What is a Harmful Algal Bloom?

Algal Bloom Definition

An algal bloom is defined as the rapid growth or accumulation of algae in aquatic ecosystems. Harmful algal blooms (HAB) are algal blooms composed of phytoplankton known to naturally produce bio-toxins that are harmful to the resident population, as well as humans. The presence of harmful algal blooms leads to fish die-offs, fish sickness, and human sickness when affected organisms are consumed.

Causes of Algal Blooms

Algal blooms are caused by excessive amounts of nitrates, phosphates, and nutrients entering an aquatic ecosystem, often via discharges from sewage treatment plants and septic tanks, and storm water run-off from fertilized lawns and farms. Other factors that aid algal growth include sunlight and slow-moving water.

These nutrients cause a type of pollution called eutrophication. Eutrophication entails excess nutrients stimulating an explosive growth of algae. As these algae grow, out-competed plants die off and become food for the bacteria that decompose them. With more food available now, the bacteria also experience an explosive growth, rapidly using up all the oxygen in the water until many fish and aquatic insects can no longer survive. The end result of an algal bloom is a dead zone.

Algal Bloom Effects

The presence of algal blooms can induce a variety of damaging effects on the surrounding environment and its inhabitants. Toxins released from the algal bloom can cause anything from tissue deterioration and illness, to death and extirpation. How these toxins induce their effects on their victims vary by mechanism, but can include malnutrition, reduced appetite, suffocation, and respiratory failure. The results are devastating—not just for the resident aquatic populations, but also to many other plants and animals in the community, including people. In humans, direct exposure to toxins via drinking water can cause a series of health issues, such as rashes, stomach or liver illnesses, respiratory problems, and neurological effects.


A few of the harmful algal bloom species are known to naturally produce biotoxins, organic poisonous substances that affect aquatic organisms and humans alike. Pfiesteria piscicida, to name one, is known to produce dermonecrotic and neurologic toxins that attack the skin and nervous system of fish and humans. A more widely known bacteria notorious for highly-visible blue-green algal blooms that produces neurotoxins and hepatoxins is cyanobacteria. Cyanobacteria algal blooms are enough of a threat to humans to warrant the shut down of recreational water activities when identified. These cyanotoxins can cause a range of symptoms in affected organisms, including vomiting, diarrhea, loss of appetite, seizures, convulsions, paralysis, increased heart rate, excessive tiredness, and difficulty breathing.


Some harmful algal bloom bacteria are, in a sense, predators of the resident fish population. Their neurotoxins and dermanecrotic toxins assist in predation by killing the neurosystems and the skin of the affected fish. These toxic components can cause lesions to form in the fish’s skins, leading to a sloughing off of skin layers and, ultimately, to infection. When the fish die, their organic compounds become food for the bacteria that killed it.

Particle Irritation

Other harmful algal bloom species, such as the spine-forming diatoms, get stuck in the gills of animals. The result is an accumulation of mucus and respiratory failure, as well as internal bleeding and bacterial infection. People often get sick from eating shellfish containing toxins produced by these algae.

Induced Starvation

Harmful algal bloom species can starve animals to death through nutritional and size mismatch. Basically, animals that feed on the harmful algal bloom species don’t get enough nutrition, despite eating large quantities. The result is nutritional deprivation and, ultimately, death. Other harmful algal bloom species reduce the appetites of affected animals, leading to death by starvation.

Localized Anoxic Conditions

Excessive algae growth and the subsequent death of out-competed plants leads to increased numbers of decomposing bacteria. With an abundant food supply of dead organic material, the newly enlarged bacteria population uses up all the oxygen in the water, causing hypoxic and anoxic conditions. The result is mass mortality of fish and shellfish. The area becomes a dead zone, unable to sustain life.

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