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Primary productivity on Sturgeon Bank Ross, Lauren Ruth


The Iona Island sewage treatment plant opened in 1963 and discharged primary treated wastewater onto the high intertidal area of Sturgeon Bank in the Fraser River estuary. In 1988, the effluent was diverted to the Strait of Georgia by the implementation of a deep sea outfall, intended to restore the ecological health of the intertidal area. This study investigated phytoplankton and benthic microalgal productivity on Sturgeon Bank, at a contaminated site adjacent to the sewage treatment plant and two reference sites further removed, to determine whether an effect of sewage pollution was present at the primary producer level of the food web. Primary productivity was determined by measuring the production and consumption of oxygen in benthic chambers over approximately 20 hour incubation periods, during 1995 and 1996. A variety of environmental parameters, each with a potential influence on primary productivity, was measured: nutrient concentrations, solar irradiance and attenuation, water turbidity, temperature, salinity and the biomass of primary producers. The contaminated site revealed the highest primary productivity, despite a lower biomass of benthic microalgae. These producers were more productive at this site, most likely due to the high level of organics from the sewage effluent, which enhanced nutrient availability. The low benthic biomass was most likely due to grazing by benthic macroinvertebrates in this area. The reference sites, however both supported a higher benthic microalgal biomass with lower productivity. Although the intermediate reference site, located approximately 2 km south of the previous sewage effluent outfall, exhibited the lowest benthic productivity, this may have been a consequence of surface freshwater used to inadvertently fill the incubation chambers during the summer months. However, the influence of the Fraser River is greatest at this site and may have been a factor controlling productivity. Correlations were not found between primary productivity and light, temperature or benthic biomass, although the effect of these environmental conditions may have been masked by other factors. Results from collaborating studies revealed information about the sediment composition, heavy and trace metal concentrations and organic content at the contaminated site that most likely did not impact productivity. As there is a lack of information regarding primary productivity in the Fraser River estuary, this study establishes baseline data useful for comparisons with future studies of benthic microalgal productivity.

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