Algal blooms in rivers, creeks and lakes are an increasing occurrence in California, threatening human health and safety as well as pets. Blue-green algal blooms (cyanobacteria) occur in California during the summer months because hot temperatures combined with low water levels stimulate growth. If there are excess nutrients present in their environment, especially nitrogen and phosphorus, algae populations grow at accelerated rates, creating algal blooms. Wet winters that deposit nutrient-rich sediment into water bodies aid the formation of algal blooms.

Exposure to cyanobacteria from harmful algal blooms 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. Even livestock and other animals can get sick or die, sometimes within minutes, from swallowing toxins from cyanobacterial blooms.

Because of the risk, swimmers or pets that enter water with a bloom occurrence should be rinsed immediately with tap water from a sink, shower, hose or outdoor spigot. It is not possible to know if a bloom is harmful just by looking at it. Some blooms produce toxins that can still be in the water even when you can’t see a bloom, according to the federal Centers for Disease Control and Prevention.

Some, but not all, algal blooms are toxic. Some produce dangerous levels of the neurotoxin domoic acid. While commercial fish are generally under 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.

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.

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 is California responding?

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

State guidelines on algal blooms recommend people avoid body contact with waters where blooms are present and to ensure that pets and livestock do not drink the water, swim through algal blooms, scum or mats, or lick their fur after going in the water.

People should not drink, cook or wash dishes with untreated surface water from these areas. Common water purification techniques such as camping filters, tablets and boiling do not remove toxins.