Steve Marshall , Dr Melissa Carew , Jennifer Anson (Hons), Dr Vincent Pettigrove (Melbourne Water) .

Rapid bioassessment is widely used to evaluate stream health. An ongoing challenge is the identification of the causal factors of ecological impairment. We employed the field-based microcosm approach developed at CESAR to complement information derived from traditional rapid bioassessment. Using rapid bioassessment in conjunction with the field-based microcosm approach, provides information on the relative importance of sediment toxicity in aquatic ecosystems compared with other potential stressors such as degraded habitat or altered hydrology. We applied this joint assessment approach to 25 stream sites spread over 3 catchments. As measured by conventional rapid bioassessment, sites with industrial catchments tended to have the most severe ecological impairment, but also tended to have the most degraded habitat and most extensively altered hydrology. The field-based microcosm approach separated the effect of sediment toxicity from these potentially confounding factors, indicating that heavy metals and hydrocarbons were a significant factor contributing to the ecological impairment observed at the most urbanized sites. We were also able to identify sites where sediment toxicity was apparently less than predicted from comparison with sediment quality guidelines. The results of this project will continue to support Melbourne Water’s efforts to improve the condition of Melbourne’s streams.