St Johns River algae bloom history
Florida's Department of Environmental Protection sampled St Johns River 2 times between 2022 and 2023. Staff saw a bloom on 2 visits (100%).
Microcystin, the most common algal toxin, was not detected in any sample tested here.
- 2022 1 of 1 visit
- 2023 1 of 1 visit
When do algae blooms happen at St Johns River?
About 100% of reports fall between May and September, when warm, calm, sunny weather lets blue-green algae multiply.
Blooms can appear, move with the wind and fade within hours or days, so the time of year is only a guide.
Recent St Johns River algae reports
Recent reports came from 2 different locations around St Johns River, including St. Johns River - Astor Park Boat Ramp and St. Johns River - off Astor City Boat Ramp. A bloom in one cove or beach doesn't mean the whole lake is affected, and conditions can differ a short distance away.
| Date | Location | Result |
|---|---|---|
| 6 September 2023 | St. Johns River - Astor Park Boat Ramp | Bloom seen · microcystin not detected |
| 6 July 2022 | St. Johns River - off Astor City Boat Ramp | Bloom seen · microcystin not detected |
What the Florida results mean
Florida records whether staff saw a bloom at each visit and, when a sample is tested, how much microcystin it contained.
- Bloom seen
- Staff saw a bloom at the sampling site, and no toxin was found or tested.
- Toxin detected
- Microcystin, a liver toxin made by blue-green algae, was found in the sample.
- 8 µg/L or more
- At or above the level where the US EPA recommends swimming advisories.
Is St Johns River 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.
- 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
- Silver Glen Springs14 km away, 5 reports, last in 2025
- Spring Garden Lake15 km away, 2 reports, last in 2025
- Lake George18 km away, 121 reports, last in 2025
- Lake Dorr20 km away, 4 reports, last in 2025
- Lake Disston21 km away, 2 reports, last in 2024
Questions about St Johns River
Is there algae in St Johns River right now?
Florida's most recent sample from St Johns River, on 6 September 2023 at St. Johns River - Astor Park Boat Ramp, found a bloom and no microcystin toxin was detected. Florida publishes its data in yearly batches, so for today's conditions check the DEP Algal Bloom Dashboard and the signs at the shore.
When was the last algae bloom at St Johns River?
The most recent entry in Florida's data is from 6 September 2023 at St. Johns River - Astor Park Boat Ramp: a bloom seen during sampling.
Has St Johns River had toxic algae before?
Yes. Blooms are recorded every year from 2022 to 2023.
Can dogs swim in St Johns River?
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 St Johns River 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.
About this data
This page uses official records from the Florida Department of Environmental Protection (DEP), which published as a yearly file; current sampling is shown on the DEP dashboard. Reports are grouped by waterbody. The site was last rebuilt on 24 October 2026.
States only test some lakes, some of the time, so a lake without a recent report hasn't necessarily been checked. This page is independent, isn't affiliated with the State of Florida, and isn't health advice. View the source data.