Lake Merritt Harmful Algae Bloom Updates and Management

Check this page for information about harmful algae blooms in Oakland waterways.

Posted: August 4th, 2023 4:03 PM

Last Updated: March 20th, 2024 11:06 AM

Latest Updates

For March 2024 updates on Lake Merritt water quality and safety guidance, visit this page.

On August 21, 2023, the City of Oakland issued this press release updating status of the Lake Merritt Water Quality Management Pilot Project.

On August 18, 2023, the San Francisco Bay Regional Water Quality Control Board reported on that the red tide began to dissipate in early August and that “It has been over a week since we received a new report of discolored water and samples confirm there is a low presence of the species responsible for the red tide, Heterosigma akashiwo, in Bay waters. However, there are several months of warm weather ahead, and it is possible another bloom may form. We will continue to work with our partners to monitor the Bay. Discolored water and other signs of potential harmful algae blooms should be reported to the Water Boards by filling out the online HAB Report form or by contacting the HAB Hotline: Email:; Phone: 1-844-729-6466 (toll free). Observations of dead fish should also be reported to the SF Bay Harmful Algae Bloom 2023 project on iNaturalist set up by California Department of Fish and Wildlife for tracking and response.”

On August 15, 2023, the City of Oakland installed a new aeration fountain in Lake Merritt. The new fountain is located near the Pergola by El Embarcadero Avenue and replaces an out-of-service fountain at that location. By mixing air into the water, the fountain will improve dissolved oxygen (DO) levels in the Lake, helping to maintain an oxygenated refuge for aquatic life near the fountain. The design and spray settings on the new fountain maximize benefit to wildlife.

On August 1, 2023 the City of Oakland was notified that the same harmful algal bloom organism that caused red tides and fish kills throughout San Francisco Bay, including Lake Merritt last August and September, has been detected again. The Water Board posted this advisory. The August-September 2022 Lake Merritt fish kill is largely attributed to low dissolved oxygen brought on by the red tide. The City of Oakland is working to fast track the Lake Merritt Water Quality Management Pilot Project which will install an oxygenation device and an aeration device in the Lake to improve dissolved oxygen levels in an effort to reduce the risk of another fish kill. The City of Oakland is working quickly to order and install equipment and obtain necessary permits from regulatory agencies. The City will install the devices as soon as possible.

Aeration Fountain installed Aug 15, 2023 at Lake Merritt
Aeration Fountain installed Aug 15, 2023 at Lake Merritt

Lake Merritt Water Quality Management Pilot Project

Lake Merritt Water Quality Monitoring Data

Background on 2022 Algal Bloom

In August of 2022 parts of San Francisco Bay suffered the effects of a harmful algal bloom (HAB), from an outbreak of Heterosigma akashiwo, an invasive marine alga associated with red tides. Parts of the Bay turned reddish-brown, and in an abundance of caution, public health warnings were issued to avoid contact with toxins that could be released by the algae. Excessive algae in some places depleted oxygen levels in the water too low for fish to breath. The drop in oxygen levels caused massive fish kills along the California coast and in Lake Merritt, where tens of thousands of dead fish washed up on the shoreline. The City of Oakland conducted a large scale cleanup, posted advisory signs around the Lake, and began planning for ways to improve future conditions at the Lake.

The City of Oakland initiated the Lake Merritt Water Quality Management Pilot Project.

Click here to see a detailed account of the 2022 Harmful Algal Bloom in Oakland

What is an algal bloom?

Algae are simple, green aquatic plants that have no roots, leaves, or flowers. Algae and other phytoplankton are important because they form the basis for many aquatic food webs. They also produce most of the oxygen present in Earth’s atmosphere. However, too many algae growing too quickly can wreak havoc on an ecosystem.

Algae make their food through photosynthesis, a process that involves capturing light energy from the sun and converting it to carbohydrates; photosynthesis requires carbon dioxide and water, and releases oxygen. When algae metabolize the carbohydrates through respiration, they get chemical energy to fuel growth, reproduction, and other life processes.

An “algal bloom” is a rapid rise of the algae population in a body of water. Algae blooms are usually caused by moderate increases in water temperature and excess nutrients—primarily phosphorus and nitrogen from crop fertilizers, animal wastes, or sewage facilities. As such, algal blooms can be unintended consequences of some of the methods we use for growing food or processing waste.

The rapid increase of algae can have a drastic effect on an aquatic ecosystem. The algae cover the surface of the water, blocking sunlight from beneficial underwater plants that provide food and shelter for fish and other animals. When the algae proliferate, they start to deplete the nutrient supply and die off. This, in turn, causes a reduction of dissolved oxygen as the dead algae decompose, a process that uses oxygen. Many fish and other aquatic organisms die when the oxygen level lowers.

Courtesy of the Center for Ecoliteracy