Residents shocked to find aquatic fern completely covering Tilden Regional Park's Lake Anza

People hoping to cool off or enjoy the scenery at Lake Anza at Tilden Regional Park were shocked to find it blanketed in some sort 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 little surprising."

According to the East Bay Regional Park District, these are small aquatic ferns called "Azolla" or "mosquito fern." Azolla grows extremely fast, and in a matter of weeks completely covered the 10-acre lake.

"It is not harmful. It is not hazardous. But it is a nuisance," said Becky Tuden, the ecological services manager at EBRPD.

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

"What we’re seeing with climate change, where we had a relatively minor winter, the water isn’t moving so it just gets to grow and grow and grow," said Tuden.

It’s unknown how Azolla got into the lake. A bird could have dropped it, or someone could have emptied their fish tank into the lank, since Azolla is sometimes used for tank drainage.

SEE ALSO: Berkeley famous falcon finds new mate one day after death of longtime paramour

Tuden said they will continue to monitor the lake, the oxygen levels, and water temperature. The Azolla could die during a cold snap, or workers could rake it out. But for now, it’s staying. Tuden said it’s keeping the water cold for the fish, and the ferns are taking nutrients away from toxic algae. "At this point it is beneficial to us," said Tuden. "It’s actually doing us a favor. It’s out-competing those harmful algae blooms."