OAKLAND, Calif. (KRON) — The stinky smell, brown sludge, and dead floating fish are enough to keep most people from touching the water in Lake Merritt. But as a precaution, Oakland public works crews posted more signs this week to alert visitors about the lake’s toxic algae bloom, which exponentially grew over the summer.

The troubled waters along Oakland and Lake Merritt’s shorelines are part of a larger natural phenomenon, with algae blooms and fish die-offs reported across many miles to the north and south.

The city was alerted that large numbers of fish were washing up around Lake Merritt in late August. Crews removed thousands of pounds of dead aquatic wildlife in response.

(Image courtesy City of Oakland)

“At Lake Merritt, which is connected to San Francisco Bay, reports suggest as many as 10,000 fish died in late August. San Francisco Bay Regional Water Board staff conducted a field investigation at Lake Merritt, where very low dissolved oxygen levels were measured in the water,” the San Francisco Bay Regional Water Quality Control Board wrote.

Over the Labor Day weekend, city staff posted additional signs warning residents: do not touch the water, scum, or fish — live or dead.

Sludge clumps together in Lake Merritt. (Image courtesy City of Oakland)

According to Oakland city leaders, algae blooms are generated by a combination of factors. “Increased inputs of nutrients like nitrogen and phosphorus (from fertilizers and human or animal wastes), low water flows, stagnant water, increased intensity and duration of sunlight, and sustained high temperatures create the ideal conditions for these blooms. Current research suggests that the rising temperatures and changing precipitation patterns caused by climate change are a catalyst for their growth,” the city wrote in a recent community update.

Lucia Ross of BlueGreen Water Technologies said cities’ water management boards could do more to keep lakes like Lake Merritt cleaner through proactive treatments.

Ross explained that a bloom is made up of cyanobacteria. “Cyanobacteria have been around for millions of years and their ideal temperature is warm, still water. They breed at a phenomenal rate. A colony can double within five-seven hours. With heat, they photosynthesize on the top surface of the water column.”

When left unchecked during extended periods of warm weather, cyanobacteria can take over a lake, dominating the ecosystem and killing off many animal and plant species.

(Image courtesy City of Oakland)

To eradicate a toxic bloom effectively, “You want to treat the water early. Be proactive. Some cities choose to do nothing, they wait it out. Next summer, if left unchecked, it will be back, and even worse than before,” Ross said.

Ross said severe algae blooms are hazardous even for people walking near them. “When the cells burst, you can breathe them in. Those toxins are being released into the air,” Ross said. Negative health impacts can include vomiting and extreme gastric distress.

Ross said lakes should be regularly tested for water quality and treated before blooms even become visible.

The city recently received results from Lake Merritt water monitoring tests that showed “low levels of contaminants associated with harmful algae blooms.” Oakland Public Works said it will continue monitoring Lake Merritt’s shoreline for changes as tidal shifts and other factors might produce evolving conditions.