Phytoplankton and phosphorus in southern Georgian Bay, 1973–1982, and implications for phosphorus loading controls

Abstract

The phytoplankton and nutrient status of the embayments between Penetanguishene and Waubaushene in southern Georgian Bay (Severn Sound) were examined during the ice-free periods of 1973–1982 because the area showed symptoms of excessive nutrient enrichment. Four wastewater treatment plants currently discharge to the area, another is under construction and a sixth plant has been proposed. Except for Penetang Bay, the area is well-mixed by prevailing winds but is somewhat isolated from the main part of Georgian Bay. Average phytoplankton biomasses throughout the area were 10~20× higher than values from adjacent Nottawasaga Bay where, during 1980, total phytoplankton biomass ranged between 0.15 and 0.25 mm³·l⁻¹. Total phosphorus concentrations were highest in Penetang Bay ranging between 30 and 49 HR μ P·l⁻¹ (May–September means) over the 10 year period. Phosphorus concentrations in Nottawasaga Bay averaged 4–6 μ P·l^-1 and no significant differences were detected among the sampling stations; however, phytoplankton densities at stations near urban centres and river inflows were significantly higher than at more remote offshore sites and attests to the use of phytoplankton as a sensitive measure of trophic status in Georgian Bay.

Although improvement of Severn Sound water quality to a level comparable to that presently existing in Nottawasaga Bay may never be practical, steps are being taken to control high industrial phosphorus loading and to lessen bypassing of sewage treatment facilities previously hydraulically overloaded during periods of heavy runoff. These measures, along with an evaluation of other major sources of nutrients to Severn Sound, should enable a refinement of the nutrient management programme for Severn Sound and some improvement in trophic status.