LIFE-HISTORY AND PRODUCTION OF DELEATIDIUM (EPHEMEROPTERA, LEPTOPHLEBIIDAE) IN AN UNSTABLE NEW-ZEALAND RIVER

Life history and production of a Deleatidium species (lilli group) (Ephemeroptera: Leptophlebiidae) was studied for a 12-month period in a flood-prone river in the South Island, New Zealand in 1985-86. Mean larval density was highest (9170-18 580 m-2) following a long period (> 5 months) of low stable flow (< 30 m3 s-1), but typically ranged from 3550 to 4310 m-2. The annual cycle of Deleatidium was difficult to interpret but there appeared to be two overlapping, poorly synchronised generations each year. Production of the winter generation calculated using the Size-frequency method (51.40 g DW m-2), was five times higher than that of the summer generation (9.17 g DW m-2) and annual production was 60.57 g DW m-2. Mean biomass of the winter generation (7.59 g DW m-2) was higher than that of the summer generation (0.87 g DW m-2) but the production to biomass ratio (P/B) of the summer generation (10.54) exceeded that of the winter generation (6.77). The ability to feed on low-biomass epilithic layers, behavioural responses that minimise population losses during floods, and flexible life history patterns enable this species to attain high annual production in a physically harsh environment.