Strategy for Phosphorus Control from Estimated Phosphorus Loads and Predicted Total Phosphorus Concentration in Penetang Bay, Georgian Bay, 1973 to 1992

Abstract

Abstract Cultural eutrophication has affected Penetang Bay, in southeastern Georgian Bay, since the 1960s. External sources of phosphorus were estimated and open water total phosphorus concentration was predicted in order to establish a rationale for an effective source control strategy for the bay. Seasonal loadings are greatest in the spring due mainly to the watershed sources, with 33-58% of the total annual load entering during the March to May period. Apart from this period, the two sewage plant effluent discharges are the largest sources of phosphorus loading to the bay. On an annual basis, the largest source of phosphorus to the bay is from treated sewage effluent (60%), followed by watershed, shoreline development and urban storm water, respectively. More than 80% of the estimated total annual loads was from sewage effluent, watershed and urban storm sources alone. These sources were estimated annually for the historical period of record (1973-1992). The trophic model predicted the long-term average total phosphorus concentration ([TP]) reasonably well. Predictions of [TP]Bay for individual years were within 1-49% of the measured [TP]Bay. The model predicted [TP]Bay to within 20% of measured values for 14 of the 20 years of record. The model as applied to Penetang Bay is sensitive to changes in sewage plant effluent loading and watershed loading. Model predictions indicate that significant reductions in open water [TP] can be expected following implementation of proposed source control actions. Total loading of phosphorus to the Bay of less than 1,000 kg/year should result in open water [TP]Bay of less than 15 µg/L.