Phosphorus removal at Gravenhurst Bay, Ontario: An 8 year study on water quality changes

Summary

As a result of frequent algal blooms and severe hypolimnetic oxygen depletions in Gravenhurst Bay, P removal at the town's sewage treatment plant was initiated in late 1971, resulting in a 60% reduction in the P load to the bay. By 1975, the P concentration in the open water had dropped from a pre-removal 44 mg m⁻³ to 20 mg m⁻³, a level very close to that predicted by a nutrient budget model, and average chlorophyll a concentration and algal biomass were both halved. The most important changes in phytoplankton composition included a drastic reduction in the bloom-forming blue-green algae Aphanizomenon, Anabaena and Microcystis from 33% of the total biomass prior to P removal to 15% by 1975. In 1975, a second sewage treatment plant was built but the P removal techniques employed were initially unsuccessful and an effluent of 3.3 g m⁻³ of total P was discharged for six months. Conditions rapidly deteriorated in early 1976 and a bloom of Aphanizomenon climaxed at 57 mm³l⁻¹ (43,000 ASU ml⁻¹). However, following correction of the technical difficulties at the new plant, the water quality deterioration was quickly reversed and low algal biomass with a species composition typical of oligotrophic Precambrian Shield Lakes, returned to the bay. Thus, reversal of the cultural eutrophication of Gravenhurst Bay leading to the establishment of a new steady state as well as the ability to respond quickly to both a sudden increase and decrease in P input has been demonstrated.