Phosphorus in aquatic ecosystems: chapter 1

Concerns and Issues

For decades, phosphorus, a crucial nutrient for growth of aquatic plants and algae, has been considered a key driver of the overall productivity of freshwater ecosystems. Other nutrients, such as nitrogen in the form of nitrates and ammonia can intensify this fertilizing effect, especially in nutrient poor rivers in remote areas as well as estuaries or coastal areas.

Today, the delivery of phosphorus to Canada’s surface waters occurs via natural and human inputs (Nutrients and Their Impact on the Canadian Environment). Human inputs to Canada’s waterways include:

• runoff from land cleared for agriculture, especially where fertilizers and manure have been applied in quantities that exceed nutritional requirements of crops
• runoff from forestry and urban expansion
• industrial emissions to soils and water (e.g. pulp and paper and mining)
• municipal and household wastewater discharge, including septic systems
• wind blown dust from bare soils

In the absence of human development, phosphorus exists only in phosphate-bearing rock and is introduced into water through soil and rock erosion. Consequently, the natural level of phosphorus in water is influenced by the amounts and types of rock and soil in the area. Water bodies in regions with a lot of soil, such as the Prairies, naturally have high phosphorus levels compared to water bodies in areas with little soil, such as the Canadian Shield.

Excess phosphorus can result in abundant growth of aquatic plants. This can lead to a shift in the assemblages of fish and invertebrates toward less desirable species, including pollution tolerant ones which may include invasive species.

Potentially toxic cyanobacteria (Threats to Sources of Drinking Water and Aquatic Ecosystem Health in Canada) (also known as blue green algae) can form blooms under certain conditions, such as high nutrient loadings and warm temperatures, and cause unpleasant taste and odour problems in drinking water. Some of these bacteria can release toxins in the water which can pose health risks to humans and animals (Blue-Green Algae (Cyanobacteria) and their Toxins).

Decaying and unsightly algal and aquatic plant growths can also clog intake pipes and impair navigation reducing the aesthetic and recreational value of aquatic ecosystems. Additionally, fish kills can occur as a result of concurrent declines in dissolved oxygen.

Conceptual diagram comparing a balanced ecosystem with one receiving excess nutrients

Source: Adapted from Bricker et al., 2007

Did you know?
Phosphorus enrichment of surface waters continues to be a national issue in Canada. Between 2005 and 2007, 32% of all water quality monitoring stations (108 out of 336 stations in total) in Canada exceeded water quality objectives for phosphorus more than half of the time it was measured (Freshwater quality in Canadian Rivers).