Monitoring and Modeling Total Phosphorus Contributions to a Freshwater Lake with Cage-Aquaculture

A mass-balance modeling approach has been applied to gain an improved understanding of the relative contributions of phosphorus loading from various sources into a freshwater lake with cage-aquaculture in Ontario. All cage-aquaculture in Ontario is located within Lake Huron and Georgian Bay of the Great Lakes where concerns about potential environmental issues are constraining growth of the industry. Phosphorus is of particular concern for cage-aquaculture operations in Ontario since this nutrient may accelerate eutrophication of freshwater. Sound scientific information is needed for the Ministry of Environment lake managers to make defensible decisions to move the industry forward in a sustainable manner. Lake Wolsey is located on Manitoulin Island in Lake Huron, Ontario. The lake is connected to the North Channel by a small inlet where water exchanges periodically. A cage-aquaculture operation was established in 1986 and has an average annual production of approximately 250 metric tonnes of rainbow trout. We have estimated total phosphorus loadings from eight sources of inputs and three sinks from the lake. We then applied a sensitivity analysis to establish parameters that require empirical measurement and field validation. Results show non-point sources as the leading contributor of total phosphorus to Lake Wolsey (40%, 1120 kg) followed by the farm (32%, 915 kg), groundwater (11%, 305 kg), and dwellings (8%, 219 kg), internal phosphorus load from the hypoxic hypolimnion mid-lake (7%, 186 kg), precipitation (3%, 79 kg) and leaf litter (0.3%, 8 kg). Results of the sensitivity analysis show non point sources to be the most significant parameter in terms of total phosphorus loading, followed by the lake sedimentation, then the contribution by the aquaculture operation. This is followed by the exchange via the inlet, groundwater, dwellings, the internal phosphorus, sportfishing, precipitation and leaf litter. Information from this project will provide water quality managers with scientific information to aid in decisions pertaining to policy and regulatory approaches for water quality risk assessment and management of cage-aquaculture in Ontario.