Drastic change in estuarine ostracod assemblages after the 2020 kyushu floods, Japan: A fundamental approach with application to past flood events

Floods resulting from heavy rainfall can induce short- and/or long-term changes in aquatic environments, leading to sequential decreases in living organisms. However, no reports have documented how ostracod assemblages change following floods and how they recover. The 2020 Kyushu Flood caused substantial damage from July 3rd to July 31st, particularly affecting Kumamoto Prefecture in Kyushu. Here we report a change of ostracod assemblages during and after the event at a fixed point. The frequency of species and the similarity of assemblages recovered within the four months. Among the characteristic species, Spinileberis pulchra, with fewer environmental constraints on its distribution, became the dominant species at the fixed point post-flood, while Loxoconcha ocellata, constrained by more environmental factors, disappeared but recovered within a year. The flood event is also preserved in sediment near the fixed point, and the event is characterized by the ostracod assemblages based on low marine, high brackish and freshwater species frequency. Ostracod assemblages serve as a useful indicator for identifying past flood events.