Effects of water level regulation on algal biofilms in the River Murray, South Australia

The composition and growth of algal biofilms were monitored over 90 days at two littoral sites on the River Murray at Lock 1 (Blanchetown). Sites included the pool impounded by a 3m weir, where water levels are relatively stable, and the tailwater, where levels fluctuate daily. Depth profiles of organic biomass above the sediment and biofilm composition were similar between sites. Algal biomass peaked in the zones of maximum light and sustained inundation. Biofilm composition was affected more by temporal environmental changes common to both sites than by differences between sites. Filamentous Cyanobacteria (Lyngbya) were prevalent early in succession, but by day 90 were replaced by filamentous Chlorophyta (Spirogyra). If river levels are managed to maintain diverse successional stages as resources for grazing invertebrates, the magnitude and duration of inundation in the littoral zone should exceed the desiccation tolerances of biofilm organisms.