Toxic algal blooms at Higher Level: This statement is Higher Level only. It looks at the few algal blooms that make poisons, with one freshwater case and one marine case.
Practise this as you read
- Explain how a toxic bloom threatens human and animal health.
- Explain how a bloom lowers the recreational value of a lake, river or beach.
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Most blooms are harmless; a few are poisonous: When nutrients pour into water, tiny producers can multiply into a bloom that colours the water. Most blooms only use up oxygen when they die. A few species also make poisons.
What a harmful algal bloom is
- An algal bloom is a sudden, huge growth of tiny producers in the water.
- It follows extra nitrate and phosphate, in warm, calm, sunny weather.
- Harmful algal blooms (HABs) contain cyanobacteria, protists, algae and dinoflagellates.
- Only a small number of these make toxins, but some toxins can kill.
- Toxins harm people, pets, livestock, fish, birds and marine mammals.
Remember it as: Feed the water, grow a bloom; the wrong species brings poison.
Cyanobacteria
- Bacteria that photosynthesise.
- Often called blue-green algae.
- Mostly in fresh water.
- Make cyanotoxins.
Dinoflagellates
- Protists with two whip-like tails.
- Mostly in the sea.
- Some make neurotoxins.
- Can turn the sea red.
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Freshwater example: Lake Erie, one of North America's Great Lakes, between the USA and Canada.
Fresh water: cyanotoxins
- In fresh water the most common toxins are cyanotoxins, made by cyanobacteria.
- Microcystin damages the liver; others attack the nerves or skin.
- People are harmed by drinking the water, swimming in it or eating fish from it.
- Dogs and cattle die after drinking from a lake or licking scum off their fur.
- Ordinary water treatment may not remove the toxin.
Remember it as: Fresh water: cyanobacteria, cyanotoxins, the liver.
Nutrients
- The Maumee River carries phosphate from farm fertiliser into the shallow western end of Lake Erie.
Bloom
- In hot, calm weather the cyanobacterium Microcystis bloomed around the city's water intake.
Toxin
- Microcystin got through the treatment plant into the tap water, above the World Health Organization's limit of 1 microgram per litre.
Ban
- From 2 to 4 August 2014, about 400,000 people were told not to drink, cook with or even boil their tap water. Shops ran out of bottled water.
Boiling does not help: Boiling kills bacteria, but it does not destroy cyanotoxins. Boiling water with microcystin in it makes the toxin more concentrated.
Marine example: the Gulf coast of Florida, USA, where the dinoflagellate Karenia brevis blooms most years.
The sea: dinoflagellates and neurotoxins
- Along coasts, many HABs are made of dinoflagellates.
- Their toxins include neurotoxins: poisons that attack the nerves.
- Some blooms turn the sea red or brown: a red tide.
- Shellfish filter the algae and store the toxin; people who eat them are poisoned.
- Waves spray the toxin into the air, so people on beaches cough.
Remember it as: Sea: dinoflagellates, neurotoxins, the nerves.
Bloom
- From October 2017 to January 2019, about 16 months, Karenia brevis turned the sea reddish-brown from Tampa Bay to Marco Island.
Wildlife
- Its neurotoxins killed tonnes of fish. In 2018 a record 589 sea turtles and 213 manatees died.
People
- Waves sprayed toxin into the air: beach visitors coughed and had sore eyes. Shellfish beds were closed to prevent poisoning.
Coast
- Dead fish piled up on the beaches, and hotels and restaurants lost visitors for months.
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A bloom also harms the recreational value of a lake, river or beach.
How a bloom spoils a lake or a beach
- Toxins: swimming, water sports and dogs are banned: a health risk.
- Smell: rotting algae smell bad, so visitors stay away.
- Murky water: scum and turbidity spoil the look of the water.
- Fewer fish: dead fish and low oxygen mean less angling.
- Drought makes it worse: low, slow, warm water concentrates nutrients and toxins.
Remember it as: Toxic, smelly, murky, fishless: four reasons to stay away.
Health
- Toxins: swimming bans.
- Dogs kept out of the water.
Senses
- Smell of rotting algae.
- Green scum, cloudy water.
Activities
- Fewer fish to catch.
- Boating and canoeing stop.
Link each effect to the visitor: Say what changes in the water AND why people stop coming: 'the toxins are a health risk, so swimming is banned', not just 'the water is toxic'.
How this comes up: Paper 1: a fact file on a lake or river with blooms, with 'explain how ... may affect its recreational value' [3] (May 2026). Paper 2: 'outline how algal blooms threaten health', with one freshwater and one marine example.
In August 2014 a bloom of the cyanobacterium Microcystis in Lake Erie formed around the water intake of the city of Toledo, Ohio.
Explain how the bloom threatened the health of the people of Toledo.
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