Algal bloom
You might have heard of how a red tide can poison shellfish and other marine creatures. Let’s look at this natural phenomenon
ARED tide is actually an algal bloom – a natural phenomenon in which coastal waters or an inland sea changes colour due to concentrated algae growth. When the water turns rust-brown or red it’s called a red tide – even though it’s not actually a tide – but it can also turn brown, purple, green or yellow. These colourations aren’t all harmful to the animals that live in the water, but red tide is a harmful algal bloom that can poison shellfish and other marine life.
WHAT CAUSES RED TIDE?
Seawater contains tiny plant-like organisms called algae. These organisms are so small they’re usually visible only under a microscope. Under certain conditions algae called dinoflagellates can suddenly drastically increase in number. These conditions cause the algae to “bloom”. During an algal bloom a single litre of water can contain up to 20 million dinoflagellates.
Favourable salt levels, wind and sea currents that cause the algae to cluster, a high concentration of nutrients in the water and water temperatures that are higher than usual are the conditions that together can result in a red tide.
Red tides occur in coastal areas, usually to- wards the end of summer and the beginning of autumn. An algal bloom can last anything from a few hours to several weeks, depending on conditions in the area where it occurs.
IMPACT ON THE ENVIRONMENT
Most algal blooms aren’t harmful. In fact an algal bloom can often be a good thing because it provides extra food for sea animals that live on algae. Algae are an important energy source in the ocean’s food chain.
Only a small percentage of the thousands of types of algae are poisonous. These algae produce harmful toxins that can kill fish, shellfish such as oysters, mussels, prawns and crabs, marine animals and birds. In a poisonous red tide you often see dead fish and other sea animals floating in the water.
In February 2015 at Elands Bay in the Western Cape about 100 tons of crayfish walked out onto the beach because the seawater didn’t have enough oxygen for them to survive. This happens when vast amounts of algae die be-
cause there’s not enough food and start rotting. The oxygen in the water gets depleted in the process, forcing the crayfish to catch their breath on the beach.
Dinoflagellates in a poisonous red tide contain a toxin that’s harmful to the central nervous systems of animals and fish. When birds and other sea creatures eat the poisoned fish they too can be poisoned. Even larger animals that eat fish, shellfish or other sea animals can die if they’ve ingested enough of the poison.
RED-TIDE POISONING
A harmful algal bloom is also dangerous for humans. You can get extremely ill if you eat shellfish that’s ingested poisonous algae. Wave action can also cause the poison to contaminate the air, making it difficult to breathe, especially when you’re already ill. Some scientists say it’s even dangerous to swim in the sea during a harmful algal bloom.
Although rare, red-tide poisoning in humans is extremely dangerous. Saxitoxin, one of the toxins released by harmful dinoflagellates, attacks the human central nervous system. For example, if you were to eat mussels from an area where there’s a red tide, you could get paralytic shellfish poisoning (PSP).
Within 30 minutes you might experience symptoms such as a headache and dizziness, nausea, vomiting and diarrhoea, numbing and stinging sensations in your lips, gums, tongue and face, intense thirst and a high pulse rate.
If you don’t get the right medical treatment PSP could paralyse your respiratory system and result in death.
At present there’s no antidote for red-tide poisoning but the patient’s stomach is usually pumped to remove the poison from the body as quickly as possible. So it’s safest not to eat seafood from areas affected by a red tide!
CAN A RED TIDE BE PREVENTED?
Because a red tide poses health risks for humans and marine animals and can have a negative impact on coastal areas and fishing grounds, scientists have been studying the phenomenon for years trying to find ways to forecast it. This way communities could be alerted ahead of time so they can avoid the health risks.
Many scientists believe red tides would occur less often if we used fewer pesticides and prevented them from washing into the sea. But on the whole algal blooms are beyond human control. They’re part of a natural process and nature should take its course.
Governments can monitor red tides though, for example by conducting regular tests on shellfish to ensure safety for human consumption.