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Discovery Bay Blue-green Algae

July 19, 2018

Harmful algal blooms can occur in warm, slow-moving, nutrient-rich water, such as Discovery Bay. These blooms can persist throughout the warmer seasons. Certain species of algae produce cyanotoxins, which are harmful to humans and pets. Contra Costa Environmental Health collected water samples from twelve locations in Discovery Bay on June 27, 2018. All the samples were positive for cyanotoxin. The sample from the Yacht Harbor exceeded the Danger level for cyanotoxins. Eight of the eleven remaining samples exceeded the Caution level. Both these levels recommend that humans and animals avoid contact with the water. Since a high percentage (75%) were above Caution or Danger levels, and areas affected can fluctuate, it is recommended that throughout the warm seasons residents and their animals avoid contact with water.

Since blooms persistent during warm weather, Environmental Health did an initial round of sampling, and will do another round when colder weather arrives and the algal blooms subside.

The following signage provides recommendations depending on the cyanotoxin level, with all three recommending that humans and animals avoid water contact:


Is it safe for me to go in the water in Discovery Bay?

It depends where you are and if there are advisories posted in the area.

A CAUTION sign means:

  • Do not swim or wade near algae or scum
  • Keep your children away from algae in the water or on the shore
  • Do not drink the water or use it for cooking
  • Do not let pets or livestock go into or drink the water or eat scum on the shoreline
  • Do not eat shellfish from the water

A WARNING sign means:

  • No swimming
  • Stay away from scum, and cloudy or discolored water
  • Do not use these waters for drinking or cooking
  • Do not let pets or livestock go into or drink the water, or go near the scum
  • Do not eat shellfish from these waters
  • For fish caught here, throw away guts and clean fillets with tap water or bottled water before cooking

A DANGER sign means:

  • Stay out of the water until further notice. Do not touch scum in the water or on shore.
  • Do not let pets or livestock drink or go into the water or go near the scum
  • Do not eat fish or shellfish from these waters
  • Do not use the water for drinking or cooking. Boiling or filtering will not make the water safe.

Exposure to blue-green algae can cause rashes, skin and eye irritation, allergic reactions, gastrointestinal upset, and other effects. At high levels, exposure can result in serious illness or death, according to the California Department of Public Health.

What about my pets?

Keep pets out of water with blue-green algae blooms. Dogs are especially vulnerable to getting sick and there have been reports through the years of dogs dying following exposure associated with drinking the water and licking algae from their fur after wading/swimming in water with blue-green algae blooms.

Is there an issue with drinking water?

No, contamination only affects recreational water areas, not drinking water from the tap in Discovery Bay.

Who should I contact if I swam in the water and now have symptoms?

If you think you or someone else is displaying symptoms of blue-green algae poisoning, contact your health care provider or the California Poison Center Help Hotline at 1-800-222-1222.

What can I do to get rid of blue-green algae in Discovery Bay?

We do not recommend attempting to treat or remove the algae. Blue-green algae blooms are natural to the environment's food chain, and eventually dissipate on their own. Chemical treatment is often ineffective and can result in more toxins being released into the water as algae cells die, as well as unintended effects on other species in the area.

Algae blooms result from changes in water conditions that encourage sudden growth of the species. The best way to reduce and prevent blooms is to reduce water pollution, particularly from runoff containing fertilizers or pesticides. Pick up trash dumped in waterways, and make sure all household sewer systems are working properly.

Treating aquariums with a hydrogen peroxide solution is a common way to get rid of some kinds of algae. But the Delta is not a closed system like an aquarium, so the results would be unpredictable, both in terms of efficacy and impact on the environment.