Blue-green algae in Sloan's Lake kills hundreds to thousands of fish, poses health risk

Team members from Denver Parks and Recreation cleaned up around 400 black crappie carcasses from Sloan's Lake on Monday.

Sloan’s Lake Park Foundation, nonprofit organization committed to restoring the popular Denver lake, estimated that around 2,000 fish have died over the past week and that 400 are still floating in it, according to Staff Member Kurt Weaver.

“The cause of death of the fish is likely a combination of factors including, but not limited to, the heat we have experienced this summer as well as potential impacts of algae and other environment processes happening in the lake,” said Courtney Ronner, a spokesperson for Denver Department of Public Health and Environment (DDPHE).

What is blue-green algae?

Blue-green algae is type of bacteria called cyanobacteria, or cyanoHAB, that are found naturally in water, according to Colorado Department of Public Health and Environment (CDPHE).

Algae blooms have distinct features, and CDPHE advised people to look out for areas of the water that:

• Are discolored, typically green, turquoise, red, or gold

• Resemble thick pea soup or spilled paint on the water’s surface

• Have foam, scum, or algae mats

• Are accompanied by dead fish or other animals washed up on shore

Although these organisms naturally occur in Colorado waters, they become a problem when they multiply rapidly and bloom — usually as a result from hot temperatures—according to an article from CDPHE.

What makes blue-green algae dangerous?

Certain species of cyanobacteria can produce toxins that become very harmful to people, animals, and the environment.

“We don't know why some blooms start producing toxins while others don't. The trigger that initiates toxin production is still a mystery and an area of ongoing research,” John Michael, a spokesperson for CDPHE, told The Denver Gazette via email.

When blue-green algae blooms in a body of water, it reduces the amount of oxygen within the water, which can eventually kill fish living there, according to Joey Livingston, a spokesperson for Colorado Parks and Wildlife (CPW).

The blue-green algae can affect humans and animals as well. Those who come in contact with the algae are susceptible to liver, skin, or neurological damages, and symptoms of exposure may include skin irritation, gastrointestinal upset, fever, headache, sore throat, muscle and joint pain, and liver damage, according to an article from CDPHE.

“Pets can get very sick, and even die, after swallowing these toxins. If pets have been in the water, immediately wash them off with clean water to keep them from licking cyanobacteria off their fur,” Ronner told The Denver Gazette in an email.

Extra caution around blue-green algae should be taken because there is no antidote for exposure to toxic cyanobacteria. Treatments can alleviate symptoms, but there is no cure, according to VCA Animal Hospitals.

Some organizations, such as CDPHE, recommend staying out of the water entirely and avoiding any activities that could lead to ingesting the blue-green algae, such as boating.

On the other hand, some organizations are less concerned about contact with blue-green algae: “As long as people are not drinking the water and are cleaning fish well if they’re eating them (i.e. removing the organs) it is safe," said Ronner. “DDPHE recommends usual precautions of not exposing skin with cuts or injuries that would make a person more vulnerable to infections, and washing after exposure to lake water.”

Recreational water activities are still allowed on Sloan Lake, but DPR encouraged both people and pets to avoid contact with the water.

What other lakes are affected?

CDPHE monitors 20 bodies of water in Colorado, and nine of those water bodies have announced blue-green algae advisories as of July 31, which is less than a month into the “blue-green algae season”.

In the Denver area, Cherry Creek Reservoir and Barr Lake detected toxins at high levels above the safety threshold, according to an interactive map of affected areas from CDPHE.

Zooming out, Windsor Lake in Windsor and Prospect Lake in Colorado Springs are both closed due to a large presence of blue-green algae, and North-Sterling Reservoir also reported that algae toxins were at high levels.

CPW released a notice yesterday urging the public to stay out of DeWeese Reservoir in Westcliffe, Colo., due to a blue-green algae bloom. No water recreation activities are allowed at this time.

How to help?

Nutrient pollution from human activities makes the problem worse, leading to more severe and frequent blooms, Michael said.

As seen in the numerous fish deaths, Sloan's Lake is especially affected by the blue-green algae bloom, and the lake itself has a history of pollution from human activities, according to Weaver with the Sloan’s Lake Park Foundation.

“We hate that it took an event like all these fish dying to make people realize that there’s a problem at Sloan’s Lake,” added Weaver.

Weaver works with the foundation to help educate the public on the current conditions of Sloan's Lake and to work with government officials to help revitalize the lake.

The public can also help reduce the occurrence of blue-green algae blooms by preventing nutrients (nitrogen and phosphorus) from entering waterways through responsible use of lawn fertilizers, picking up pet waste, and avoiding using deicers that contain urea, according to a news release from CPW.