Lake Merritt Algal Bloom – In May 2022, Oakland Public Works (OPW) posted warning signs along Lake Merritt near the 1200 to 1400 blocks of Lakeshore Avenue, after testing determined low levels of contaminants associated with harmful algal bloom. These caution signs alert the public that harmful algae may be present in the water and to stay away -- and keep children and pets away -- from the algae, scum, and water.
On Aug. 4, OPW received a resident’s report of brownish red water in the same area. The reddish color of the material in the water indicates it could be a Cyanobacteria bloom (also known as a harmful algal bloom, which can produce toxins that are harmful to humans and animals). OPW staff are conducting additional inspection of the area and will submit a report to the San Francisco Bay Regional Water Quality Control Board. The City anticipates ongoing testing and will continue coordinating to provide appropriate notification and warnings to the public.
What causes harmful algal blooms at the Lake: Generally, 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.
How you can help: We can all play a part in keeping the Lake safer for everyone. For example, property owners can avoid over-irrigation of landscapes (such as overwatering lawns) to prevent excess nutrients from entering waterways. And we all need to continue working to keep Lake Merritt clean – littering, especially food and food containers, may contribute nutrients to the Lake that feed these harmful algal blooms.
It is also important to note that algae is a normal and regularly occurring organism in Lake Merritt. Most summers the City employs an algae skimmer to harvest the larger concentrations of algae from the lake.
State of California information about harmful algae blooms, including the signage OPW has posted, is available online at the State's My Water Quality web page.