Algal bloom

An algal bloom or algae bloom is a fast increase in the number of algae in fresh water or marine water. These blooms can be safe or harmful, depending on the type of algae and the place. We often see algal blooms when the water changes color because of the pigments in the algae.

Algae include many types of water organisms, from large plants like seaweed to tiny single-celled organisms like cyanobacteria. When we talk about algal blooms, we usually mean the quick growth of these tiny algae, not the larger seaweed. One example of a large algae bloom is a kelp forest.

Algal blooms happen when nutrients like nitrogen or phosphorus from sources such as fertilizer runoff or other nutrient pollution enter the water. This makes the algae grow too quickly, which can harm the whole ecosystem. Some effects are safe, like providing food for other animals, but harmful blooms can block sunlight and lower oxygen levels in the water.

Algae are also very important because they help make about 70 % of Earth's oxygen, which keeps life on land alive.

Description

An algal bloom is when lots of tiny plants called algae grow very fast in water, like in lakes, rivers, or the ocean. This can make the water look green, red, or brown.

Algae need food to grow, like nutrients such as nitrogen or phosphorus. When these nutrients get into the water — from rain washing farm dirt into rivers or from sewage — the algae can grow faster. Sometimes, this fast growth helps other sea creatures by giving them food. But sometimes, the blooms can be harmful because some algae can make toxins or hurt fish and other water animals.

Freshwater algal blooms

Further information: Eutrophication § Freshwater systems

Freshwater algal blooms happen when there are too many nutrients, like phosphates, in the water. These nutrients often come from fertilizers on land or cleaning products in homes.

When there are too many nutrients, algae can grow very fast. When the algae die, tiny creatures in the water break them down. This uses up oxygen in the water, which can harm fish and other water animals. In some cases, like in fish tanks with too much food, this can also cause problems for the fish.

Algal blooms can sometimes happen naturally, even in lakes that don’t usually have many nutrients. Some types of algae, called cyanobacteria, are good at getting nutrients from the air and water. They can move around in the water to find nutrients, which helps them grow even when nutrients are scarce. Natural changes in the water, like when the surface freezes or melts, can also mix nutrients and cause blooms.

Marine algal blooms

Further information: Eutrophication § Coastal waters

In the summer, storms mix the ocean and bring extra nutrients to the surface. This helps tiny plants called phytoplankton grow quickly in large numbers. These plants have green pigments called chlorophyll that help them use sunlight. This green color helps scientists see these blooms from space.

Satellites can find these blooms by measuring how much chlorophyll is in the water. Usually, the blooms last until late spring or early summer. Small animals called zooplankton then start eating them. A big research project called NAAMES, done between 2015 and 2019, studied how these tiny plants affect the air and weather.

In France, people help scientists by telling them when they see colorful water. This helps learn more about these blooms. Sometimes, wildfires can also cause these blooms by sending particles into the ocean.

The NAAMES study looked at how phytoplankton affect atmospheric aerosols and clouds.

Wildfires can lead to phytoplankton blooms by adding particles to the ocean.

Harmful algal blooms

Main article: Harmful algal blooms

A harmful algal bloom (HAB) is when algae grow very fast and can hurt other living things. This can happen because the algae make natural toxins or damage other organisms. These blooms can be hard to manage and cause problems, especially near coasts. They can be linked to events where many sea animals die and can make people sick if they eat shellfish.

People can get sick from HABs by eating seafood with toxins, swimming in the water, or breathing in tiny droplets in the air that contain toxins. This can lead to illnesses affecting different parts of the body. Beachgoers sometimes get sick with breathing problems, eye and nose irritation, fever, and need medical help. A common illness from these blooms is ciguatera fish poisoning (CFP). Drinking water can also get contaminated by toxins from these algae.

When there is a lot of algae, the water can change color, looking purple, pink, red, or green. However, not all algal blooms make the water look different.

Bioluminescence

Some types of algae called Dinoflagellates can glow in the dark and also make toxins. They create a blue light at night, and during the day, the water can look red or brown, which is sometimes called Red Tides. These algae have been linked to poisoning in seafood because of the toxins they produce.

Management

There are three main ways to manage algal blooms: fixing problems after they happen, stopping them before they start, and controlling them directly.

To fix problems after they happen, we watch for harmful algae and toxins. This helps us keep unsafe food away from people. We can also move fish away from harmful areas.

To stop algal blooms before they start, we can reduce dirty water that flows into rivers and lakes. Using special surfaces that let water soak through, like certain streets and parks, helps keep nutrients out of the water. Planting more trees, gardens, and native plants also helps. Farmers can help by using fewer fertilizers and keeping animals away from streams.

To control algal blooms directly, there are several methods. We can spread special clay in the water to gather up the algae. We can also use natural ways, like special smells or adding certain fish, to reduce algae growth. There are also ways to change the genes of some plants, but these can sometimes harm other plants and animals. Changing how we move water can also help.

Environmental impacts

Harmful algae blooms can cause problems for the environment and are happening more often. These blooms can create areas in the water with very little oxygen, which can harm fish and other creatures. They can also make the water unsafe for people to swim in.

These blooms affect places like the Great Lakes and the St. Lawrence River. Some small creatures in the water, like mussels, can make these blooms worse by changing the water’s chemistry. In August 2024, a type of toxic algae along the coast of California made sea lions sick. These sea lions acted strangely because they were not feeling well. The toxins from the algae can move through the food chain, affecting many animals.

Impact of Climate Change

Algae blooms have become more common and stronger in recent years. This is because oceans and other waters are getting warmer, which helps algae grow more and for a longer time. Climate change can also sometimes reduce algae blooms if heavy rains wash them away from lakes and seas.

Scientists think that in the future, algae blooms might spread further north, especially in areas like the North Sea.

Human Interference

People can sometimes help cause algal blooms without meaning to. When farmers use fertilizers on their crops, rain can carry these fertilizers into rivers and lakes. This extra "food" from the fertilizers can make algae grow faster and in bigger groups.

Fossil record

The first known algal bloom is called Eoseira wilsonii. It lived in a time called the Eocene and was found in places named Horsefly Shale and the Eocene Okanagan Highlands. This algae is a special kind called a diatom.