of these dominant species changed markedly with the seasons, particularly around the midpoint of the tidal river, due to the flooding of the river in the summer. However, the total number of species and mean biomass of the macrobenthic animals in the tidal river as a whole were very stable, 39 to 43 species and 51.8 to 68.9gWW/m2, respectively, throughout the year.
