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Fig 1: Commercial fishing boats on Lake Winnipeg |
Lake Winnipeg is the largest aquatic life support system in Manitoba, covering some 24,500 sq km. It sustains a complex food web and the largest commercial fishery west of the Laurentian Great Lakes. Our inland sea is Manitoba’s recreational playground and as the world’s third largest hydro reservoir, it energizes our economy. Like many natural systems, the lake is shaped by environmental forces and disturbed by human activities. Weather, climate, and geology regulate the growth and reproduction of its plants and animals.
But excess nutrients from urbanization, agriculture and economic development in the Lake Winnipeg watershed are impairing its water quality and, in spite of its size, Lake Winnipeg is susceptible to the impacts of climate change.
This natural wonder, reminiscent of an inland-sea, is the 10th largest freshwater lake in the world (1), easily visible from space as the province's dominant geographical feature. |
Evolution
Lake Winnipeg is the largest remnant of giant Lake Agassiz that covered much of central Canada after glacial retreat about 10,000 years ago.
Lake Winnipeg arose as three separate basins that remained isolated during the mid-Holocene (6000 BP) when temperatures were 1-2°C warmer than during the 1950s. The lake took its present shape about 2500 BP when the northern and southern basins combined.
Watershed
Lake Winnipeg is inextricably linked to a vast area of North America from which it collects surface runoff and groundwater. Its watershed covers nearly a million square kilometers stretching from the Rocky Mountain foothills to within 80 kilometers of Lake Superior.
This land area is 40 times greater than the surface of Lake Winnipeg, a ratio surpassing that of any other large world lake. Consequently, the amount of dissolved and particulate material (inorganic and organic chemicals, sediments, and detritus) received by Lake Winnipeg is comparable to that in more densely populated regions such as the Great Lakes basin. For example, the amount of phosphorous (a plant nutrient) flowing into Lake Winnipeg today, mainly via the Red River, is similar to that which entered Lake Erie during the 1960s and 1970s prior to phosphorous controls.
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Fig 2: Lake Winnipeg watershed |
Monitoring of watershed nutrient sources (Lake Winnipeg Stewardship Board Report, 2006) has revealed that crop land fertilizers, susceptible to surface runoff and flooding within the Red River Basin, provide the largest amounts of phosphorus to the lake with human and livestock wastes, particularly those in close proximity to water, and the atmosphere, contributing additional loads. |
Fig 3: Most of the phosphorous in Lake Winnipeg originates from the 6 million people, 17 million livestock, and agriculture in the drainage basin. |
Altered precipitation patterns and river flows associated with climate warming can potentially influence the supply of watershed nutrients to Lake Winnipeg.
Climate is the single most important factor controlling the biodiversity in Canadian lakes, (2) with warmer southern lakes more biologically diverse than colder northern ones. Consequently, any change in climatic conditions will elicit a response in ecosystem structure and function.
Scientists have gained a good understanding of the response to climate warming of smaller, undisturbed lakes in the Experimental Lakes Area, north-western Ontario (3). Several unexpected changes in the physical, chemical and biological processes in these lakes were caused by modified interactions between weather, watersheds and stream flows.
Recent investigations on Lake Winnipeg have also revealed the importance of climatic factors even in large aquatic ecosystems.
Water temperatures
From 1909 to present, August water temperatures in the South and North Basins of Lake Winnipeg have increased by 1.9°C and 1.0°C, respectively (4).
Further warming of Lake Winnipeg is expected in the second half of this century as air temperatures increase with doubling of global CO2. Based on our knowledge of air and water temperature relationships (5), South Basin summer water temperatures could rise from their present range of 20° - 25°C to between 25° and 30°C by 2085.
Warming will affect the entire water column, not just upper surfaces, in this relatively shallow well-mixed lake.
Open water seasons will lengthen and ice cover periods will shorten.
The capacity of Lake Winnipeg water to hold dissolved oxygen will decline as water temperatures increase.
Water quantity
River flow monitoring (Environment Canada) indicates that river flows to Lake Winnipeg have changed substantially over the last several decades.
Relatively less water is now delivered to the North Basin by the Saskatchewan River because of increased irrigation and declining glacier-sourced flows (6). Conversely, increased precipitation and runoff yields (7) in the Winnipeg and Red River catchment area has resulted in increased flows into the South Basin.
These changes appear to correspond with predictions of future water supplies based on climate data analyses which suggest that precipitation will increase in regions where it is already high and decrease in more arid regions (8). As a consequence, mid-continental summer droughts may become more frequent and intense (9) with prairie river flows into Lake Winnipeg declining even further (10).
In contrast, rainfall in the Boreal Shield east of Lake Winnipeg is predicted to increase about 15% (11). Consequently, higher flows in the Winnipeg River, currently providing 43% of flow to Lake Winnipeg, may compensate for losses of the Saskatchewan River.
Red River flows, predicted to increase by 50% (11) will have the most important impact on Lake Winnipeg because, in turn, the river will deliver more phosphorus to the lake even though human and animal populations in the basin may not increase appreciably.
Annual evaporative losses, equivalent to 2-12% of Lake Winnipeg's south basin volume (12) will increase with warming, and together with altered river dynamics, may complicate lake management strategies for water quality improvement.
Due to its’ relative shallowness and reduced volume, Lake Winnipeg is strongly characterized by its inflowing rivers. River flows determine the length of time water remains in the lake (water residence time) before it is diluted or flushed out by new, incoming water.
Closely tied to this water residence time is the growth and accumulation of planktonic animals (zooplankton) and algae (phytoplankton). With sufficient nutrients and sunlight, the longer water stays in the lake the more algae and zooplankton will accumulate because they are essentially floating organisms whose distribution is controlled by water currents and mixing.
Natural variability in Lake Winnipeg water residence time is now complicated by lake regulation for power production and may be further compromised by climate induced changes in hydrological cycles.
Water quality
The quality of water in Lake Winnipeg has deteriorated during the past three decades principally as a result of excess phosphorus loaded into the lake from watershed sources. Phosphorus is the crucial element controlling the proliferation of blue-green algae that now accumulate as surface scums and contribute to Lake Winnipeg’s ranking as the most eutrophic large lake in the world (13).
The more extreme and variable weather events predicted by global climate models may exacerbate the water quality decline in Lake Winnipeg by enhancing nutrient loading. For example, extreme flooding of the Red River in 1997 carried large amounts of agricultural soil, manure, and organic detritus into the South Basin, elevating phosphorous and nitrogen to record high levels and reducing water clarity to its lowest point in the summer of 1998. By 1999, these nutrients had moved into the more transparent North Basin where thick mats of blue-green algae developed and covered 8,000 sq km of its surface.
Even more recently, record Winnipeg River and Red River summer flows in 2005 carrying high phosphorus loads were followed in 2006, a warm dry year, by the most extensive blue-green blooms ever observed throughout Lake Winnipeg.
Periods of prairie drought, with reduced river flows, will likely not bring immediate respite from blue-greens as these species proliferate in hot, sunny conditions. High in-lake phosphorus concentrations will be maintained through internal recycling from sediments and intensified feedback from zooplankton. In addition, the increased need to retain lake water during summer months for power production in winter may exacerbate blue-green algal growth in low water years.
Community structure
The types and abundance of biological species living in a water body or preserved in its sediments can inform us about historical or existing habitat conditions, the overall state of its health or the impacts the system has received.
The plant and animal communities presently inhabiting Lake Winnipeg are comprised almost entirely of species that invaded central Canada from ice-free refugia to the south (Mississippi) and north (Beringia) after glacial retreat several thousand years ago (14, 15). Species able to adapt successfully to the physical and chemical conditions in the lake survived and formed a food web that continued essentially intact to the present.
Research suggests that the diversity of aquatic species in Lake Winnipeg will decline as water temperature rises |
As the climate warms, the assemblage of species in the Lake Winnipeg food web will change with unclear consequences to overall ecosystem performance. Taxa with a preference for warm water will flourish while cool water forms will decline or disappear.
Among the phytoplankton community, blue-green algae with a tolerance for higher temperatures will become much more prevalent. Blue-greens are known to produce toxins that are harmful to animals, including fish and humans (16). Warmer water will favour cladocerans, a group of zooplankton that feed on algae, but the prevalence of inedible blue-greens may limit the success of larger cladoceran species to the advantage of smaller sized cladocerans that can utilize other algal species.
The loss of cooler water habitat will negatively impact larger bodied calanoid zooplankton and several fish species. Of the 56 fish species presently found in Lake Winnipeg, approximately one half could face thermal stress and possible extirpation if predicted water temperature increases are actualized by 2089 (17).
As Lake Winnipeg warms, cool-water species of zooplankton that are important food items for plankton-eating fish, such as young whitefish, will probably disappear |
One commercial species, lake whitefish (Coregonus clupeaformis) is directly dependent on cool water for survival and reproduction and may become extinct if the warming of Lake Winnipeg is excessive.
Temperature sensitive life cycles of aquatic insects may be sufficiently altered by warming to cause shifts in emergence, abundance, species composition and loss of diversity that will disrupt food web functioning (18).
Recently observed changes in the lower trophic levels of Lake Winnipeg may be the first signs of the impact of climate warming on lake water temperatures. For example, the species composition of the summer phytoplankton community has shifted from a diverse assemblage in 1969 to one dominated since 1990 almost exclusively by thermophilic blue-green species (19). Higher water temperatures also appear to responsible for the increased abundance and dispersion of the microcrustacean Mesocyclops edax.
Warming of central Canada will increase the likelihood of establishment of exotic species in Lake Winnipeg from southern and eastern regions. The recent invasion of two exotic species, rainbow smelt (Osmerus mordax) and a small, algae-eating zooplankton (Eubosmina coregoni), from the Great Lakes into Lake of the Woods and Lake Winnipeg illustrates the ease of transfer of species between these regions.
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Fig 4: Lake Winnipeg. 26 Sept 2001 at 13:49 CST. AVHRR NDVI image [normalized difference (Channel 2 - Channel 1) vegetation index]. Colours range from brown = low chlorophyll through yellow to green = high chlorophyll. Courtesy of: Greg McCullough, University of Manitoba |
Exotic species represent the greatest single threat to freshwaters in North America. |
Kyoto Protocol
The importance of Lake Winnipeg will increase under the Kyoto protocol because it is a storage reservoir, third largest in the world, for the production of low-emission hydroelectric energy, important for helping to achieve emission reduction targets. Additionally, it has the potential to store CO2 in the form of algal cells (organic carbon) that die and are incorporated into bottom sediments. Research underway will help to determine the lake's effectiveness as a carbon sink.
The Bottom Line
The essential message from Lake Winnipeg, Kyoto and climate change is simple. All life on earth is inseparably linked and inter-dependent. A single human breath is ultimately shared with the entire biosphere through our common atmosphere. That breath was rooted in the past and will be connected to the future. There is a link between the microscopic plankton in Lake Winnipeg and your automobile exhaust.
For eons the earth has evolved into a self-regulating home for all life on earth. How humans, situated at the top of the food chain, behave ultimately affects how our planet can perform. Our lifestyles, from the onset of the Industrial Revolution to now, are threatening to upset the earth's natural ability to sustain life.
For information on how to live a more climate-friendly lifestyle, and to help mitigate the impact on Lake Winnipeg, contact Climate Change Connection.
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Page last updated: November 14, 2007
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