An algal bloom is when algae grow rapidly in aquatic ecosystems. Although algae are crucial members of many ecosystems, these areas of rapid population growth threaten both fresh and marine aquatic life around the world.

Algae is the name for a diverse group of microscopic organisms. One type of algae commonly involved in blooms is known as “green algae.” Green algae is another broad term that includes many kinds of algae that photosynthesize, or get their energy from the sun. Like land plants, they use a pigment called chlorophyll to photosynthesize, which gives them their characteristic green color. Even though algae are microscopic, algal blooms contain so many organisms that they are easily seen. These algal blooms look like large areas of green slime covering bodies of water.

Like all organisms, algae need specific nutrients to thrive. In most environments, algae will grow until they run out of nitrogen or phosphorus. In other words, algal growth is limited by the amount of these two nutrients.

Lakes and other bodies of water near urban centers receive a lot of run-off that is high in nitrogen and phosphorus. Run-off is made up of waste, like discharge from sewage treatments or fertilizer, that drains into bodies of water. When run-off is added to aquatic environments, algae suddenly have plenty of nutrients to grow. If other conditions like temperature, light, and water movement are optimal, algae will grow very rapidly. This process, which is often called “nutrient pollution” or “eutrophication,” is one well-documented cause of algal blooms.

However, the story doesn’t end with algal growth. Algae have short lifespans, and when they die, they sink to the bottom of the water. Bacteria in the sediment break down the algae with processes that use up oxygen. If enough algae grow, die, and are decomposed, this starts to use up the oxygen in the water. Many animals cannot survive in water that doesn’t have enough oxygen. Because of this, algal blooms are said to create “dead zones,” which are areas in the water where oxygen is so low that other life cannot survive.

Algal blooms can also be harmful to people. Some algae can produce chemicals that are toxic to people and other animals. Many ecological departments in the US track toxic algal blooms to inform people where it is unsafe to swim. By harming aquatic life, algal blooms threaten resources that many people rely on, like fish and shellfish. Beyond that, too much algae can also make water green, slimy, and smelly, which is unappealing to tourists and can harm communities that rely on tourism. Environmental advocacy works to reduce urban run-off in order to protect aquatic ecosystems from severe algal blooms.