What are the impacts of harmful algal blooms?

Fish killed by toxins — Harmful algal blooms can produce toxins that have harmful effects on wildlife. Even if they are not toxic, algal blooms can block light from penetrating the water, cause decreases in oxygen in the water, or even clog or harm fish gills. This can kill fish or other marine species in the area (Credit: Tom Archer).

HABs harm human and animal health, coastal economies, fishing and aquaculture, and long-term ecosystem health. Some HABs are toxic, can poison animals, and make the surrounding air hard to breathe. The toxins produced by algae vary by species and region, and impact wildlife and people in different ways. While HABs occur naturally, they are becoming more frequent and lasting longer. HABs are also occurring in new areas because of human-influenced ecosystem changes like nutrient pollution and extreme weather.

How do harmful algal blooms impact wildlife?

A bloom does not have to produce toxins in order to be harmful. In fact, only 1% of algal species produce toxins. Blooms of nontoxic species can block light to aquatic plants deeper in the water, and even clog or harm fish gills. Some HABs can also result in dead zones. When blooms occur, the overgrowth of algae can quickly die off. When large amounts of algae die, sink, and decompose in the water, the decomposition consumes oxygen and depletes the amount available to healthy marine plants and animals. This process is known as eutrophication. If the organisms are unable to leave the area, they may die.

There are hundreds of dead zones in coastal waters around the world. One of the largest and best-known occurs in the Gulf of Mexico. In 2019, it grew to cover more than 6,900 square miles of the sea floor, making it the 8th largest in NOAA’s 33-year record. Scientists at NOAA can predict how severe the dead zone will be each year. Water temperature, nitrogen and phosphorous concentrations, and changes in currents and tides are some of the conditions that they monitor to make these predictions. However, they are still learning about the many factors and how they interact to cause dead zones, so the forecasts may not always match reality.

How do harmful algal blooms impact people?

Although it is rare, people can get sick by eating shellfish containing toxins produced by algae. Airborne HAB toxins may also cause eye irritation and breathing problems and, in some cases, trigger asthma attacks.

*A water sample taken from Lake Erie during a harmful algal bloom in 2017. The sample in the beaker was taken directly from the lake in its natural concentration (Credit: NOAA).*

HABs can also be costly in economic terms. HABs can result in reduced tourism, closed beaches and shellfish beds, and decreased catch from recreational and commercial fisheries. Experts estimate that HABs cost the U.S. economy at least $82 million a year.

Depending on where they occur, there are even more risks from HABs. In 2014, over half a million people in Ohio were left without clean drinking water when Lake Erie experienced a major HAB near a water treatment plant.

Our pets can also be harmed by HABs. Dogs can be exposed to toxins from drinking or swimming in contaminated water, or from eating washed up algae mats. They can also be poisoned after leaving the water from grooming their fur or paws.

HABs have been reported in all 50 U.S. states, and their occurrence may be on the rise. HAB species and their impacts vary significantly from region to region, so the HAB one person experiences in Florida is completely different from a HAB in the Great Lakes.