Life history and secondary production of Ephemera orientalis (Ephemeroptera: Ephemeridae) from the Han River in Seoul, Korea

In this study, we investigated the life history of Ephemera orientalis, a common lowland burrowing mayfly which resides in temperate East Asia and often causes a serious nuisance to people due to mass emergence from the Hall River in Seoul, Korea. Larvae were sampled monthly (every two weeks during the emergence period) from April 2006 to June 2007 using a Surber sampler (50 x 50 cm, mesh 0.25 mm,,two replicates). The mean density of E. orientalis was found to be 105.88 +/- 42.14 indiv/m(2) during the study period. On the basis of the larval body size distribution and emergence time, we concluded that E. orientalis has a univoltine life cycle with two distinct cohort groups, S1-group emerging in May-June and S2-group in August-September. The estimated annual production of the larvae was 1350.84 mg DW/m(2); the mean biomass was 325.17 mg DW/m(2); the annual production to mean biomass ratio (P/B) was 4.15. Mean water temperature during the study period was 15.02 +/- 6.84 degrees C. The accumulated degree days for the S1- and S2-groups were 2565.07 and 2,621.69 degree days, respectively. This study shows that the Hall River population of E. orientalis lacks the fast growing cohort (F-group) ill May-August, and demonstrates a relatively higher secondary production than that of other stream mayflies (e.g. Gapycong stream population of E. orientalis).