Responses of stream invertebrate and fish assemblages to an extreme flood event in the Shigenobu River, southwestern Japan

Abstract

Flood disturbance is amongst the most universal event in rivers and streams, and known to exert an overriding effect on lotic ecosystems. Ongoing climate change is causing the frequent occurrence of massive floods worldwide. However, there is limited information on the impact of such extreme flood disturbances on lotic organisms owing to its rarity of occurrence and thus the difficulty in observation. In September 2017, an unprecedented flood occurred in the Shigenobu River on Sikoku Island, southwestern Japan. This paper reports the changes in invertebrate and fish assemblages caused by the extreme flood based on a rare dataset obtained 10-11 days before and 60-61 days after the peak flow. We also compared the response of the assemblages among four representative channel unit types in a targeted river segment. We observed significant decrease in invertebrate density (88.7％) and taxon richness (41.0％) after the flood. Invertebrate communities after the flood were characterized by the dominance of mobile taxa, that were supposed to recolonize faster than others. In contrast, fish differently responded to the disturbance event compared with invertebrates. While fish species richness did not show clear change after the flood, the abundance decreased in riffles and rather increased in backwater habitat. Analyses of the individual species showed the aggregation of Opsariichthys platypus at backwaters after the flood, implying that such semi-lentic habitats serve as a flow refugia for water-column fishes during high flow. Consequently, we showed strong impacts of an extreme flood event on invertebrate and fish assemblages, and that the effects differed between these taxa and between channel unit types. Our results give support to the conservation and the restoration of lentic habitat in low water channel from the viewpoint of disturbance ecology.