The effect of sewage on two bioindicators at Port Stephens, New South Wales, Australia

The effect of secondary treated sewage discharged from a recently commissioned extended ocean outfall at Boulder Bay, New South Wales, Australia on two bioindicators (oysters and kelp) was examined. Sydney rock oysters, Saccostrea commercialis Iredale and Roughley, were deployed at the study outfall location and control locations for three months after which time they were retrieved and analysed for trace metal and organochlorine concentrations. This process was repeated every six months on a total of eight sampling times, three times before and five times after the commissioning of the extended ocean outfall. The abundance and recruitment of adult and juvenile kelp plants, Ecklonia radiata were also investigated. At the outfall and control locations counts were made for a total of three periods, one before and two after the commissioning of the outfall. Within each period kelp abundance was determined on three random occasions. Univariate statistics were used to test the hypothesis of an outfall effect over and above variation between the control locations. Only three organochlorines (technical chlordane and the DDT metabolites DDE and DDD) were detected in oysters across the entire sampling period. Due to the low frequency or low mean concentrations of organochlorines an impact versus control comparison was not feasible for this study. Mean concentrations of trace metals in oysters were highly variable across all sampling periods. No obvious changes in the contaminant concentrations were noticed over time. Statistical comparisons of the data collected before and after commissioning of the extended ocean outfall revealed no short-term differences in trace metal concentrations between outfall and control locations. Analysis of variance results for both adult and juvenile kelp abundance revealed no outfall effect over and above the variation found at control locations. Student Newman-Keuls tests, however, revealed a significantly higher abundance of both adult and juvenile kelp plants immediately after the commissioning of the outfall. The value of these bioindicators for detecting impacts at small secondary treated outfalls is discussed.