Seasonal variation in the vertical distribution of hyporheic invertebrates in an Australian upland river

Abstract

The vertical distribution of the hyporheos in the Acheron River, Victoria, was examined in each of four seasons using a freeze-coring technique. Cores were 30 cm long and were split into 10 cm sections. Most individuals (>77 %) and species (>64 %) occurred in the surface (0-10 cm) layer except during winter (July) and once in spring (October) when approximately 50 % of both individuals and species occurred below 10 cm. Chironomidae, other Diptera, Elmidae, Hydracarina and Harpacticoida were the groups that commonly moved down into the bed sediments, whereas Ephemeroptera, Plecoptera, and Trichoptera remained largely on the surface in all seasons. Downward migration seemed to be a response to increased discharge during the winter and spring ; such a hydrological pattern is typical for rivers in this region. The degree to which the hyporheos can supply colonists after a flood is thus dependent on season : far more potential recruits are available in winter than in summer.