What are algal blooms? | WWF

What are algal blooms?

Sailing boat slices through an algal bloom off the coast of Poland.
© WWF Poland
Algal blooms are intensified when excess nutrients are deposited in the water and massively over stimulate growth. The nutrients mostly come from unsustainable farming practices in the countries around the Baltic. The algae can make people and animals sick and severely damage the Baltic ecosystem. Read what WWF is doing to reduce the effects of eutrophication.
Phytoplankton blooms, mass developments of microscopic algae, are a natural phenomenon in the sea. Due to the eutrophication process, however, phytoplankton blooms have become more frequent and intense. Today, harmful blooms occur annually in the Baltic Sea.
In most of the Baltic Sea, there are two annual blooms, the spring bloom and the cyanobacterial (also called blue-green algae) bloom in late summer. In the southern Baltic Sea, autumnal blooms are regular, too. Additionally, exceptional blooms formed by various species can occur locally.

When large quantities of microscopic, free-floating algae (phytoplankton) perform photosynthesis, multiply and lump together we see them as a greenish, yellowish, brownish or reddish layer on the sea surface or as a thick ‘soup’ in the water. These visible accumulations (or aggregations) of algae are usually referred to as algal blooms.

The term algal bloom is used for events when phytoplankton multiply rapidly during a limited time period due to the rich supply of plant-available nutrients in the water. Algal blooms are a normal and essential part of life in the sea, but they become a problem when they occur too often, are too large, last for too long time, and/or produce toxic substances.

What to do if an algal bloom occurs

  • Avoid bathing
  • Never drink water containing high concentrations of al­gae. Boiling the water doesn’t help – it doesn’t get rid of the toxins
  • Don’t let children swim or play near the shore
  • Keep pets and livestock away from affected shores and waters

Harm to animals

Fish, birds and mammals can all be killed by algal toxins.

A sig­nificant number of domestic animals are reported to have died as a result of algal blooms in lakes and in the Baltic Sea. In other sea areas, blooms have also claimed human lives. In the Baltic Sea, it is unusual for algal toxins to be transmitted from fish to humans.

The main reason why fish and other aquatic animals die in the wake of a bloom is the shortage of oxygen that arises when large masses of algae decompose.
Young common (or harbour) seals (<i>Phoca vitulina</i> on sand beach. Wadden Sea, ... 
Young common (or harbour) seals (Phoca vitulina). The harbour seal is under severe threat in the Baltic sea.

Be careful!

The best thing is to avoid waters where algae are blooming.

Take particular care with young children and animals, and keep them away from affected waters and shores. Young children can easily swallow large amounts of water by accident when swimming. You can also end up with an itching rash if you sit on a beach where algae have been washed ashore.

Domestic animals can’t tell safe water from toxic. As a result, they may ingest lethal doses of algal toxins if they drink conta­minated water or lick their fur after swimming in it.

Some algal toxins are taken up by shellfish, making them poi­sonous to humans. This is mainly a problem in waters that are saltier than the Baltic Sea, where eating mussels after a bloom can pose a health risk.

Remember that toxins can linger for some time after the algae have disappeared.
	© Germund Sellgren
When algal blooms are active, keep children away from affected waters and shores
© Germund Sellgren

Common symptoms

Symptoms of poisoning include nausea, irritated skin and eyes, gastrointestinal problems and fever.

If you notice such symp­toms, contact a doctor. If you have bathed in water affected by an algal bloom, take a shower afterwards.

The first symptoms in animals that have swallowed toxic water are vomiting and diarrhea. A pet showing symptoms must be treated by a vet without delay. If the animal has been in the water, it must be rinsed off and dried as soon as possible.

(Source: The Information Office for the Baltic Proper, at County Administrative Board of Stockholm)
Exposure to an algal bloom can lead to nausea, irritated skin and eyes, gastrointestinal problems ... 
	© WWF / Paivi Rosqvist
Exposure to an algal bloom can lead to nausea, irritated skin and eyes, gastrointestinal problems and fever
© WWF / Paivi Rosqvist
	©  .

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