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A guide to understanding algal blooms

A guide to understanding algal blooms

There are many benefits of cold water swimming, but also many risks. An algal bloom is one of the many hazards when swimming in open waters.

It is of utmost importance that any swimmer recognises harmful algae and understands the potential risks of swimming in infected waters.

Algal blooms are common in coastal areas, such as the shoreline of the Pacific Ocean, so take care if you are planning to swim in these areas.

Our article will cover all you need to know about algal blooms, so you can safely identify algae and know when and where to swim.

What is an algal bloom?

An algal bloom is a rapid growth of cyanobacteria in a body of water that can be deemed dangerous.

Blue-green algae is a natural phenomenon that is a common occurrence in many lakes across the UK and does not always pose a problem. However, once the algae form a bloom, you need to take care.

Harmful algal blooms (HAB) can make the water appear scummy, painted or foamy; it may even change the colour of the water to:

  • Green
  • Blue
  • Red/brown

If the water has an unusual appearance, smells strange or tastes bad, check the safety of the water with a local before swimming.

What causes algal blooms?

What causes algal blooms?
Algal bloom in lake

The most common cause of an algal bloom is excess nutrients, such as nitrogen and phosphorus, entering the water via marine ecosystems – otherwise known as eutrophication.

Cyanobacteria use nutrients and sunlight to reproduce and control water bodies, such as great lakes and coastal waters.

In areas where sewage fertilizers leak into the water, there is a high chance of algal blooms. For example, Lake Windermere frequently suffers from algal infections due to runoff from local sewage treatment centres and livestock farmlands.

Cyanobacteria also thrive in warmer conditions. Due to climate change increasing the temperatures of our waters in the UK, we can expect to see algal blooms presenting themselves more often.

Why are algal blooms dangerous?

Blue-green algal blooms can become dangerous for swimmers and aquatic life due to the toxins they produce; a harmful algal bloom can even be fatal.

Human health

Swimmers should seek medical attention if they swallow or swim in water with harmful algal bloom and experience any of the following symptoms:

  • Nausea
  • Vomiting
  • Rashes
  • Itchiness
  • Fever
  • Muscle pain
  • Diarrhoea

The algal toxins can be neurotoxins and hepatotoxins, affecting your nervous and respiratory systems or your liver. The likelihood of a serious reaction to a HAB is low, but not zero, so always contact a medical professional if you have any concerns.

Ecosystem health

Eutrophication can negatively impact aquatic ecosystems when different algae grow in freshwater systems; they block sunlight from reaching underwater plants.

When the aquatic system is blocked from the sun, the plants beneath die, as photosynthesis cannot occur, meaning the wildlife has limited food. Without foliage, the entire ecosystem will be disrupted and even abolished.

Additionally, when the HAB dies, oxygen levels deplete, and toxin levels rise, which can increase the amount of natural fish kills, disrupting the ecology further.

Ecosystem health
Clear seas meeting a coastal forest

How long do algal blooms last?

If a large body of water has an algal bloom, it can take several weeks to disappear.

Eutrophication is hard to predict, but recent studies into monitoring algal blooms have suggested windy and cooler weather helps reduce build-up. Cyanobacteria thrive in warm, calm, and sunny areas.

In areas where eutrophication is common, public health professionals tend to conduct seasonal research into HAB detection, taking water samples to evaluate the different nutrients present.

Information regarding infected waters can be found on local or government sites, such as Citizen Science.

If you are swimming in an area without public information regarding algal blooms, ensure you do your due diligence to avoid health concerns.

How long do algal blooms last?
Weeds in lake

How do you get rid of algal blooms?

The most common methods for removing and preventing an algal bloom include the following:

  • Aeration
  • Chemical and biological additives
  • Ultrasonic technology
  • Algaecides

However, treating algal cells can have negative environmental and economic impacts on coastal communities. So, many sites tend to wait until the HAB passes naturally.

Can you swim in algal blooms?

We do not recommend swimming in locations with suspected HAB, as the low water quality, nutrient pollution, and eutrophication can result in serious illness.

Some types of algae are not harmful; however, unless you strongly understand different algae, we suggest avoiding swimming in any area with suspected algae.

Even unharmful algae can cause skin rashes as algae is a breeding ground for other dangerous bacteria, so it is always better to be safe and swim in clean waters.

Do algae cause paralytic shellfish poisoning?

Phytoplankton called Gonyaulacoid dinoflagellates are the primary cause of paralytic shellfish poisoning toxins and contribute to developing HABs.

Paralytic shellfish poisoning (PSP) is extremely dangerous; if humans eat any seafood infected with PSP and domoic acid, they could get extremely sick.

Therefore, local councils must recognise and deal with algal bloom and PSP before it becomes a wider problem.

If you come across an area of HAB, please report the location to your local council so they can address the matter.


Harmful algal bloom, or HAB, can be a serious health concern for swimmers and the underwater ecosystem. All open-water swimmers must understand the risks associated with swimming in infected waters.

As climate change is a key contributor towards HAB, we cold water swimmers can do our part by ensuring we keep the environment we swim in clean and waste-free.

To learn more about the harmful effects of this natural phenomenon, the National Oceanic and Atmospheric Administration provides more information.