KEY FINDING 4.Trends over the past 40 years influencing biodiversity in lakes and rivers include seasonal changes in magnitude of stream flows, increases in river and lake temperatures, decreases in lake levels, and habitat loss and fragmentation.
Over 8,500 rivers and 2 million lakes cover almost 9% of Canada's total area.1, 2 The hydrology of these rivers and lakes influences the structure of aquatic habitats and the composition of ecological communities, including plankton, plants, benthic macroinvertebrates, and vertebrates such as fish, amphibians, reptiles, and birds.2 In North America, species living in aquatic ecosystems have a higher risk of extinction than species living in other ecosystems.3
Oldman River, Alberta
More than 99% of the world's freshwater is frozen in glaciers, permafrost or permanent snow, or locked in underground aquifers.15 The remaining accessible freshwater is not distributed evenly, with 40% of the world's population projected to live in water-scarce regions by 2020.16
Most rivers in Canada show pronounced seasonal variation in flows. Minimum annual flow occurs in late summer when precipitation is low and evaporation is high, and in late winter when precipitation is frozen in ice and snow. Minimum flows can limit the availability of specific aquatic habitats and also influence water temperatures and dissolved oxygen levels. For example, a decrease in minimum flow can affect the quantity and temperature of water for late-spawning fish and increase thermal stress and exposure to predation for all fish.
In a study of 172 sites in naturally flowing rivers, the lowest annual flow increased between 1970 to 2005 at 13% of the sites. These sites were generally in the northern Montane Cordillera, Boreal Cordillera, Taiga Plains, Taiga Shield, and Arctic ecozones+. Twenty–eight percent of the sites had decreases in minimum flow, generally in the southern Pacific Maritime, southern Montane Cordillera, Boreal Shield, Mixedwood Plains, Atlantic Maritime, and Newfoundland Boreal ecozones+. Twelve percent of the sites, mostly in eastern Canada, Great Lakes, and the North, had later minimum flows, while 9%, mostly in the South and along the western coast, had earlier minimum flows.2
Maximum annual flow, or spring freshet, generally occurs in late spring and in early summer and is driven by snow melt and seasonal rainstorms. A change in maximum flow can affect species with life cycles synchronized to the spring freshet and the rich foods provided by flood plains.
Almost 20% of the sites showed a decrease in maximum flow. These sites were distributed across almost all ecozones+. About 5% showed an increase in maximum flow, mostly in the Atlantic Maritime Ecozone+. Maximum flow occurred earlier at about 10% of the sites and later at about 8% of the sites.2
The average flow of prairie rivers has been declining over the past 50 to 100 years, including:
20% reduction from 1958 to 2003 – 33% since 1970 – for the Athabasca River at Fort McMurray, Alberta;
42% reduction from 1915 to 2003 for the Peace River, near the town of Peace River, Alberta;
57% reduction from 1912 to 2003 for the Oldman River at Lethbridge, Alberta;
84% reduction from 1912 to 2003 for the South Saskatchewan River at Saskatoon, Saskatchewan.4
Reduced flows like these can impact biodiversity in many ways, including reducing habitat availability, not meeting the minimum flow requirements for aquatic species, and increasing summer temperatures.5
In the Prairies, a combination of glaciation and dry climate has resulted in numerous closed-basin saline lakes, that drain internally, rarely spilling runoff. These lakes are sensitive to climate, with water levels and salinity driven by precipitation on the lake, local runoff to the lake, and evaporation off the lake.6 Aquatic communities within these closed-basin lakes are sensitive to chemical changes that can be a result of changes in water levels. For example, water levels affect salinity and the diversity of aquatic species declines as salinity increases. When salinities reach extremely high values, species diversity becomes very low.7
From 1910 to 2006, water levels in 16 representative closed-basin lakes showed an overall pattern of decline by 4 to 10 metres.6 Declines can be explained in part by climate,6 including increases in spring temperatures, for example from 1950 to 2007,8 potentially resulting in increased evaporation rates and declining stream runoff9 to the lakes. However, climate variables alone cannot explain the declines, for example no significant change was evident in precipitation or in an index of drought severity, from 1950 to 2007.8 Other contributing factors that reduce surface runoff to the lakes include land use changes, such as dams, ditches, wetland drainage, and dugouts, and changes in agricultural use and practices,6 such as the decline in summer fallow,10 increase in conservation till, and continuous cropping.6
Loss of variability in Great Lakes water levels
Metres, 1918 to 2007
Note: Metres are in relation to the International Great Lakes Datum (IGLD), 1985, which is a lake reference level adjusted every 25 to 30 years to account for movement of the Earth's crust.
Source: adapted from Environment Canada and U.S. Environmental Protection Agency, 200917
Diverse and varied plant communities inhabiting Great Lakes wetlands are dependent on the high seasonal and year-to-year variability in water levels found naturally,18 in, for example lakes Huron and Michigan, which are unregulated. Natural water levels are affected by precipitation, evaporation from the lake surface, inflow from upstream, and outflow to the downstream lakes.
Water levels are also affected by direct regulation as well as dredging, control structures, dams, canals, and diversions.19 The regulation of water levels in Lake Superior since 1914 and in Lake Ontario since about 1960 has reduced the variability of water levels. In Lake Ontario, this has adversely affected coastal wetland ecosystems, reduced plant species diversity, and altered habitat values for many animals that depend wholly or partly on wetlands to thrive.18,20 As water shortages become more common in the southern U.S., there may be pressure for water diversions from the Great Lakes, which could, if allowed, result in further impacts on biodiversity.
Freshwater discharge from Canadian rivers into the Arctic and North Atlantic Oceans has decreased by about 10% over the past 40 years. This has been attributed to a decrease in precipitation over the same period.11, 12 In spite of this, an overall 5.3% increase in river discharge to the Arctic Ocean has been documented. The net increase is due to significant increases in annual discharge from the six largest Eurasian rivers.13, 14 Freshwater discharge to northern seas can influence ocean processes that, in turn, influence the population dynamics of marine species.