Phytoplankton responses to wastewater discharges at two sites in Western Australia

The two main wastewater discharges on the west coast of Australia, near the city of Perth, were monitored for their potential impact on phytoplankton communities between 1996 and 2000. The wastewater outlets varied in their location, length and depth (at point of discharge), volume of discharge and nutrient load. Phytoplankton responses were markedly different in the vicinity of the two outlets. In the vicinity of the southern outlet with the greatest effluent volume and nutrient load, phytoplankton responses were at or near the level of detection, except during summer. In the vicinity of the northern outlet significant environmental impacts were noted: light attenuation was 32% greater, phosphate and nitrate concentrations were 301% and 548% greater, respectively, and phytoplankton biomass was 221% greater than the relevant control site. The magnitude of the environmental responses at these two outlets is hypothesized to be associated with differences in current speed and stratification. At the northern outlet, where a greater phytoplankton response was detected, biomass was negatively correlated with wind speed, which suggests that measurable biological impacts were associated with the retention of the buoyant effluent in the vicinity of the outlet during periods of slower along-shore transport.