Red Tide Facts For Kids

Harmful algal bloom (HAB) is a name for when tiny water plants and microbes grow too much in lakes, rivers, or the sea. Because they grow so fast and in large numbers, they can make life harder for fish, birds, and people who use the water. Some HABs make chemicals called toxins that can be harmful. Other problems happen because lots of dead plants use up the water’s oxygen, so animals can’t breathe. People sometimes call some of these events a red tide, but that name only fits some kinds of blooms.

A HAB happens when conditions in the water help tiny plants and microbes multiply quickly. This can last a few days or go on for many weeks or months. When a bloom grows it may change the color of the water—red, brown, green, or even pink—so people notice it from the shore or from boats.

When a bloom ends, the dead material is broken down by other tiny organisms. This can use up lots of oxygen and create a "dead zone" where fish and other animals may not survive. Some blooms make toxins that can affect animals and people, so scientists watch them closely.

Cyanobacteria are tiny living things often called blue-green algae, but they are actually bacteria. They live in freshwater lakes and rivers and can form thick, green or blue-green scum on the surface when they grow too much. Some kinds, like Microcystis, can make chemicals called microcystins that can hurt the liver or make people and animals sick if swallowed.

Because toxins can float or stay in the water, health officials sometimes issue water-use warnings. Boating, fishing, and drinking water can be affected, so people are told to stay away from water that looks like a bloom.

Dinoflagellates are tiny plant-like cells that live mostly in the ocean. When they grow into large groups along coasts, people often call the event a red tide. A famous example is Karenia brevis, which can make the water look reddish or brownish. But not all dinoflagellate blooms are red, and the word "tide" is misleading because these blooms are not caused by tides.

Some marine blooms make toxins that can affect fish, shellfish, and sometimes cause breathing irritation near shore. Other colorful blooms are caused by different organisms, for example Mesodinium rubrum can look very red because it steals parts of other algae to make its color.

Nutrients are tiny food bits that plants need to grow. In the ocean and lakes, algae also eat these nutrients. When people use lots of fertilizer on farms, let waste flow into rivers, or when rain washes soil and chemicals from cities into water, those nutrients can pile up. This extra feeding of the water is called eutrophication, which just means the water gets too full of food for algae.

Because algae have lots to eat, some kinds can grow very quickly and form a red tide. Too many nutrients make blooms start more often and cover larger areas than they would naturally.

Climate change means the planet is getting warmer in many places. Warmer air makes ocean and lake surfaces warmer too. Many algae grow faster in warmer water, so when waters heat up, blooms can begin sooner and last longer. Also, calmer seas or weaker currents can let algae stay near the surface where sunlight helps them grow.

Scientists have noticed that as waters warm, harmful blooms happen more often in some places. That is why warmer temperatures and changing weather patterns can help red tides become bigger and more common.

Some algal blooms make toxins, which are poisonous chemicals that can make people and animals sick. If water has a bloom, signs or warnings may tell you not to swim, fish, or let pets drink or play in the water. Boiling the water does not remove these toxins, so it is not safe to drink.

If someone touches or breathes spray from the water, they might feel sick within a few hours with stomachaches, vomiting, headaches, or coughing. If that happens, a grown-up should call a doctor or animal clinic for help and follow the posted safety advice.

When red tides happen, beaches and lakes are often closed and signs are posted to keep people safe. That means fewer visitors, which can hurt local businesses that rely on swimming, fishing, and tourism. Towns that expect summer visitors may lose money when people stay away.

Algal blooms can also harm wildlife. When very large blooms die, they use up the water’s oxygen as they rot. Low oxygen, or hypoxia, makes it hard for fish and other animals to live and can create “dead zones.” These places are hard to fix and can change how the whole coastal area works for a long time.

Lake Okeechobee is a very large, shallow lake in Florida. Because it is shallow and gets lots of sunshine, and because nearby farms add extra plant food (called nutrients) to the water, tiny water plants and bacteria can grow fast there. When these tiny plants grow too much they form Harmful algal blooms, which means large patches of algae or bacteria that can cause trouble for animals and water use.

A long water route called the Okeechobee Waterway links the lake to the ocean. It sends water out through the St. Lucie River to the Atlantic and the Caloosahatchee River to the Gulf of Mexico. In the wet summer months, managers often release lake water. Those releases can carry blooms down the rivers and into estuaries and bays, so coastal areas sometimes get affected by the same blooms that started in the lake.