Algal blooms in rivers, creeks and lakes are an increasing occurrence in California, threatening human health and safety as well as pets. Exposure to toxic blue-green algae, also known as cyanobacteria, can cause eye irritation, allergic skin rash, mouth ulcers, vomiting, diarrhea and cold- and flu-like symptoms. Young children are most likely to be affected by harmful algal blooms because of their small body size and tendency to play in the water for longer periods. Dogs also are susceptible because they tend to drink while in the water and lick their fur afterward.

Blue-green algal blooms occur in California during the summer months because hot temperatures combined with low water levels stimulate growth. Algal blooms are aided by wet winters that deposit nutrient-rich sediment into water bodies.

What are algae?

Organisms that photosynthesize but lack the formal water circulation structure of land plants are placed in the broad category of “algae.” These can grow in either freshwater or saltwater environments, existing as single cells (often congregating in colonies) or multicellular organisms such as giant kelp. 

Algae are so prolific that their photosynthesis is responsible for producing approximately half of the planet’s oxygen, making them essential to our survival. Old algae deposits also make up a portion of petroleum products, and living or recently dead algae serve as both food and shelter for innumerable species.

If there are excess nutrients present in their environment, especially nitrogen and phosphorus, algae populations grow at accelerated rates. These sudden population surges are referred to as “algal blooms.” Algal blooms can pose a health risk to people and pets. 

How are algae dangerous?

Algal blooms often occur faster than their ecosystem can compensate for. Although algae produce oxygen to grow, when they die, they remove oxygen from their environment in order to decay.

Additionally, at night algae undergo respiration in order to develop, a process requiring still more oxygen. Thus, especially in large masses, algae remove much of the finite amount of oxygen in the water.

Fish, bacteria, and even plants require certain amounts of oxygen to live, so algal blooms of even nontoxic species can cause fish kills and otherwise disrupt biodiversity.

Beyond their effects from overpopulating bodies of water, some algae species themselves are toxic. Several species of blue-green algae in particular are notorious for releasing dangerous neurotoxins. Despite their name, these algae are not restricted to blue-green colors and in fact appear as green, yellowish-brown, or red.

Since nontoxic algae are also green, the colors of blooms are not always reliable indicators of toxicity. Because red tides are unmistakably a result of toxic algae, harmful blooms in saltwater environments are often referred to as “red tides” (regardless of their pigment).

How can I determine if an algal bloom is harmful?

Freshwater harmful algal blooms are often more difficult to detect than colorful red tides in marine environments, and generally appear green. Nevertheless, these species usually conform to characteristic patterns that can be recognized from the surface. Grouping in parallel streaks or clumped dots usually indicates toxicity, as does the semblance of spilled green paint or thick pea soup. Alternatively, nontoxic algae generally appear as floating rafts, scum, tangled or sprawled hair and thick mats.

How are fish affected?

Some algal blooms produce dangerous levels of the neurotoxin “domoic acid.” While commercial fish are generally under strict-enough regulatory restrictions for safe consumption, recreationally caught fish are unregulated and could introduce dangerous levels of domoic acid. Many fisheries have been temporarily closed, resulting in millions of dollars of lost revenue. The California Department of Fish and Wildlife releases updated fishing restrictions that include safe locations to avoid harmful domoic acid.

The State Water Resources Control Board works with state and local agencies to identify and respond to hazardous algal blooms throughout California. Formed in 2008, the California Harmful Algal Bloom Monitoring and Alert Program includes algal bloom researchers, water quality and shellfish managers, universities, state and local agencies. The program is a coordinated response network for efficient algal bloom mitigation, prediction and prevention and provides updates on current algal blooms.