Warnings placed at Arroyo Pond due to bacteria bloom

People and pets are being asked to use caution when at the Arroyo Pond on the Rancho Santa Fe Association-owned Arroyo Preserve. Signs have been posted to stay away from the water due to the detection of cyanobacteria, also known as blue-green algae.

According to Caitlin Kreutz, RSF Association environmental resource coordinator, the bacteria is common and often found in shallow, warm and stagnant water—the pond experienced the same bacteria bloom last summer.

According to the Center for Disease Control, some cyanobacterial blooms can make people and animals sick. Humans can be exposed by ingestion, skin or eye contact. Animals are often the first to be affected because they are more likely than humans to swim in or drink contaminated water, even if it looks or smells bad.

The pond is fed by the San Dieguito River, an Army Corps of Engineers-regulated “blue line stream”. With its blue line stream status, periodic water quality testing is done by state agencies such as the California Regional Water Quality Control Board and the California Department of Fish and Wildlife.

With the San Dieguito River at base flow, the pond gets drier and warm which causes the algae bloom. Fertilizer-run off from the golf course above raises the nitrate level of the pond really high, Kreutz said, and those nitrates fertilize the cyanobacteria algae and suck the oxygen out of the water.

The Water Quality Board identified cyanobacteria in the sample of water taken from the Arroyo Pond, however, the cyanotoxins were all just at the detection limit.

“Due to the recent fish and aquatic life die off, we decided to post signs and keep people away from it just to be safe,” Kreutz said. “This includes fishing, dogs swimming, basically any interaction with the pond.”

The pond will be off-limits until bio-remediation can be done, which might take a couple of months.

Kreutz said they can’t just use a traditional algaecide to kill it, because it sometimes makes the water toxicity worse. The RSF Association is considering other common bio-remediation strategies such as aeration with nano-bubblers.

“The hope is that with remediation efforts, it won’t be a problem next year,” Kreutz said. “It will make the water cleaner and the wildlife will come back.”