Harmful algal blooms (HABs) are rapid overgrowths of algae in water systems, often forming thick, green or red scums on the surface of lakes, rivers, or coastal waters. While algae are a natural part of aquatic ecosystems, certain types—especially cyanobacteria (also known as blue-green algae)—can produce dangerous toxins when they multiply excessively.

Several factors contribute to the growth of HABs. Chief among them is nutrient pollution, particularly an overabundance of nitrogen and phosphorus from fertilizers, sewage discharge, stormwater runoff, and agricultural runoff. Warm temperatures, stagnant water, and sunlight further accelerate bloom formation. Climate change is also playing a role, with warmer waters and changing rainfall patterns that create ideal conditions for HABs to thrive in more regions and for longer periods each year. While all coastal areas in the U.S. experience HABs, different algal species have different impacts on their host environment.

Downstream impacts: One of the most damaging consequences of harmful algal blooms is hypoxia—a condition in which oxygen levels in the water drop so low that aquatic life cannot survive. When a bloom dies off, the decomposing algae consume massive amounts of oxygen during the breakdown process, creating these low-oxygen zones. 

Hypoxia can lead to fish kills, loss of biodiversity, and the collapse of entire ecosystems. Large hypoxic areas, sometimes called “dead zones,” are particularly common in places like the Gulf of Mexico/America and the Chesapeake Bay. These zones devastate fisheries, disrupt food chains, and make recovery for native species extremely difficult. Scientists predict the size of the dead zone in the Gulf of America that occurs in late summer each year by assessing the Mississippi River discharge and nutrient loading during the month of May. 

In addition to natural aquatic life suffering, hypoxia can also cause economic losses for communities that depend on fishing, tourism, and recreation. Boating and swimming may be restricted in areas impacted by blooms, and water quality concerns can deter visitors.

Human health: The toxins produced by HABs, such as microcystins, anatoxins, and saxitoxins, pose serious health risks to humans. Exposure can occur through direct contact with contaminated water, inhalation of water droplets, or ingestion of tainted drinking water or seafood.

Health symptoms vary depending on the type of toxin and level of exposure, but may include:

• Skin rashes, irritation, or blisters after swimming
• Eye and throat irritation
• Respiratory issues from inhaling aerosolized toxins
• Nausea, vomiting, or liver damage from drinking contaminated water

Communities that rely on untreated surface water sources are especially at risk. During severe HAB events, water utilities may need to issue "Do Not Drink" advisories or shut down water intakes entirely to prevent health hazards. Boiling water does not remove algal toxins—in fact, it can concentrate them—making proper treatment and early detection essential. Cyanobacteria overgrowth was the cause of the water crisis in Toledo, Ohio in 2014, cutting off water supply to the half-million residents who receive water from the western basin of Lake Erie. 

Moreover, seafood harvested from contaminated waters can accumulate toxins, leading to shellfish poisoning in humans. Regulatory agencies monitor commercial shellfish beds, but recreational harvesting remains a risky venture during bloom periods.

Animal and pet health: While humans may avoid obviously contaminated water, animals—especially pets—often aren't so discerning. Dogs are particularly vulnerable to HAB exposure, often drinking from or playing in scummy water without hesitation. Reports of dog fatalities due to HAB toxin ingestion have increased in recent years. Symptoms can appear within minutes and include drooling, weakness, vomiting, seizures, and in severe cases, death. Wildlife such as fish, birds, and marine mammals are also affected, either directly through contact or indirectly through the food chain. Livestock that drink contaminated water can experience weight loss, reproductive problems, or even death, posing serious concerns for agricultural operations. 

Protecting animals means raising awareness. Pet owners should learn to recognize the signs of HABs and avoid letting animals near questionable water bodies. Local authorities should also post warnings around affected lakes and rivers during bloom events. Veterinary offices may consider educating staff about the generalized symptoms of HABs exposure in animals and include possible HABs exposure as an intake question for pet owners.

Prevention efforts: HABs are a clear sign that our water ecosystems are under stress. By understanding their causes and impacts—and taking proactive steps to reduce our contribution—we can help protect public health, pets, and the environment.

Preventing and mitigating HABs starts with us:

• Reduce fertilizer use and opt for eco-friendly lawn care.
• Maintain septic systems and dispose of waste properly.
• Support green infrastructure like rain gardens and buffer zones.
• Stay informed through local water quality reports and advisories.