Harmful algal blooms

What is an algal bloom?#

Algae and cyanobacteria are simple organisms that live in water. They can grow rapidly out of control, or “bloom,” when water is warm, slow-moving and rich in nutrients (for example, when runoff carries fertilisers or sewage into it). Blooms can occur in fresh water, marine water and brackish water.

A bloom can look like foam, scum, mats or paint on the surface of the water. It can also change the colour of the water to green, blue, brown, red or another colour.

Some algal blooms may not be visible#

Algae and cyanobacteria can sometimes harm people, animals and the environment. A build-up of algae that may be harmful is known as a harmful algal bloom, and these can occur in both salt water and fresh water.

Not all algae and cyanobacteria produce toxins, and not all blooms are harmful. Some types cause irritation and allergic reactions on direct contact, while others release toxins (poisons) that can affect the health of people and animals. Seafood harvested from algae-affected water can also put your health at risk.

Harmful algal blooms persist as long as conditions favour them, including warmth, sunlight and low water flow. A bloom can last from weeks to months, and it is difficult to predict when it will clear. Cooler, windy weather or increased water flow may reduce or stop a bloom.

How harmful algae can affect your health#

Contact with harmful algae can cause a range of illnesses and symptoms, depending on the type of algae and how you are exposed.

• Direct skin contact can cause skin and eye irritation and allergic reactions.
• Inhaling fine spray or droplets from affected water can cause mild respiratory effects. This most often happens during recreational water activities such as swimming, water-skiing, jet-skiing and boating.
• Drinking affected water, or eating food such as fish or shellfish that contains toxins, can lead to gastroenteritis, with vomiting, diarrhoea, fevers and headaches. Some toxins may also affect the liver or nervous system.

If you feel unwell after contact with affected water, seek medical advice.

Reducing your exposure#

To lower your risk around water that may be affected:

• Look out for water that is discoloured or where visible scums are present. Scums are a build-up of algae that settles along the edge of the water, where you are most likely to make contact.
• Do not swim, and avoid water sports where you may make direct contact with affected water.
• Do not let children touch, swallow or swim in affected water, and always supervise them near water.
• Follow any warning signs about harmful algal blooms, and avoid contact until authorities advise the risk has passed.

Children tend to have more sensitive skin than adults. They are also more likely to accidentally swallow or inhale water, which puts them at greater risk of algae-related poisoning. Try to make sure children in your care understand the dangers of harmful algae.

If you suspect a local waterway is affected, contact the body responsible for managing it for further advice. Remember that water affected by a harmful algal bloom may not always look or smell any different.

If you come into contact with affected water#

• Leave the water immediately.
• Thoroughly wash and rinse your skin and hair in clean water, and remove any traces of algae.
• Wash and dry all clothing and equipment, including wetsuits, in clean water.

Wetsuits used for water sports may increase the risk of rashes, because algal material can become trapped inside and stay in contact with the skin for long periods. If you feel ill after contact, seek medical advice.

Telling whether a waterway is affected#

• Look for warning signage and follow the advice given.
• If there is no signage, look for discolouration or scums on the surface of the water.
• Consider whether the area is prone to harmful algal blooms.

If in doubt, take a precautionary approach and follow the advice in this article.

Keeping animals safe#

Pets and livestock can be poisoned by contact with, or ingestion of, harmful algae. Precautions include:

• Do not let animals swim in affected areas.
• Avoid walks along banks where scums accumulate.
• Do not let animals drink affected water.

If you keep livestock, check water supplies frequently for harmful algae and keep animals away from blooms. If an animal comes into contact with affected water or scums, wash it thoroughly with fresh water before drying, so it does not swallow algae while grooming. If you are concerned about an animal’s health, consult a vet as soon as possible.

Seafood and algal toxins#

Fish, shellfish and crustaceans can take up the toxins produced by harmful algae. These toxins can concentrate in shellfish and in the internal organs of fish and some crustaceans, and levels can stay high for several weeks to months after a bloom has dissipated. Eating affected seafood can cause serious illness, and cooking or freezing does not destroy the toxins.

Seafood can be affected even where there are no warning signs, because it is not always easy to tell whether a bloom is present now or has occurred previously. Some seafood also moves in and out of affected waters and may be caught where a bloom is not apparent. If you are unsure, take a precautionary approach.

You can still catch and release, or harvest fish from affected waters, as long as you take certain precautions:

• Wash the fish in clean water.
• Gill and gut (fillet) the fish before freezing or cooking.
• Discard the internal organs in the rubbish; the fish has likely swallowed algal toxins, so do not feed leftovers to animals.
• Do not eat whole fish harvested from affected waters.

When algal blooms die and break down, they use up oxygen in the water and create conditions that can suffocate fish. If you see fish that are dead, dying or swimming erratically, do not touch or eat them.

Crustaceans such as crabs, prawns and crayfish may also concentrate toxins in their internal organs. Take care when consuming recreationally caught crustaceans, especially when water quality is poor, and avoid them altogether where there is a harmful algal bloom.

Shellfish are more likely to accumulate dangerous levels of algal toxins and other harmful substances from the water. You can develop shellfish poisoning if you collect and eat shellfish affected by toxins from a bloom, and this can cause serious illness and potentially death. Symptoms can appear from minutes to hours after eating affected shellfish and may include tingling or numbness, weakness, blurred vision, difficulty breathing, vomiting and diarrhoea. Cooking or freezing does not destroy the toxins, and there is always a risk of illness from eating wild shellfish harvested where water quality is poor.

Harmful algae and water supply#

If you have any concerns about the quality of your drinking water, contact your water supplier.

If you rely on a private water supply, you can help prevent algae growing by:

• Sealing your water tank.
• Ensuring pipes and fittings are not exposed to sunlight.

Do not use untreated, affected water for drinking, showering or washing. Boiling affected water does not remove the toxins.

Avoid using affected water on your lawn and garden beds. It may affect plant growth and poses a risk to anything that contacts the wet surface. Spray irrigation can also produce very fine, airborne droplets that can lead to toxins being inhaled. If there is no other water source, minimise spray by using drip irrigation, soaker hoses or a watering can, and avoid directly watering the edible parts of plants.

Harmful algae and motor neurone disease#

There is no consistent evidence that exposure to harmful algae causes motor neurone disease, and no evidence that people living near affected water are at increased risk of developing it.

Key points#

• Algae and cyanobacteria can sometimes cause harm to people, animals and the environment.
• Seafood harvested from algae-affected water can also put your health at risk.
• Not all algae and cyanobacteria produce toxins, and not all blooms are harmful.
• Contact with harmful algae can cause various illnesses and symptoms depending on the type of algae and your exposure.
• Avoid contact with the water until authorities advise there is no longer a risk.

Where to get help#