Lake Merced algae bloom status

Lake Merced algae bloom history

California's Water Boards hold 22 bloom reports for Lake Merced going back to 2020. Reports came in during 5 of the 7 years since then, with the busiest year being 2022 (14 reports).

State staff recommended an advisory 15 times: 11 Danger and 4 Warning. Danger is the highest level and means people and pets were told to stay out of the water and off the shore.

Reports have fallen in the last three years compared with the three before.

That puts Lake Merced in the top 15% of the 318 California lakes on this site for the number of years with blooms.

When do algae blooms happen at Lake Merced?

At Lake Merced, reports are most often logged in January, March and October.

Unusually, about 45% of reports here fall between November and February, so blooms aren't only a summer problem at Lake Merced. Some of this reflects when the lake is monitored, not just when blooms happen.

Blooms can appear, move with the wind and fade within hours or days, so the time of year is only a guide.

Recent Lake Merced algae reports

Recent reports came from 10 different locations around Lake Merced, including South end of lake near Daly City, by meadow on east side and Lake Merced East "E". A bloom in one cove or beach doesn't mean the whole lake is affected, and conditions can differ a short distance away.

What the California results mean

California recommends one of three advisory levels when a bloom is checked, matched to the signs posted at the shore.

Caution

Toxins may be present. Stay away from scum and keep pets and livestock from drinking the water.

Warning

Toxins are at higher levels. No swimming, and keep children and pets away from the water.

Danger

High toxin levels. Stay out of the water and off the shore until further notice.

Bloom reported

A bloom was reported, often by a member of the public, without a formal advisory level.

Is Lake Merced safe for dogs?

Dogs are at the greatest risk from toxic algae. They swallow water while swimming, eat scum on the shore and lick algae off their fur afterwards, and a small amount can kill a dog within hours. With blooms recorded in 5 different years here, take particular care at Lake Merced in January, March and October.

• Keep your dog on a lead near water that looks green, scummy, foamy or discoloured, and away from algae washed up on the shore.
• If your dog gets in, rinse it straight away with clean water and don't let it lick its coat.
• Call a vet immediately if your dog vomits, drools, has diarrhoea, staggers, shakes or seems weak after being near the water.

Read the full guide to toxic algae and dogs: symptoms, what to do in the first minutes and how vets treat it.

Health effects of toxic algae

Blue-green algae (cyanobacteria) can produce toxins that affect the liver, nervous system and skin. Swallowing water during a bloom can cause stomach pain, vomiting and diarrhoea. Skin contact can cause rashes, and breathing spray from the water can irritate the eyes, nose and throat. Children are at higher risk because they are smaller and more likely to swallow water.

How to spot a bloom

Toxic blooms can look like pea soup, spilled paint or green streaks on the surface, and they can also be blue, brown, white or red. Scum often collects along the shore downwind. If you can't see your feet in knee-deep water, or you aren't sure, stay out.

Don't drink untreated lake water, and don't boil it to make it safe: boiling doesn't destroy algal toxins. If you feel unwell after contact with the water, contact a doctor or Poison Control on 1-800-222-1222.

Nearby lakes with algae reports

• Richardson Bay16 km away, 7 reports, last in 2023
• San Francisco Bay18 km away, 12 reports, last in 2026
• San Francisco Bay Central22 km away, 7 reports, last in 2024
• Lake Merritt23 km away, 15 reports, last in 2025
• Seal Slough26 km away, 3 reports, last in 2023

All San Francisco County lakes with algae reports

Questions about Lake Merced

Is there algae in Lake Merced right now?

There are no bloom reports for Lake Merced in the last 30 days of California's data. The most recent was a bloom report on 25 August 2026 at South end of lake near Daly City. California's data is checked every day, so for today's conditions check the California HABs Portal map and the signs at the shore.

When was the last algae bloom at Lake Merced?

The most recent entry in California's data is from 25 August 2026 at South end of lake near Daly City: a bloom report.

Has Lake Merced had toxic algae before?

Yes. Blooms are recorded in 5 of the 7 years from 2020 to 2026, most often in January, March and October.

Can dogs swim in Lake Merced?

Keep dogs out of the water whenever a bloom is reported or the water looks green, scummy or discoloured. Dogs are the animals most often killed by algal toxins because they swallow water and lick their fur. If your dog swims in water that might have algae, rinse it with clean water and call a vet if it vomits, drools, staggers or seems weak.

Can you eat fish caught in Lake Merced during a bloom?

If you keep fish caught during a bloom, health agencies advise removing the guts and liver and rinsing the fillets in clean water before cooking, because toxins collect most in the organs. Some agencies advise not eating fish from an active bloom at all, so follow local guidance.