Algal Blooms: Formation, Reasons and Impact on Aquatic Life

Algal blooms occur when algae grow rapidly and disturb the natural balance of lakes, rivers, and oceans. They often change the colour of the water and reduce its quality. In many cases, they appear suddenly and spread quickly across the surface. Although algae are essential for aquatic life, uncontrolled growth can harm both the environment and human health.

This article provides insights into what is algal blooms are in easy words, with clear examples and practical insights into their effects on living organisms and water ecosystems. It also shows why preventing water pollution is important to keep water bodies healthy.

Table of Contents

• What Are Algal Blooms?
• Types of Algal Blooms
• What Causes Algal Blooms?
• Negative Impacts of Harmful Algal Blooms

What Are Algal Blooms?

Ever noticed a pond turning green or the sea appearing red. These colour changes are often a sign of algal blooms. An algal bloom is the rapid increase in algae in a water body such as a pond, lake, river, or sea.

Algal blooms can appear in both freshwater and marine environments. They are easy to spot because the water changes colour to green, red, brown, or bluish shades. This happens due to pigments found in different types of algae.

The word 'algae' includes both single-celled and multicelled organisms that use sunlight to make food.

For example, the green colour in pond water comes from green algae. Red tides in oceans are caused by dinoflagellates such as Gonyaulax. Giant kelp forests are an example of multicellular algal blooms.

So what are the algal blooms causes?

In most cases, algal blooms occur when large amounts of nutrients like nitrogen and phosphorus enter water bodies. These nutrients help algae grow very quickly.

Now, where do algal blooms usually form?

Let’s find out.

They can appear on the water surface, in shallow regions, on sediments, in the littoral zone near the shore, or even deep within the water column.

Different algae live at different depths depending on how much light and nutrients are available.

Tiny floating plant-like organisms in water are called phytoplankton, while animal life in water is known as metazoa.

Algae reproduce in different ways. Some grow by fragmentation, like pond scum.

Others multiply through cell division or spontaneous multiple division. Some species also reproduce sexually through meiosis.

Before moving on, here are two important terms you should understand.

1. A red tide is a type of harmful algal bloom commonly seen in coastal areas. It changes the colour of the water and releases toxins.

2. Eutrophication is the process in which too many nutrients enter a water body, causing excessive algal growth and oxygen depletion.

Types of Algal Blooms

Now that you know algal bloom causes, let us look at the main types based on where they occur and how harmful they can be.

Type of Algal Bloom	Where It Occurs	Main Cause	Major Effects
Freshwater Algal Bloom	Lakes, ponds, rivers, and aquariums	Excess fertilisers and nutrient runoff	Oxygen depletion and fish death
Ocean Bloom	Seas and oceans	Natural ocean processes and nutrient rise	Causes red tides, kills marine organisms, and lowers oxygen
Harmful Algal Bloom	Freshwater and marine bodies	Toxic algae growth	Releases toxins, causes health disorders, and leads to mass aquatic death

What Causes Algal Blooms?

Algal blooms usually occur when water bodies receive too many nutrients along with the right environmental conditions. Let us look at the main causes one by one to understand this better.

1. One of the most common algal blooms is the high amount of nutrients, such as nitrogen and phosphorus, entering water bodies. These nutrients come from places like farms, factories, and homes.

They mainly enter water through agricultural runoff, sewage discharge, industrial waste, fertilisers, untreated wastewater, and rainwater.

When it rains, nutrients are washed from the soil into rivers, lakes, and oceans. Once inside, they act as food for algae and help them multiply very fast.

2. Not all algal blooms are caused by human activity. Some occur naturally in seas and oceans.

Ocean currents sometimes bring nutrient-rich water from deep layers up to the surface. When this water also gets enough sunlight, algae grow quickly and form blooms.

3. Another major reason is rising water temperature. Due to global warming and ozone layer depletion, water becomes warmer.

Warm conditions break nutrients down faster and make them easier for algae to absorb. As a result, algae grow more quickly than usual.

4. Dead plants and animals also play a role. As they decay, bacteria break them down and release nutrients into the water.

This makes the environment even more suitable for algae to grow and spread.

5. Algal blooms grow more easily in large, calm water bodies. When water movement is low, algae remain undisturbed and spread faster. Still, conditions give algae enough time and space to multiply.

Together, these conditions create the perfect environment for algae to grow out of control.

Negative Impacts of Harmful Algal Blooms

And it’s important to note that harmful algal blooms (HABs) occur when certain algae grow quickly and release toxins or disturb aquatic ecosystems. As a result, they affect wildlife, humans, and the environment.

1. First, HABs block sunlight and reduce oxygen in water, making survival difficult for aquatic organisms. When algae die and decompose, oxygen demand increases further, which leads to fish suffocation and higher mortality.

Because of this, seabirds, sea turtles, marine mammals, and fin fish are commonly affected.

2. Next, humans are exposed when toxins enter the food chain, mainly through shellfish. Since shellfish store these toxins, eating contaminated shellfish can cause neurotoxic, paralytic, diarrhoeal, or amnesic shellfish poisoning.

Symptoms may range from stomach discomfort to paralysis and nervous system damage.

3. In addition, HABs harm the environment and economy. They lower water clarity, damage aquatic habitats, affect tourism and fishing, and reduce recreational use of water bodies.

4. However, not all algae are harmful. About 2% of phytoplankton species are toxic and are classified as HABs. A common example is red tides caused by Gonyaulax.

5. Finally, the impact of blooms depends on nutrient levels, water depth, sunlight, and water movement. As blooms grow rapidly, they reduce light for bottom-dwelling organisms, lower oxygen levels, and disrupt the entire ecosystem.

Overall, harmful algal blooms trigger a chain reaction that threatens aquatic life, public health, and environmental balance.

So far, we have discussed how algal blooms may seem natural, but they are deeply connected to pollution and climate change. While some blooms occur naturally, most are a result of human activities. Protecting water bodies, reducing fertiliser use, and proper waste treatment are the only ways to control their impact. Knowing what is algal blooms are today can help protect water resources tomorrow.