Long-term and seasonal large-scale disturbances of a small lowland stream

Abstract

The Pranjip-Creightons Creek system, a lowland stream system in north-central Victoria, contains large amounts of sand derived from agricultural activities in the upper catchment. The sand has caused long-term changes to the morphology of the upper and middle sections of the stream system-a press disturbance. During predictable winter and spring spates, sand substrata underwent regular scouring, causing large seasonal declines in macroinvertebrate species richness and numbers of individuals and marked changes in community structure. These regular short-term seasonal disturbances may be termed pulse disturbances, and their effects were most severe at mid-reach sites where sand deposits were most recent. At these sites, the press disturbance of increased sand storage also rendered the stream bed more susceptible to pulse disturbances. When winter and spring scouring spates ceased, stable communities of macroinvertebrates developed. At sampling sites on lower reaches, where the sand had yet to reach, there was little seasonal change in macroinvertebrate community structure or numbers of individuals. Seasonal variation in benthic species richness at these structurally heterogeneous sites was due to changes in the numbers of less abundant species associated with macrophytes. Current stream restoration works aimed at stemming the input of sediment should increase the seasonal stability of macroinvertebrate communities by decreasing the extent and intensity of substratum scour during winter and spring spates.