Berkeley parks

Residents are shocked to find a water fern completely covering Lake Anza in Tilden Regional Park

People hoping to cool off or enjoy the scenery of Lake Anza in Tilden Regional Park were shocked to find it covered with some kind of green and brown plant.

“It’s literally green. I thought it was a football field,” said Berkeley resident Ahmed Hamida.

“Just a little disappointing,” said Berkeley resident Anna Victoria. “I just wanted a day to swim. It’s a bit surprising.”

According to the East Bay Regional Park District, these are small water ferns called “Azolla” or “mosquito fern.” Azola grows extremely fast and within weeks has completely covered the 10 acre lake.

“It’s not harmful. It’s not dangerous. But it’s a nuisance,” said Becky Tuden, environmental services manager at EBRPD.

Tuden said the Azolla is a native plant and they have seen it in Lake Anza before, but usually in small patches.

“What we’re seeing with climate change, where we’ve had a relatively minor winter, the water isn’t moving, so it’s just growing and growing and growing,” Tuden said.

It is not known how Azolla entered the lake. A bird might have dropped it, or someone might have emptied their aquarium into the lake, as the Azolla is sometimes used for aquarium drainage.

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Tuden said they will continue to monitor the lake, oxygen levels and water temperature. The Azolla could die during a cold spell, or workers could rake it. But for now, it remains. Tuden said it kept the water cold for fish and the ferns stripped nutrients from toxic algae. “At this point, it benefits us,” Tuden said. “It’s actually doing us a favor. It’s overriding those harmful algal blooms.”