Macrozoobenthos distribution after flood events offshore the Mimi River estuary, Japan

Abstract

To assess the distribution of macrozoobenthos and environmental conditions after river flooding events offshore of the Mimi River, the water quality, benthic conditions and macrozoobenthos communities were investigated at five stations over a 6-year period. At the station closest to the river mouth, sediment median grain size was coarser than that at the other stations. Also, the macrozoobenthos community at this particular station was dominated by crustaceans and differed from that observed at other stations. Because these changes in sediment and macrozoobenthos were related to the size of floods caused by typhoon rains, it is suggested that the outflow of relatively coarse sand from the river affected benthic communities at this station by changing the sediment conditions. The three offshore stations detected relatively fine sediment and polychaete-dominated communities, and they found no evidence of flood effects on both their environment and macrozoobenthos. At the remaining station, the environmental conditions and the macrozoobenthos community were somewhat changed as a result of the flood events, and the median grain size and community composition differed from those of the other stations. These changes may have depended on the transport capacity of coastal currents for coarse sediments at each flood event. Our results show that sediment supply derived from river flooding could significantly modify benthic conditions, resulting in increased macrobenthic biodiversity offshore a river mouth.