A rusty red color in Lake Merritt that left lake stewards scrambling to sample the water on Mar. 7 has tested positive for the same algae that caused the devastating harmful algal bloom in 2022.
On Friday, lake stewards sent water samples to labs run by the California Department of Public Health and San Francisco Bay Regional Water Control Board. Unofficial field testing initially detected no harmful algae. However, lab testing confirmed over the weekend the presence of Heterosigma akashiwo, a type of algae often associated with harmful blooms.
Harmful algal blooms, or HABs, occur when certain types of algae grow rapidly and release toxins, lower oxygen levels, and cause other changes in water quality that can kill fish and other marine creatures.
While the water samples collected on Friday show trace amounts of Heterosigma akashiwo in the lake, officials say the algae isn’t present at a level as severe as those associated with past HAB events.
Typically, HAB events occur during summer months, when the weather is warmer. Algal blooms usually prefer warmer water temperatures. However, Eli Kersh, president of Lake Tech, one of the organizations contracted to track the water quality at Lake Merritt, said that recent weather patterns and warmer temperatures associated with climate change could be linked to this event.
“I’d say that what we’ve seen in the last few years with the changes of weather patterns, going from drought to wet years, we’ve seen changes in the normal seasonality that we’ve become used to,” said Kersh. “With milder winters and seasons starting earlier and going later — like the growing season — if you think about our gardens, lakes are no different. They follow the same seasonal patterns of plant and algae growth. We see it in other landscapes as well: gardens growing sooner in the year, flowers seeming to bloom a little earlier. The lake seems to be following those same trends. ”
City of Oakland officials have placed precautionary signs around the lake which will remain until there are no longer any visual signs of the bloom and water quality monitoring partners have deemed the levels low enough.
“OPW is recommending that visitors to Lake Merritt exercise caution by avoiding direct contact with algae or scum in the water and by keeping pets out of the Lake and away from the water’s edge,” said Sean Maher, a spokesperson for the city.
While the algae is relatively harmless to most people, it can cause respiratory problems in sensitive groups and irritation of the eyes and skin.
Community members can keep tabs on the lake’s water quality through the State Water Resources Control Board’s HABs report map which provides updates on water quality issues across the state, including the current event at Lake Merritt.
