Algae are a beneficial part of natural environments and only a few are harmful. Harmful algal blooms (HABs) can occur in fresh, marine (salt), and brackish (a mixture of fresh and salt) water bodies around the world. They are caused by diverse organisms, including toxic and noxious phytoplankton, cyanobacteria, benthic algae, and macroalgae. Some HABs produce toxins that have harmful effects on people, fish, marine mammals, and birds. The toxins produced by algae vary by species and region, and impact organisms in different ways. HAB species and their impacts vary significantly from region to region. Information on the health effects of HABs on marine animals, shellfish, and people is available from NOAA, the Woods Hole Oceanographic Institution, and the Centers for Disease Control and Prevention.
HABs occur in all U.S. waters. According to the Harmful Algal Bloom and Hypoxia Research and Control Act, NOAA has purview over HABs in marine environments, whereas the Environmental Protection Agency is focused on freshwater HABs that occur in all U.S. states. NOAA and EPA share authority over the Great Lakes with regards to HABs. Visit the NOAA Ecological Forecasting page to see ecological forecasting “hotspots” in marine/coastal environments or the Great Lakes, or select a region at the top of this page to learn more about region-specific resources.

Algae are the bottom of the food chain in all natural environments. Only a few are harmful. HABs can occur in fresh, marine (salt), and brackish (a mixture of fresh and salt) water bodies around the world. They are caused by diverse organisms, including toxic and noxious phytoplankton, cyanobacteria, benthic algae, and macroalgae.

Cyanobacteria, a type of photosynthetic bacteria also known as blue-green algae, are often the cause of algal blooms in freshwater and occasionally in marine water. As the name implies, they often (but not always) discolor the water bright green or blue green and form scum. A combination of environmental factors such as the presence of nutrients, warm temperatures, and lots of light encourage the natural increase in the numbers of cyanobacteria.  Cyanobacteria HABs produce multiple toxins, including liver, nerve, and skin toxins, which can affect human and animal health. The most common cyanobacterial HAB toxins in the U.S. are microcystins, a group of liver toxins that can cause gastrointestinal illness in humans, and mortality in pets, livestock, and wildlife.

Alternatively, dinoflagellates and diatoms, different types of phytoplankton, are the most common HAB species in marine and brackish waters, including estuaries. Some of these blooms discolor the water different shades of red and brown and a few are bioluminescent. While the causes of individual marine HAB events have been shown to vary across years and locations, nutrient inputs may help sustain an event once the bloom reaches the shoreline. Some dinoflagellate species produce cysts, which are dormant “seeds” that are created during a bloom and are capable of living through harsh conditions. When favorable conditions resume, the cysts rupture, germinate, and populate the water column with a new generation of photosynthetically active cells primed for another bloom. Marine HABs produce toxins that may result in neurotoxic shellfish poisoning, amnesic shellfish poisoning,diarrhetic shellfish poisoning, ciguatera poisoning in fish, respiratory irritation in humans, or shellfish contamination. Some marine HABs also affect the central nervous system of fish, marine mammals, and birds.

In both marine and freshwater environments, there are also HABs that do not directly affect humans but can kill fish, shellfish, and invertebrates or cause other ecosystem impacts such as shading submerged aquatic vegetation, smothering corals, or causing oxygen in the water to be depleted. For example, fish kills in freshwater may be caused by golden algae and brown tides cause many ecosystems impacts in estuaries.

HABs can’t be stopped but control methods are currently being investigated. In the early 2000s, NOAA funded studies to test the use of clays to control red tides. Since that time, different types of clays have been developed that have fewer environmental effects and are in the initial phases of testing. This method involves a modified clay mineral that, when dispersed into the ocean surface, binds with red tide cells and the toxins they produce and carries them to the seafloor where they are eventually buried.

Additionally, many other control methods are being tested. For example, engineers at Nano Air Bubble Aeration System Technology are researching an environmentally sustainable method to eliminate harmful freshwater algae and their toxins. The method uses nanobubbles — bubbles smaller than the width of a single human hair — to aerate water bodies and create ozone that helps dissolve harmful algae.

Another instance is the examination of a chemical control agent isolated from a bacteria species that may provide a mechanism for halting the growth of certain types of toxic dinoflagellate harmful algal blooms while having no negative impacts on other organisms.

Because of their behavior, dogs are much more susceptible than humans to cyanobacterial poisoning, which is typically found in freshwater bodies and some estuaries. When toxins are present, dogs can be exposed to toxins by drinking the water, by eating washed up mats or scum of toxic cyanobacteria, and by having skin contact with water. Dogs are often attracted to algal scum odors. After leaving the water, dogs can also be poisoned by grooming their fur and paws. If your dog has been swimming in a lake or pond with a suspected or identified HAB, closely monitor your dog for any signs of cyanobacterial poisoning and be sure to rinse the dog with clean water. Signs of poisoning can occur within 30 minutes to a few hours after exposure, depending on the size of the dog, the type of toxin, the toxin concentration, and how much toxin the dog has ingested. In severe cases, dogs can show signs of cyanobacterial poisoning within a few minutes and can die within an hour of toxin exposure. NOAA’s National Centers for Coastal Ocean Science provides additional information and resources for protecting your dog from HABs.

Although there is not a large body of data, there are reports of other HABs affecting dogs. As a precaution, do not allow dogs to swim in discolored water, roll in or eat algae piled on the beach, or run and play on beaches where humans are experiencing respiratory problems.