One of the world's largest lakes is experiencing worsening toxic algal blooms from agricultural runoff. The blooms are now visible from space. Scientists study it as a preview of what climate change and nutrient loading will do to freshwater globally.
European Space Agency, CC BY-SA 3.0 igo
The Emerald Sickness
From orbit, Lake Winnipeg can appear as a colossal emerald and turquoise swirl painted across central North America. This image is not of mineral deposits, but of a massive, recurring bloom of photosynthetic bacteria. Lake Winnipeg, the world's 10th largest freshwater lake by surface area, is an example of eutrophication—the over-enrichment of water by nutrients. The problem has escalated dramatically since the 1990s, leading to the Global Nature Fund declaring it the "Threatened Lake of the Year" in 2013.
The blooms are composed of cyanobacteria, often called blue-green algae, which are single-celled organisms that thrive in nutrient-rich water. Satellite imagery shows these blooms can cover vast areas of the lake's 24,514 square-kilometer surface. In September 2023, blooms covered an estimated 8,400 square kilometers, about a third of the lake. In some years, the blooms have covered more than half the lake, with the 2006 bloom blanketing nearly the entire surface. The dominant cyanobacteria genera include Aphanizomenon, Dolichospermum, and Microcystis.
A Watershed-Sized Problem
The primary cause of the blooms is an excess of phosphorus. Between 1990 and 2000, phosphorus concentrations in the lake nearly doubled. This nutrient is a powerful fertilizer for the cyanobacteria. The lake's watershed is immense, covering nearly one million square kilometers and spanning four Canadian provinces and four U.S. states. This vast drainage area, where nearly seven million people, is dominated by agriculture.
Runoff from fertilized fields and livestock operations carries phosphorus into rivers that feed the lake. The Red River is the largest contributor, delivering an average of almost 5,000 tonnes of phosphorus per year. The Winnipeg River contributes over 1,000 tonnes annually. Municipal wastewater also adds to the nutrient load. The lake's shallow nature, with a mean depth of only 12 meters, makes it particularly susceptible to the effects of these inputs.
The consequences of these blooms are severe. Some cyanobacteria species produce potent toxins. Studies have detected microcystins, which can cause liver damage, and BMAA, a neurotoxin linked to degenerative diseases, in Lake Winnipeg's blooms. When the massive blooms die and decompose, the process consumes vast amounts of oxygen, creating hypoxic "dead zones" where fish cannot survive. This threatens the lake's commercial fishing industry, valued at $25 million annually, and a tourism industry worth $100 million a year.
💡Fun Facts
Lake Winnipeg's watershed is enormous—at nearly 1 million square kilometers, it is about 40 times larger than the lake surface area.
The name "Winnipeg" is derived from the Cree word "winipek," meaning "muddy waters," an ironically appropriate name given the current algae situation.
Algal blooms are not a new phenomenon in the lake, with records of them dating back to the 1930s, but their frequency, size, and toxicity have increased dramatically since the 1990s.
The lake is a remnant of the prehistoric glacial Lake Agassiz, which was once the largest lake in North America.
The lake is a natural, public area accessible 24/7. Algal blooms are most common in the late summer months of August and September. Health advisories often warn against swimming or contact with water at public beaches during severe blooms.
Admission
Free to access public beaches and parks.
Accessibility
Access points vary. Many public areas like Grand Beach have sandy, flat terrain. Other parts of the shoreline can be rocky and rugged. During blooms, the water is unsafe for contact.