Harmful Algal Blooms (HABs)
Imagine a pond in summer. The water turns a vivid green, almost like pea soup, and a foul smell hangs in the air. Fish float to the surface, dead. This is not just "a lot of algae" — it is a Harmful Algal Bloom.
The Intuition: Too Much of a Good Thing
Algae are tiny, plant-like organisms that live in water. They form the base of the aquatic food web, producing oxygen and feeding small creatures. Normally they are kept in check by limited nutrients (such as nitrogen and phosphorus) and by grazers — tiny animals that eat them.
Now pour in a sudden flood of nutrients, from fertiliser runoff, untreated sewage or industrial waste. The algae multiply explosively, forming a dense, visible scum or discolouration on the water surface. That is the bloom.
This becomes deadly for two main reasons:
- The oxygen crash (hypoxia/anoxia): when the bloom dies, bacteria decompose the dead algae. This decomposition consumes large amounts of dissolved oxygen in the water, so fish, crabs and other aquatic animals suffocate, creating a "dead zone" where little can survive.
- The toxins: some bloom-forming algae, particularly certain cyanobacteria (blue-green algae), produce potent toxins. These can kill fish and birds directly, and can contaminate drinking water, causing liver damage or neurological problems in humans and livestock. Even contact with the water can cause skin rashes.
A Common Mistake
Not every algal bloom is harmful. A bloom of non-toxic algae can still be a nuisance — it blocks sunlight and clogs filters — but the term Harmful Algal Bloom (HAB) specifically refers to a bloom that causes harm, whether by producing toxins, creating dead zones, or physically damaging the ecosystem.
The Precise Statement
A Harmful Algal Bloom (HAB) is a rapid, excessive proliferation of algae (most often cyanobacteria, dinoflagellates or diatoms) in a water body, driven mainly by eutrophication — the enrichment of water with nutrients, especially nitrogen and phosphorus. This proliferation causes harm through one or more of the following:
- Toxin production: the algae release potent biotoxins (for example, microcystins and saxitoxins) that poison aquatic life, wildlife, livestock and humans. …