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.
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