Cyanobacteria - Frequently Asked Questions

A eutrophic waterbody has naturally or artificially elevated nutrient levels, especially nitrogen and phosphorus. Elevated levels of phosphorus can lead to a proliferation, or ‘bloom’ of  cyanobacteria (blue-green algae) in a waterbody. Such blooms can create ecosystem dynamics in which the oxygen concentration in the lake is too low to sustain fish and other aquatic organisms.

Cyanobacteria blooms are commonly seen in urban lakes on Vancouver Island and across the globe. In the case of Quamichan Lake, human activity since the turn of the last century has elevated nutrient levels in Quamichan Lake such that it is now hyper eutrophic. The earliest reports of cyanobacteria blooms in Quamichan Lake date back to the 1930s.

What is blue-green algae?

Cyanobacteria is a photosynthetic bacteria. They use energy from the sun to make their own food. Like other photosynthetic organisms, they also produce oxygen. Unfortunately, this oxygen production is more than offset by the digestion of cyanobacteria by other bacteria living in the lake. The oxygen consumed by these other bacteria usually removes all the oxygen from water deeper than 3 meters during most summers. Cyanobacteria blooms cover the entire lake surfaces and in some cases produce toxins. Of particular concern in Quamichan lake is the microcystin, a toxin that is harmful to people, pets, fish, and other wildlife.

Why is cyanobacteria growing in Quamichan Lake?

Cyanobacteria has the capacity to grow quickly and outcompete other aquatic algae and plants for sunlight and nutrients. The high concentrations of phosphorus in Quamichan Lake provides an ideal nutrient balance for cyanobacteria. The phosphorus fueling cyanobacteria blooms in Quamichan Lake is present in the nutrient-rich sediment at the bottom of the lake and in the run-off that enters the lake.

Are there methods for controlling cyanobacteria growth?

There are many different methods that can be used to reduce nutrient concentrations, and increase dissolved oxygen in the water. All of these methods come with varying degrees of involvement and cost. Because of this, it is necessary to collect baseline data to ensure that the remediation actions are appropriate for a given waterbody. Currently, North Cowichan has worked with a local consultant and BCIT's Ecological Restoration Program to determine the best combination of remediation options to improve lake health and ultimately eliminate the presence of cyanobacteria. This baseline survey work is being used to develop a research, monitoring and management plan to mitigate persistent cyanobacteria blooms.

Who is responsible for improving conditions in the lake?

The bottom of Quamichan Lake is owned by a private landowner. Water resources in BC are currently under the mandate of the Ministry of Forests, Lands and Natural Resource Operations (FLNRO). As such, North Cowichan has no jurisdiction on the lake. Despite this, the municipality understands the importance of this lake to residents and First Nations. People use Quamichan Lake for various forms of recreation in addition to it being an important resource for local wildlife. Council has therefore directed staff to research options to improve the health of the Lake with the intent of attracting support from higher levels of government.

Is this a problem in other nearby lakes?

According to Island Health, cyanobacteria is present in many urban lakes on Vancouver Island and the Gulf Islands. The abundance and species mix of cyanobacteria in a lake can vary dramatically depending on the environmental conditions present in and around a lake. Prominent examples of cyanobacteria outbreaks in our area are St. Mary Lake on Salt Spring Island and Elk Lake in Victoria.

Is North Cowichan testing the lake water for toxins and cyanobacteria?

Yes, North Cowichan is conducting such testing in the lake. However, the presence or absence of toxins can vary at any given location or time so the testing methods available are not a reliable means of determining whether or the water is harmful at a particular location or time. Furthermore, weather, boat traffic, and other environmental factors can move algae and toxins around the lake unpredictably. For this reason North Cowichan has been advised by Island Health to place permanent signage at access points around the Quamichan Lake in order to advise the public of the ongoing potential for exposure to toxins. 

Is it safe to recreate on the lake?

Island Health is responsible for advising the public as to whether or not a particular waterbody is safe for use by the public. Those who are recreating on or around Quamichan Lake should be aware of Island Health’s advisories relating to cyanobacteria. 

I have seen several dead fish in the water. Why?

Fish Kills in our area are fairly common in shallow low elevation lakes and can usually be tied to either high temperature or low oxygen. Either of these two factors can lead to fish mortality and, because they often occur at the same time of the year, they can magnify each other’s negative effects. Neither of these pose an increased risk to human health when safely recreating on the lake. In most summers fish in the lake are confronted with zero oxygen (‘anoxia’) in the deeper lake water as a result of dying algae sinking and being digested by bacteria. Although the surface remains oxygenated, due to atmospheric mixing and oxygen production by plants and algae, the surface layer can become too hot for fish. In the case of trout, many will die once the temperature goes over 21ºC for a few days. Fish are thus killed by the temperature or low oxygen if they cannot find refuge in surrounding shaded backwaters, springs or creeks.  North Cowichan is currently monitoring temperature and oxygen changes in the lake with temperature and oxygen sensors. North Cowichan is also sampling for nutrient concentrations. Both programs will help our community to determine the best ways to improve lake water quality in the future.

Oxygen and temperature sensor being pulled onto a boat. There are some hands working on the sensor.

Where can I report observations or information about Quamichan Lake?

You can report directly to the environmental department at