Nutrient Loading and Algal Blooms

Status and Trends
improving status where nutrient inputs successfully reduced
Healthy, improving at a slow to moderate rate
deteriorating status where algal blooms increasing and nutrients not controlled
Concern, getting worse at a slow to moderate rate
some good data but spatial coverage limited; temporal trends often short
Medium confidence in finding
algal blooms reappearing in some areas where nutrient inputs have been reduced
Red flag

KEY FINDING 12. Inputs of nutrients to both freshwater and marine systems, particularly in urban and agriculture-dominated landscapes, have led to algal blooms that may be a nuisance and/or may be harmful. Nutrient inputs have been increasing in some places and decreasing in others.

This key finding is divided into three sections:

Nutrient loading refers to the release, through human activities, of nitrogen, phosphorus, and other nutrients into the environment.1 Fertilizers from agriculture, phosphates from detergents, and sewage from urban development are examples of nutrients that can be loaded into aquatic systems. Although increased nutrients stimulate the growth of phytoplankton – the bacteria and algae at the foundation of aquatic food webs – this can have negative impacts on aquatic ecosystems.2

Nutrient loading can result in algal blooms – rapid increases in phytoplankton growth – and sometimes dead zones. Algal blooms can cause dead zones through two mechanisms: 1) they can consume so much oxygen that other plants and animals can no longer survive; and, 2) a few species of phytoplankton – primarily blue-green algae in freshwater and dinoflagellates in the ocean – can form harmful blooms that produce toxic compounds that kill other organisms.3 Algal blooms have been the cause of many massive fish kills. However, only about 2% of the 2,000 described phytoplankton species in freshwater, and of the estimated 3,400-4,000 known phytoplankton species in marine systems, are toxic.4,5

Although algal blooms do occur naturally, nutrient loading contributes to increases in the frequency, areal extent, and intensity of algal blooms.6 Increasing water temperatures may also contribute, and climate change is expected to cause changes in the distribution, seasonality, and frequency of algal blooms.7

Algal blooms – both toxic and non-toxic – occur across Canada in lakes, reservoirs, ponds, rivers, swamps, and estuaries. They have been reported in coastal and inland B.C., the Prairies (Qu’Appelle Lake system8), central Canada (Lake Winnipeg9 and Lake of the Woods), the Great Lakes and Boreal Shield of Ontario, the Mixedwood Plains, Boreal Shield and St. Lawrence River in Quebec, and the Atlantic Maritime.4


Global Trends

More than 400 dead zones have been reported in coastal waters worldwide.6 Nutrient loading to terrestrial, freshwater, and coastal waters ecosystems are projected to increase substantially in the future.1






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Photo: algal bloom © Michel Starr, Institut Maurice-Lamontagne, DFO

Nutrient loading

Nitrogen and phosphorus levels in water bodies

Percentage of sites with increasing, decreasing, and stable trends between 1990 and 2006
Two pie charts showing the percentage of sites with increasing, decreasing and stable trends in nitrogen and phosphorus. Click for graphic description (new window).
Note: these are the results of 83 sites monitored for nitrogen and 76 sites monitored for phosphorus through federal and provincial water quality monitoring programs.
Source: adapted from Environment Canada, 20102

  Photo: Skaha lake © Epp
  Skaha Lake, B.C.

Reductions in nutrient loading in Skaha Lake, B.C.

Micrograms/litre of phosphorus, chlorophyll a and milligrams/litre of dissolved oxygen, 1968 to 2008
Graph: reductions in nutrient loading in Skaha Lake, B.C. Click for graphic description (new window).
Note: left axis is phosphorus and chlorophll a; right axis is dissolved oxygen.
Source: updated from Jensen and Epp, 200226


The Okanagan River Basin drains through a chain of lakes in the southern interior of B.C., ultimately leading to the Columbia River. Since the early 1970s, controls have been introduced to reduce nutrient pollution in the region, with the most significant reductions made in agricultural and sewage treatment inputs. This has resulted in significant declines in phytoplankton (measured as chlorophyll a) and phosphorous and an increase in dissolved oxygen. Skaha Lake is one of the lakes in the Okanagan where nutrient loading has been reduced.

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Photo: algal bloom in Lake Winnipeg fouling a beach © Greg McCullough
Algal bloom in Lake Winnipeg fouling a beach

Algal blooms

Algal blooms in Lake Winnipeg

Phytoplankton biomass(mg/m3), late July to early September, 1969 to 2003
Graph: phytoplankton biomass in Lake Winnipeg. Click for graphic description (new window).
Source: adapted from Shipley and Kling, 201010

The Lake Winnipeg drainage basin is the second largest in Canada, spanning 953,000 km2 across four Canadian provinces and four U.S. states. Sixty-eight percent of the watershed is agriculture – cropland and pastureland. The watershed is also home to 6.6 million people and 20 million livestock.11 Intensification of agriculture, land clearing, wetland drainage, and rapid growth of human populations has led to an increase in nitrogen and phosphorus in the lake.11, 12 One of the most noticeable symptoms of increased nutrient loading has been the development of extensive surface algae blooms comprised largely of blue-green algae. Blooms have been as large as 10,000 km2, at times covering much of the north basin of the lake. Between 1969 and 2003, the average biomass of phytoplankton increased five-fold. A shift in species composition towards blue-green algae has been particularly pronounced since the mid-1990s.11

Algal blooms in Lake Winnipeg are a concern to recreationists and commercial fishers, as they foul beaches and cover nets. Decomposition of large algal blooms can result in low oxygen conditions, which can negatively affect fish and other aquatic life. Nevertheless, algal blooms have not resulted in a decline in the valuable Lake Winnipeg fishery, and, in fact, walleye production in Lake Winnipeg is now the highest it has ever been in the history of the commercial fishery.11

Harmful algal blooms in Quebec

Number of water bodies with harmful algal blooms, 2004 to 2009
Graph: number of water bodies with harmful algal blooms in Quebec. Click for graphic description (new window).
Source: adapted from Ministère du Développement durable, de l’Environnement et des Parcs, 200913

Harmful algal blooms appear to be increasing in lakes and reservoirs across Canada, although long-term monitoring to verify this is weak. Available trends are usually for less than 10 years and reports of increases in algal blooms are often anecdotal. In Quebec, the number of water bodies experiencing harmful algal blooms increased from 21 in 2004 to 150 in 2009.13

In Alberta, 75% of lakes and reservoirs contain harmful algal blooms at least once in the open water season.14 In Fort Smith, near the northern edge of the Boreal Plains, Aboriginal people have noticed an overabundance of algae covering river banks and clogging fishing nets.15

Great Lakes algal blooms

With the exception of shallow bays and shoreline marshes, the Great Lakes were historically cool and clear – that is, they had naturally low productivity.16 Urbanization and agricultural development have resulted in nutrient loading, particularly from sewage, phosphate detergents, and fertilizers.

In the 1920s, Lake Erie was the first of the Great Lakes to demonstrate a serious problem from nutrient loading.16 Not only is it the most vulnerable of the Great Lakes because it is the shallowest, warmest, and naturally most productive, but it was the first to have intense agricultural and urban development on its shorelines.

Photo: western Lake Erie agal bloom 25 August 2009 © NOAA, 2009By the 1960s, public alarm was raised by the appearance of filamentous algae covering beaches in green, slimy, rotting masses and people feared that Lake Erie was “dying”. Research showed that phosphorus was the main culprit, and the 1972 Great Lakes Water Quality Agreement introduced regulations that reduced point sources of phosphorus entering the lakes. Ten years later non-point sources of phosphorus were also controlled, leading to a clean-up of the lakes and one of the great success stories in international environmental cooperation.

Map: Great Lakes algal blooms. Click for graphic description (new window).
Source: adapted from Watson et al., 200824
Species composition of phytoplankton in Lake Erie
Percentage of species in samples, 2003 to 2005
Graph: species composition of phytoplankton in Lake Erie. Click for graphic description (new window).
Significant decreases in chlorophytes (green algae) and increases in cyanobacteria (blue-green algae) have occurred from 2003 to 2005. Blue-green algae cause harmful algal blooms, green algae do not.
Source: Millie et al., 200925

In the past decade, massive toxic blue-green algae, or harmful algal blooms, have reappeared in lakes Erie, Ontario, Huron, and Michigan as well as some neighbouring lakes, such as Lake Champlain. The causes of recent algal blooms are more complex than in earlier times and the effects are more detrimental. Phosphorous inputs appear to be increasing again, particularly from agricultural watersheds in Ohio,17 and an increasing proportion of the phosphorus is in a form that is biologically available to fuel near-shore algal blooms.18 Invasive quagga mussels have compounded the problem due to their capacity to selectively remove edible algae, leaving behind the toxic blue-green algae, Microcystis.19-21 Blooms of Microcystis are of particular concern for two reasons: 1) they are a poor food source for zooplankton that are, in turn, important food for fish larvae; and 2) they can contain a toxin that, when ingested by animals, including humans, may cause liver damage.22

Harmful algal blooms in the oceans

In marine systems, blooms of toxic phytoplankton are referred to as either red tides or harmful algal blooms. They can cause severe health effects in humans and they are also responsible for extensive mortality of fish and shellfish. They have been implicated in episodic mortalities of marine mammals, seabirds, and other animals dependent on the marine food web. Since the 1970s, harmful algal blooms have occurred more frequently, increased in size, and expanded their global distribution.5

Photo: toxic algal bloom off the west coast of Vancouver Island and Washington © NASA 2009. Click for graphic description (new window).
Note: toxic algal bloom off the west coast of Vancouver Island and Washington State. Left is the natural colour; right has been enhanced to reveal chlorophyll concentrations.
Source: NASA, Earth Observatory, 200929

The Bay of Fundy has a long history of algal blooms. Extended periods of low wind, fog, and warmer water conditions in the summer are conducive to algal blooms, which can discolour the water, form red tides, and result in shellfish toxicities harmful to the health of animal and human consumers.27

Harmful algal blooms have appeared in recent years on the west coast of North America, including the west coast of Vancouver Island. These algal blooms may be associated with declines in dissolved oxygen observed over the past 25 years. Massive fish kills, associated with these algal blooms, have been observed off the Washington and Oregon coasts but not off the west coast of Canada.28

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