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‘Explosive’ growth of non-toxic fern on Lake Anza closes area to swimmers

The usually expansive blue surface of Lake Anza in Tilden Regional Park has been blanketed by a squishy, non-toxic fern that resembles an algae bloom, keeping the lake closed to swimmers.

If you haven’t visited Lake Anza before, you might mistake the Azolla fern coating the water for a large, grassy field surrounding a small pond. But closer inspection reveals that the small-leaved fern (which looks unlike a typical fern) is gently bobbing right above the lake surface.

Becky Tuden, environmental services manager for the East Bay Regional Parks District, said there have been Azolla growths at the lake in the past, but nothing as “explosive” as the coverage this spring.

She said the plant began flourishing because there was little water flow and movement at the lake due to a lack of rainfall and drought conditions. The last time Lake Anza was officially open with lifeguards was in the summer of 2019, so there also haven’t been swimmers to disrupt any aquatic plant growth.

Lake Anza, Tilden Park, in 2017. Credit: Ryan Lieb

But despite its high rate of growth and invasive appearance, Tuden said the Azolla is actually helping the lake maintain a cool environment for aquatic creatures, and ridding the lake of cyanotoxins, which the lake suffered from last year.

EBRPD installed an oxygen system at the lake in 2020 in response to the cyanobacteria bloom (sometimes incorrectly called an algae bloom) that coated the lake in a gooey, blue-green substance, and took away oxygen from the fish and produced cyanotoxins.

Now, the growth of the Azolla is creating further competition for the cyanobacteria, countering cyanotoxins, keeping a cool cover for the fish underneath in warm weather and overall benefitting the lake.

“We have seen Azolla at the lake before; it usually just hung around at the corners,” Tuden said. “It’s never exploded to this level.”

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

The fern could block sunlight in normal conditions and prevent photosynthesis inside the lake, but Tuden said EBRPD is still providing oxygen to the lake. The combination of factors means the fern and the oxygen system are keeping the lake healthy, so EBRPD doesn’t have immediate plans to remove the plant and open the area for swimming.

“It’s a bit of a patience game right now, to let the oxygen and the Azolla work their magic, but we’re hoping it’ll have long-term benefits,” said Tuden, explaining that the ultimate goal is to rid the lake of cyanobacteria that produces cyanotoxins.

This would make the ecosystem healthier for the trout, otter, non-native bullfrogs, nesting birds, dragonflies, pillbugs and many other creatures that call the area home.

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A mallard goes bottoms up for lunch at Lake Anza on April 13, 2022. Credit: Supriya Yelimeli

Tuden said the lake is tested weekly for cyanotoxins, but EBPRD doesn’t have the information to know when cyanobacteria in the lake (which is harmless on its own) will produce the toxin. So they’re letting the fern and the oxygen system do its work in the meantime. Historically, Azolla has been responsible for cooling massive bodies of water, but it will take EBRPD scientists more time to determine the fern’s impact on the local environment.

There are also a number of maintenance projects ongoing at the lake right now — deferred during the pandemic — that need to be completed before the lake is officially opened.

Lake Anza was created in 1938 when the Tilden Park dam was constructed. Over time, Tuden said dammed lakes become more eutrophic, meaning they’re rich in nutrients that support both animal and plant life (including bacteria that can produce toxins).

As the ecosystem gradually changes due to climate change, shifts in water flow and naturally occurring processes, Tuden said it will be important to rid the location of cyanobacteria for the long-term health of the lake.

“Even though it looks really bad — and it’s very frustrating to not go swimming — we’re hoping it’s actually a good thing,” Tuden said.

‘Explosive’ growth of non-toxic fern on Lake Anza closes area to swimmers