What are harmful algal blooms?

Have you ever heard the phrase “too much of a good thing”? This can also be true in coastal waters. Algae are small plants that are essential to many aquatic environments. However, some algae can be toxic, and too much algae (even nontoxic species) can be harmful to the environment.

Under certain conditions, algae may grow out of control. This is called a bloom. Algal blooms can happen in fresh, marine (salt), and brackish (a mixture of fresh and salt) water. Not all algal blooms are harmful. Most blooms are beneficial, because the tiny plants are food for animals in the ocean. In fact, they are the major source of energy that fuels most ocean food webs. Phytoplankton (microscopic algae) are found at the base of almost all marine food chains, which means that the majority of life in the ocean relies on these organisms. Algae also produces huge amounts of oxygen. Scientists estimate that roughly half of Earth’s oxygen comes from algae and other photosynthetic life in the ocean.

Harmful algal blooms (HABs) have unhealthy impacts on aquatic and coastal environments, as well as humans and coastal economies. Some of these impacts can even be toxic.

One of the best known HABs in the U.S. occurs nearly every year along Florida’s Gulf Coast. The microscopic algae Karenia brevis causes this bloom. Karenia brevis produces toxins that can kill fish or marine mammals and make shellfish dangerous to eat. Its toxins can also be aerosolized (meaning particles can be suspended in and travel through the air), making it harmful to breathe. This HAB is commonly referred to as a “red tide” because the algae often turn the water red.

Although all states experience HABs, different types of algae live in different places and cause different problems. Other factors—such as the geography of the coast, amounts of runoff, tides and currents, water temperatures, and even other organisms in the water—can all change the scope and severity of HABs and their impacts.

What causes harmful algal blooms?

The growth of algae that leads to HABs happens naturally with sunlight, slow-moving water, and nutrients (nitrogen and phosphorus). Many different circumstances can bring about these conditions. Some blooms happen annually, while others can be sparked by natural events like changes in water circulation, higher water temperatures, and extreme weather events like hurricanes, floods, and drought. Human activities can cause HABs to occur more often or severely. For example, coastal development can change water circulation patterns, and human-caused climate change brings on higher temperatures and more severe weather.

Human activities also influence increases in nitrogen and phosphorus levels entering coastal waters. These chemicals are found in agricultural fertilizers and lawn care products, which wash out into our waterways. While these chemicals are necessary for plant growth—hence the term nutrients—too much of them can cause big problems. Excess nitrogen and phosphorus in air or water is called nutrient pollution.