Low oxygen in Lake Merritt, Oakland

Lake Merritt is a brackish tidal lagoon in central Oakland, 8 to 10 feet deep.¹⁰ The Lake Merritt Institute gives it 140 acres and the Water Board 150.^6,10 A 4,650-acre urban watershed drains into it through more than 60 storm drains, and Bay water reaches it through the Lake Merritt Channel.^5,9,10 Oakland's Measure DD bond, approved in 2002, paid in part for opening that channel.¹⁴ In July 2013 a dam and a twelve-lane road at 12th Street were replaced with bridges, a boulevard and a park, which SPUR reported raised the lake's tidal flow by 50 percent.^12,14 A second Measure DD project, to replace the culverts under 10th Street with a bridge, was under way in 2015.^13,14 Tide gates at the county's pump station near the channel mouth still control how much Bay water comes in, and are closed for flood control and other needs.^8,9

In August 2022 a bloom of the alga Heterosigma akashiwo spread from the Bay into the lake.^3,6 Oxygen fell to near zero across most of it for several days,⁶ and fish died in large numbers: the City says tens of thousands,³ and a later Water Board report puts the toll at up to 10,000 organisms.⁸ The lake is listed as impaired for low dissolved oxygen, and the Water Board is now writing a restoration plan for it.⁶

Who needs an answer

• The City of Oakland, which has to choose which measures to install and where. It ran a pilot of aeration and oxygenation devices from 2023 and replaced the one that did not work with a diffused aeration system in January 2026.^2,11 It has $959,757 in federal money for a Healthy Lake Initiative running to 2029.^4,11
• The San Francisco Bay Regional Water Quality Control Board, whose Advance Restoration Plan will recommend actions for a City lake management plan.⁶
• The Alameda County Flood Control and Water Conservation District, which operates the tide gates⁹ and has to weigh flood protection against flushing.
• Rowers and sailors, and the volunteers of the Lake Merritt Institute and the Rotary Nature Center Friends, who reported the two small die-offs of June 2025.^6,8

The domains it chains

Tides and gate operation in the channel.⁹ Rain and storm drains, which can lay fresh water over the salt.^5,7 Nutrients, more than 99% of which the Water Board estimates come in from the Bay, and the algae they feed.⁹ Oxygen at the surface and on the bottom, which differ most in the narrow Glen Echo arm, while both arms fall short of oxygen more often than the center.⁷ Shrimp, gobies and larger fish, and the people who row, sail and walk around the lake every day.

What a twin should let people try

• Gates: how long a closure can last in summer or winter before the bottom water of the arms runs out of oxygen. The first June 2025 die-off came two days after the gates reopened from an eleven-day closure, the second after a shorter closure of about a day. The Water Board could not tie them to gate operation and plans a fuller analysis of past closures.⁸
• Flushing against feeding: more Bay water mixes and oxygenates the lake but also brings in the nitrogen that limits algae there.⁹ What does more or less exchange do to both?
• Aeration: where fountains or diffusers would do the most, given that the oxygenation device in the Glen Echo arm did not perform as expected.²
• The next Bay bloom: how fast a red tide like 2022's would empty the lake of oxygen, and whether early gate closure or aeration would buy time.

Where to start

The City and the Lake Merritt Institute have run three continuous monitoring stations since 2023, measuring oxygen, temperature and salinity at surface and bottom.⁷ Their readings are published through the City's LakeTech dashboard.⁶ The Water Board ran two more from June to October 2024 and posts its nutrient data as a spreadsheet beside its reports.^6,7,9 Dead fish have been logged on iNaturalist, where the California Department of Fish and Wildlife set up an SF Bay Harmful Algae Bloom 2023 project for such reports.³ A coastal flow and water-quality model such as Delft3D could take the channel, the gates and the lake together.