The effect of secondary treated sewage discharged from a recently commissio
ned extended ocean outfall at Boulder Bay, New South Wales, Australia on tw
o bioindicators (oysters and kelp) was examined. Sydney rock oysters, Sacco
strea 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 tim
es, three times before and five times after the commissioning of the extend
ed 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 tw
o 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 betwee
n the control locations. Only three organochlorines (technical chlordane an
d the DDT metabolites DDE and DDD) were detected in oysters across the enti
re sampling period. Due to the low frequency or low mean concentrations of
organochlorines an impact versus control comparison was not feasible for th
is study. Mean concentrations of trace metals in oysters were highly variab
le across all sampling periods. No obvious changes in the contaminant conce
ntrations were noticed over time. Statistical comparisons of the data colle
cted 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 juvenil
e kelp abundance revealed no outfall effect over and above the variation fo
und at control locations. Student Newman-Keuls tests, however, revealed a s
ignificantly higher abundance of both adult and juvenile kelp plants immedi
ately after the commissioning of the outfall. The value of these bioindicat
ors for detecting impacts at small secondary treated outfalls is discussed.