Phosphorus and Nitrogen Levels in Lake Winnipeg
High phosphorus and nitrogen levels result in large algal blooms in Lake Winnipeg. In 2010, phosphorus levels in the north and south basins were above water quality guidelines for the protection of freshwater plants and animals, while nitrogen levels were at or below water quality guidelines.
Phosphorus levels are consistently above water quality guidelines for the protection of freshwater plants and animals in the Red River, while just over 20% of phosphorus samples in the Saskatchewan River are above guidelines for 2008 to 2010. In the Red River, almost all nitrogen samples are above water quality guidelines for the same period. Phosphorus and nitrogen levels are always below the guidelines in the Winnipeg River.
Status of phosphorus and nitrogen levels in Lake Winnipeg, Canada, 2010; and in three tributary rivers, Canada, 2008 to 2010
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How this indicator was calculated
Note: Status colours for the north and south basins of Lake Winnipeg were determined by comparing seasonally weighted average total phosphorus and total nitrogen levels in water collected across the basins to their appropriate water quality guideline for the protection of aquatic life. For the Red, Winnipeg and Saskatchewan rivers, the total phosphorus and total nitrogen water quality status was determined based on the frequency with which water quality monitoring data were above their guideline.
Source: Manitoba Conservation and Water Stewardship, and Environment Canada.
Lake Winnipeg is Canada’s sixth-largest freshwater lake and the world’s third-largest reservoir, generating hydro-electric power for all of Manitoba.[1] The lake is a source of drinking water, and supports a large commercial fishery as well as recreation. Lake Winnipeg is composed of a large, deeper North Basin and a smaller, more shallow South Basin. The two basins are separated by the Narrows, through which waters from the South Basin flow northward. With an average depth of 13 metres in the North Basin and 9 metres in the South Basin, the lake is shallow compared to the Laurentian Great Lakes. At 953 240 square kilometres, the lake’s drainage basin is the largest drainage basin of all lakes in Canada, covering four provinces and four U.S. states. The shallow waters and large amount of water flowing in from the rivers draining into the lake are major influences on water quality in the lake.
Before European settlement in the region, the lake had moderate levels of phosphorus and nitrogen. Since then, the levels of these nutrients have been affected by a range of human activities. Opening the west to agriculture brought livestock and crops, which contributed phosphorus and nitrogen from manure and fertilizer to the drainage basin. It also resulted in the draining of the thousands of wetlands that dotted the Prairies, which received and processed nutrients running off the land. Growing cities have also generated more nutrients and changed how water flows across the land.
Nutrient levels in Lake Winnipeg waters are strongly related to the amount of nutrients in the rivers that flow into the lake. The highest levels are typically found in the South Basin near the inflow from the Red River, declining as the water flows north. The levels of total phosphorus in the lake were particularly high in 2005 because of heavy spring and summer rain events causing high flows into the lake from the rivers. The highest total nitrogen levels occurred in 2006, causing algae to grow and cover much of the North Basin during the summer and fall.
Phosphorus and nitrogen are key nutrients for plant growth in lakes. An oversupply of phosphorus and nitrogen can cause nuisance aquatic plants and algae to grow to noxious levels, resulting in changes to the types of fish that live in the lakes and potentially affect human health while swimming or boating in the lake. Phosphorus and nitrogen from human activity enter Lake Winnipeg from municipal and industrial wastewaters, agricultural runoff, golf courses, city and country residential and cottage areas, and air pollution. Natural sources of phosphorus are rock weathering and the decay of dead plants and animals. Nitrogen is naturally very abundant in the atmosphere and is made available for plants and animals by specialized bacteria.
Environment Canada, Manitoba Conservation and Water Stewardship, and other partners are working closely together to more fully understand the relationships between phosphorus and nitrogen levels and nuisance algal growth in Lake Winnipeg. While this work continues, water managers are working to reduce human sources of nitrogen and phosphorus in the Lake Winnipeg drainage basin.
Related information
[1] Environment Canada and Manitoba Conservation and Water Stewardship (2010) State of Lake Winnipeg: 1999 to 2007. Retrieved on 3 January 2013.
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