What are they?
Harmful algal blooms, or HABs, occur when colonies of algae — simple plants that live in the sea and freshwater — grow out of control and produce toxic or harmful effects on people, fish, shellfish, marine mammals and birds. The human illnesses caused by HABs, though rare, can be debilitating or even fatal.
Ranging from microscopic, single-celled organisms to large seaweeds, algae are simple plants that form the base of food webs. Sometimes, however, their roles are more sinister. Under the right conditions, algae may grow out of control — and a few of these “blooms” produce toxins that can kill fish, mammals and birds, and may cause human illness or even death in extreme cases. Other algae are nontoxic, but eat up all of the oxygen in the water as they decay, clog the gills of fish and invertebrates, or smother corals and submerged aquatic vegetation. Still others discolor water, form huge, smelly piles on beaches or contaminate drinking water. Collectively, these events are called harmful algal blooms, or HABs.
With climate change and increasing nutrient pollution potentially causing HABs to occur more often and in locations not previously affected, it’s important for us to learn as much as we can about how and why they form and where they are, so that we can reduce their harmful effects.
Why do they happen?
While we know of many factors that contribute to HABs, how these factors come together to create a “bloom” of algae is not well understood. HABs occur naturally, but human activities that disturb ecosystems seem to play a role in their more frequent occurrence and intensity. Increased nutrient loadings and pollution, food web alterations, introduced species, water flow modifications and climate change all play a role.
Studies show that many algal species flourish when wind and water currents are favorable. In other cases, HABs may be linked to “overfeeding.” This occurs when nutrients (mainly phosphorus and nitrogen) from sources such as sewers and agriculture flow into bays, rivers, and the sea, and build up at a rate that “overfeeds” the algae that exist normally in the environment. Some HABs appear in the aftermath of natural phenomena like sluggish water circulation, unusually high water temperatures, and extreme weather events.
Other factors, such as the structure of the coast, runoff, oceanography, and other organisms in the water, can also change the scope and severity of HAB impacts.