Children wade into a slightly scummy Lake Anza. An adult man wearing a hat watches the three children.
Children play in a reopened Lake Anza on June 4, 2023. Credit: Sylvie Carr

Swimmers are allowed at Tilden’s Lake Anza this summer for the first time in four years. 

The popular watering hole at the top of the Berkeley Hills reopened in the spring, and officials expect the lake to remain open until mid-September, when it typically closes for the winter.

Lake Anza hadn’t been officially open with lifeguards present since summer 2019. 

The lake has struggled with intermittent harmful algae blooms since at least 2015, and  swimming was halted in 2020 because of one such spike in toxic cyanobacteria. 

Access improvement projects, such as repaving the parking lot and adding an ADA accessible path, kept the lake closed even after October 2021 storms flushed out the cyanobacteria from the lake. 

But heavy rainfall caused other problems, as Wildcat Creek swelled, sweeping in large amounts of sediment and creating hazardous conditions at the shore where a tributary of the creek enters the lake at the south end of the beach. Matthew Graul, chief of stewardship at the East Bay Regional Park District, described those conditions as “almost quicksand.” He said repair work needed to prevent the spot from becoming a “place where people could start sinking into the sand” contributed to the lake’s continued closure.

Then, in early 2022, the lake saw an explosive growth of the squishy, green-and-brown Azolla fern, which came to blanket much of its surface. Though non-toxic, the floating mat of vegetation was too dense to swim through. 

The fern, which actually had a beneficial effect on the cyanobacteria algae blooms, receded over this past winter, possibly washed away by heavy rains, though it has started to expand again over the past few weeks. 

For now, it’s time to enjoy the water. The park district opened the lake back up for weekend swimming on April 29 and weekday swimming on May 29, with a lifeguard present 11 a.m. to 6 p.m. (Weekday swimming ends Sept. 8; weekend swimming, Sept. 17 .)

Park-goers like Sylvie Carr, a parent of two young children who moved to the Berkeley Hills in 2021, have been ecstatic about the lake’s reopening, though Carr noted that personally, the lake’s water still doesn’t look “totally appealing” to dive into because the edges of the lake still appear slightly scummy. Her 3-year-old, June, however, has no such trepidation and loves to “slosh around in the beach water.”

“To have a natural place to swim and to be outside in the summertime is just amazing, and I feel so lucky,” Carr said. “We love the public pools around here, but there’s nothing like actually getting to be at a little lake. You see the weird sticks in the water and you look for fish, and it’s really special.” 

But what will happen in future years is hard to predict. Graul said park district staff are studying conditions and seeking to create a “delicate balance” between the fern, the algae and the swimming public.

Is the lake still toxic?

Scummy patches of green on the edges of Lake Anza as pictured in September 2015, when there was a bloom of toxic cyanobacteria.
Toxic algae can be highly visible, or not visible at all. In the photo above, algae can be seen at Lake Anza on Sept. 17, 2015. Credit: Sunshine Townsend/East Bay Regional Park District

Cyanobacteria, commonly called blue-green algae, is present in ocean and fresh water around the world. It tends to bloom when certain conditions are met: low water levels, limited circulation and warm temperatures. It’s not known what causes algae to become toxic. 

Cyanobacteria cells are starting to grow again in Lake Anza, but Graul said weekly water tests so far this year “haven’t detected toxins.” (The state recommends closing lakes with the detection of six more parts per billion of Microcystin toxin, 20 parts per billion of Anatoxin-A toxin or 4 parts per billion of the Cylindrospermopsin toxin.) 

According to Lake Anza’s latest weekly water quality status report, E. Coli bacteria levels are currently below the state’s health standards and there’s a “low health risk for water contact.”

Check the park website or signage posted at the lake for the latest water quality information. Do not go into the water if there is a water quality hazard in effect. 

When cyanobacteria produce toxins, they can affect the nervous system and liver, and exposure to toxic algae through ingestion or skin contact can cause rashes, skin and eye irritation, allergic reactions, gastrointestinal problems, and, at high doses, serious illness or death. 

Dog owners should exercise caution around the lake and keep their dogs out of the scummy parts of the lake. Several dog deaths in California have been attributed to toxic algae poisoning.

The park district’s first documented closure due to cyanotoxins was in 2014, in Oakland’s Lake Temescal. Lake Chabot was put under an advisory for toxins and reports of dog deaths in 2015. 

Harmful algae blooms are on the rise, Becky Tuden, the environmental services manager for EBRPD said during a presentation to the park’s Natural and Cultural Resources Committee in April 2022. Since those first documented incidents, all but one of the park district’s swimming spots have been closed off at one time or another due to the presence of cyanotoxins. 

Cyanobacteria can be found naturally growing in oceans and water bodies across the world. But man-made lakes like Lake Anza (which was created in 1938 when the Tilden Park dam was built) tend to become more eutrophic, or excessively filled with nutrients like phosphorus and nitrogen that feed cyanobacteria. That’s because when a stream channel flows into an open water body, the velocity of water slows down, depositing sediment — and nutrients — into the lake. 

The age of the park district’s reservoirs is likely contributing to more harmful algae blooms. “They’re coming to the end of their lifespan,” Tuden said during the meeting. “They’ve been around for around 100 years, so you’ve got a significant amount of sediment in those impoundments.”

Climate change, which drives extreme weather, makes harmful algae blooms more likely, and they’re increasing worldwide. Heavy storms dump nutrient-filled sediment on the bottom of the lake. When followed by periods of drought, the stagnant water and warm temperatures create the perfect conditions for a harmful algal bloom. 

“Climate change is exacerbating what makes it a happy place for those cyanobacteria,” Tuden said at the meeting.

Learn more about the impacts of eutrophication

Eutrophication sets off a chain reaction in the ecosystem. The excessive loads of nutrients result in algal blooms, which in large amounts deplete oxygen levels in the water and can result in mass marine die-offs like the one that impacted Lake Merritt in 2022.

To reduce the frequency and duration of harmful algal blooms, the district in July 2020 installed a hypolimnetic oxygenation system in Lake Anza to bring dissolved oxygen to the depths of the lake (important to maintain its rainbow trout population). Park officials believe the combination of the new system and shade from the Azolla fern helped reduce the nutrients that are available for the cyanobacteria to feed on.

What happened to the Azolla fern?

A lake covered with a brown and green fern resembling a carpet
Lake Anza covered in an aquatic fern, Azolla, on April 13, 2022. Credit: Supriya Yelimeli Credit: Supriya Yelimeli

Azolla, the non-toxic  green-red fern that covered Lake Anza’s surface in 2022, is referred to as the mosquito fern.

Becky Tuden, environmental services manager for EBRPD, told Berkeleyside last year that the fern’s sudden growth could be attributed to little water flow and movement at the lake due to a lack of rainfall and drought conditions, coupled with a lack of swimmers to disrupt plant growth. 

The mat of floating vegetation may have appeared unsightly, but park scientists like Tuden believe it likely contributed to reducing the presence of toxic algae by outcompeting the cyanobacteria for nutrients and cooling down the lake’s temperature. (Cyanobacteria thrive in warm, stagnant water.)

Graul said park officials don’t have a definitive answer for why the fern receded over the winter. It’s seasonal, so there’s a good chance it was washed out by the series of heavy winter rainstorms. The oxygenation system the district installed in July 2020 to reduce the frequency and duration of harmful algal blooms could have played a role. It could be both of those theories — or neither. For now, the park is continuing to observe lake conditions. 

Close-up views of the Azolla fern coating Lake Anza in Berkeley’s Tilden Regional Park in 2022.
Close-up view of the Azolla fern coating Lake Anza in Berkeley’s Tilden Regional Park in April 2022. Credit: Supriya Yelimeli

“The fern is beneficial, potentially, to reducing algal blooms,” Graul said, “but it’s also a safety risk if we had fern growing over the entire lake where people [are] swimming, so it’s something we’ll need to continue monitoring and managing as needed.” 

In June, EBRPD approved the purchase of a $250,000 aquatic harvester machine to to allow for management of excessive aquatic weed vegetation in the lakes across the district. The machine, Graul said, will allow staff to cut the fern to make room for recreation in the lake but has yet to be used in Lake Anza. 

Another potential concern, Graul said, is that the Azolla fern, while helpful while it is alive, could add more nutrients to the water once it decays and contribute to another toxic cyanobacteria bloom. It hasn’t played out that way in Lake Anza this year, but park officials are preparing for a future scenario where staff need to clear out a large fern growth before it decomposes.

This story was updated after publication with additional information.

Iris Kwok covers the environment for Berkeleyside through a partnership with Report for America. A former music journalist, her work has appeared in the San Francisco Chronicle, KQED, San Francisco Examiner...