Effects of flood disturbance on the cohort structure of the riverine mayfly Ephemera orientalis (Ephemeroptera: Ephemeridae) in the Han River in Seoul, Korea

The aim of the present study was to investigate the adaptation of coexisting Ephemera orientalis (Ephemeroptera: Ephemeridae) cohorts to flood timing. Larvae were collected from the Han River (Seoul, Korea) from 2012 to 2014, and their life history was assessed by measuring head capsule width and by comparing the emergence period of the insects in relation with the flood season. Two slow-growing cohorts (S1 and S2) and one fast-growing cohort (F) were identified. The S1 cohort was found to hatch in June or July and to emerge the following summer. Whereas, the S2 cohort was found to hatch in August or September and emerge the following August. Early-hatching larvae from the S1 cohort became the F cohort through the process of cohort splitting and subsequently merged with the S2 cohort. The F cohort, however, was not observed when huge floods occurred or under low temperature conditions. Furthermore, floods that resulted from >80 mm precipitation significantly reduced larval abundance; therefore, flooding affects cohorts both directly and indirectly. Most cohorts emerged before and after the flood season (July 10 to August 10). The results of the present study indicate that the emergence period of E. orientalis has evolved to avoid the flood season.