Response of periphyton to current velocities in the Nepean River, NSW

Warragamba Dam is the largest water supply dam on the Hawkesbury-Nepean River system. It greatly reduces flow variability and volume downstream, impacting water quality, periphyton, macrophyte growth and ecological processes dependent on these. The benefits of variable environmental flows from Warragamba Dam have been investigated by the NSW Government. Environmental flows can scour nuisance algae and reset periphyton communities; however the flow regime required to achieve this in the Nepean River is unknown. The study was conducted in a cobble-dominated riffle in the Nepean River at Penrith, NSW. Eighty-four algal genera were identified. Leptolynhbya dominated all samples during the study. A flow of 5440 ML/day reduced biomass, overall abundance, particularly filamentous green algae and stimulated algal succession, resulting in increased community richness. Periphyton communities are influenced by current velocity; however, the similarity if flow velocities between scouring and non-scouring events suggests that thresholds may exist. This study investigated responses of periphyton to current velocity using Threshold Indicator Taxa Analysis (TITAN). Scenedesmus, Navicula and Cymbella has loss thresholds, whilst others exhibited positive associations with increasing current velocity. This study will help finalise an environmental flow regime for Warragamba Dam and supports the existence current velocity scouring thresholds for some periphyton taxa.