Algal Blooms

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Have you ever noticed why the colour of the pond is green? Have you ever heard about the phenomenon of red tides? These are the effects of algal blooms. Let us find more about algal blooms in the following article.

What are Algal Blooms?

“An algal bloom can be defined as a rapid increase in the population of algae in an aquatic ecosystem.”

Algal blooms can be found in either a marine environment or a freshwater environment. The phenomenon can be easily recognised by the discolouration on the surface of the water. It usually occurs due to the introduction of a nutrient (such as nitrogen or phosphorus) into an aquatic ecosystem.

Algae is a term that is used to describe both unicellular and multicellular, photosynthetic organisms. Discolouration of the water characterises a unicellular algal bloom. The green colouration of pond water is caused due to photosynthetic green algae. The red colouration of coastal water known as ‘red tides’ is caused by the dinoflagellates like Gaunyolax. Giant kelp forests are examples of a multicellular algal bloom.

What Causes Algal Blooms?

The most common cause of algal blooms is a sudden increase of nutrients which are typically needed by algae for growth. These nutrients are usually introduced into an aquatic ecosystem through agricultural runoffs, sewage runoffs and other anthropogenic causes.

However, algal blooms can also be caused naturally. Ocean currents drive nutrients to the surface from the depths, and the abundance of nutrients, combined with sunlight on the ocean surface provides a very favourable environment for the algae to grow. These conditions will lead to the rapid growth of algae.

Are Algal Blooms Harmful?

In a marine environment, single-celled, autotrophic organisms form the base of the food web. These organisms are called phytoplankton, and about 5,000 species have been discovered to date.

Of these 5,000 species, 2% of the phytoplankton species are known to be toxic. These toxin-producing species are termed Harmful Algal Blooms (HABs). For example, red tides caused by Gaunyolax is a HAB. They cause damage by producing biological toxins or interfering with mechanical functions or organisms. HABs are also responsible for mass die-offs, where a large population of species dies after consumption of the toxic algae.

Harmful algal blooms most notably affect sea birds, sea turtles, marine mammals and many species of finfishes. Though marine life is affected the most, the toxins can travel through the food chain and enter the human body. This can cause adverse repercussions from gastrointestinal ailments to paralysis.

One of the most common ways humans ingest the toxin is through shellfish poisoning. Shellfish are filter-feeders, which means the toxin found in harmful algal blooms can bioaccumulate within the body of the shellfish. Consuming the affected meat can cause various syndromes such as Neurotoxic shellfish poisoning, paralytic shellfish poisoning, diarrheal shellfish poisoning and amnesic shellfish poisoning.

Algal blooms affects the aquatic organisms by reducing the penetration of sunlight. They increase fish mortality by clogging their gills. When the dead algae and other aquatic organisms decompose inside the water, the BOD of water gets reduced which further affects the aquatic organisms. The algal bloom also reduces the aesthetic value of water bodies and impacts recreation and business values.

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Frequently Asked Questions on Algal blooms


What is an algal bloom and what causes it?

The rapid increase in the population of algae in a marine or freshwater body is known as algal bloom. It is caused by an increase in nutrient (such as nitrogen or phosphorus) concentration in the aquatic ecosystem.


What are HABs?

The Harmful Algal Blooms are abbreviated as HABs. These HABs produce toxins in the water body which are harmful to the organisms consuming the water. The toxin can enter the human body through bioaccumulation in the food chain. E.g: red tides caused by Gaunyolax is a HAB.


Why is an algal bloom harmful?

Algal blooms are harmful because some algae releases toxins into the water.


How does algal blooms affect the environment?

Algal blooms increase fish mortality. When the dead algae and other aquatic organisms decompose inside the water the BOD of water gets reduced which further affects the aquatic organisms.

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