The Effects of Treated Sewage Discharge on the Biota of Port Phillip Bay, Victoria, Australia

Abstract

The Werribee sewage-treatment farm contributes more than half of the total nitrogen and phosphorus input to Port Phillip Bay. This study attempted to determine the fate of these nutrients and their effect on the biota of the Bay. This was addressed by comparing community composition, biomass, productivity, or process rate in the Werribee area of the Bay with that in Bay areas more remote from nutrient discharge. Rates of bacterial nitrification and denitrification were greatest in sediments closest to the sewage discharge point. Up to 15 percent of the inorganic nitrogen discharge may be lost via sequential nitrification-denitrification in a 4 km2 area. Epibenthic microalgal biomass and productivity were found to be five times sgreater at Werribee than at a control station. The Werribee macrophyte community showed reduced species diversity, dominance of fast growing opportunistic species, loss of large brown algal species, and occasional algal blooms, as compared to communities in nutrient-poor areas of the Bay. Classification analysis revealed offshore and nearshore groups of macrobenthos in a 2 km2 area at Werribee; species diversity was greatest offshore. The Werribee offshore macrofauna was typical of that along the whole northwestern coast of the Bay. In summer, fish biomass at Werribee was equal to that found at stations remote from sewage discharge, however community composition differed. At Werribee, the nearshore fish community was dominated by juveniles and small species, while offshore older and larger fish were more abundant. Phytoplankton productivity decreased with increasing distance from sewage outfalls only during summer. Nitrogen is probably the nutrient critical to phytoplankton biomass production in the Bay but light and/or temperature may limit productivity over much of the non-summer period. Baywide phytoplankton productivity was similar to that of non-eutrophic coastal marine waters. Zooplankton standing crop was low compared to that for many estuaries and marine embayments. Densities of zooplankton in the Werribee area were highly variable, and crustacean species found at Werribee were also distributed throughout the Bay. These findings suggest that sewage discharge has affected benthic more than planktonic communities, but that the measurable impact of the discharge is limited to a few hundred meters around the outfalls.