CHANGES IN BIOFILMS INDUCED BY FLOW REGULATION COULD EXPLAIN EXTINCTIONS OF AQUATIC SNAILS IN THE LOWER RIVER MURRAY, AUSTRALIA

Notopala sublineata, Notopala hanleyi (Viviparidae) and Thiara balonne nsis (Thiaridae) are prosobranch gastropods that were once abundant in the lower River Murray. These and other snail taxa have declined mark edly over the last 50 years, coinciding with increased flow regulation by dams and weirs. In this paper we speculate that the decline may be linked to changes in the nature of food resources. Before regulation, most of the biofilm biomass in the lower Murray probably was microbia l, as fluctuating; water levels and high turbidity would have maintain ed these communities in a state of early succession. By stabilising se asonal water levels, we suggest that regulation has promoted the growt h of filamentous algae, perhaps at the expense of bacteria. Evidence f rom gut and faecal pellet analysis, and from analysis of carbon stable -isotopes, suggests that the gastropod taxa are detritivores, feeding mainly on amorphous organic detritus. Algae have a relatively high C:N ratio (low nutritional value) and may be an inadequate food to mainta in female growth and reproduction, especially in viviparous snails.