What causes an algae bloom?

Algal blooms in BC most often occur in the late-summer and early-fall. However, they can occur at any time of year, even under ice.

Blooms typically last from several days to a few weeks, but can sometimes last as long as several months if environmental conditions are favourable.

Phosphorus and nitrogen are essential to algae production, and these nutrients encourage the growth of algae in waterbodies. Phosphorus is the key nutrient limiting algal growth in the majority of BC lakes. Increased concentrations of nutrients often lead to increased algal production, and increased risk of a bloom forming. This process of nutrient enrichment in a waterbody is called eutrophication, which can be exacerbated by anthropogenic influences such as:

• shoreline development
• certain farming practices
• wastewater and industry effluent
• or urban and stormwater runoff

Note that algae blooms can also occur in waterbodies where nutrients are fairly low, but are more common when nutrient concentrations, especially phosphorus, are high.

Blooms usually develop during the warmer months of the year when water temperatures are higher. Cyanobacteria prefer warmer water and have a competitive advantage over other algae at temperatures over 25°C.

In the spring when surface water begins to warm, many deeper lakes begin to thermally stratify (warm surface layer and colder bottom layer). As the warming progresses throughout the summer this temperature difference creates a density stratification in the water column. This creates a beneficial situation for algae and cyanobacteria to grow in a warm environment with increased light near the surface.

Sufficient light is required for algae to grow and form high enough densities to bloom. The availability of light is largely influenced by the light penetration and mixing dynamics within the water body.

• Euphotic Depth - The depth of water where enough light penetrates for algal growth. Defined as the depth where only 1% of surface light remains.
• Mixing Depth - The depth to which algae are mixed in a waterbody. This is either the depth of the waterbody when fully mixed, or is the depth of the thermocline (area of steep drop in temperature with depth) when a waterbody is thermally stratified.
• The ratio of the euphotic depth and mixing depth determines how much light is available for algae growth in a given waterbody. The greater the ratio, the more light is available for photosynthesis and algal growth.

Note that even in low light conditions, or in turbid water, cyanobacteria have higher growth rates than other algae. Many cyanobacteria can also regulate their buoyancy, enabling them to move to more favorable positions higher in the water column. These abilities to adapt to variable light conditions gives cyanobacteria a competitive advantage over other algal species.

Generally, calmer waters and low-wind conditions favour algal bloom development.

Stable surface water conditions and less water column mixing promotes thermal stratification in waterbodies. Cyanobacteria prefer these stable water condition with minimal turbulence.