What is an Algal Bloom?

Algae are protists that are capable of producing their own food via photosynthesis. They are widespread globally and present in various sizes, shapes, and colours depending upon the surrounding ecosystem. In an aquatic ecosystem, the rapid increase in the population of algae is known as an algal bloom.

An algal bloom occurs due to a sudden spike of nutrients (such as nitrogen or phosphorus) into an aquatic ecosystem, either in a marine environment or in a freshwater environment. Algal bloom might be of many types-

Unicellular algal bloom characterises just the discolouration of surface water.
Photosynthetic algae cause green colouration of the water body.
Dinoflagellates like Gonyaulax cause red colouration of coastal water- a phenomenon known as ‘red tides’.
Giant kelp forests are stable ecosystems. Blooms mainly involve unicellular/microscopic algae.

Causes of Algal Bloom

Oversupply of nutrients

Nitrogen-rich or phosphorus-rich fertilisers can get washed from fields into rivers and streams, due to irrigation and rainfall.
Untreated or partially treated sewage contains nitrogen-containing compounds, which, if leaked, flow out of the sewage treatment plant into nearby water bodies.
Water contaminated with minerals, nitrogen, and phosphorus provides a rich nutrient supply to the growing algae, leading to an algal bloom.

Presence of Dead Organisms

Algae are primary producers; they don’t feed on dead matter. Instead, decomposers release nutrients from dead matter, which algae then absorb.

Effect of Global Warming

Overavailability of mineral nutrients is not enough for an algal bloom.
An optimum temperature is necessary for the decomposition of the compounds, so that the algae can use them.
Warmer temperatures can exacerbate blooms by boosting nutrient cycling and algal metabolism, but algal blooms also depend on light and stratification of water. This increases the rate of decomposition.

Stagnant Water

If a stagnant water body is contaminated, the chemical compounds are not diluted or refreshed over time. Moreover, if it remains unchecked, the concentration keeps on increasing.
An abundance of nutrition to the algae results in an algal bloom.

Effects of Algal Bloom

Most algae species contain the necessary pigments for photosynthesis and produce oxygen as a byproduct. They are considered one of the most important species that generate vast amounts of oxygen through their photosynthesis. Thus, most algae are harmless and are an important part of the food web.

But certain algae can grow quickly to cover all or portions of a water body as an algal bloom; even if the algal bloom is large, it is not necessarily harmful. However, some species of algae, like blue-green algae (cyanobacteria), can produce toxins that can harm other organisms; blooms of such algal species are referred to as harmful algal blooms (HABs).

HABs produce toxins that can kill aquatic organisms; even if the algae are non-toxic, they can affect the coral reefs, clog the gills of aquatic organisms, and smother submerged aquatic vegetation. Decomposition of dead algae and other aquatic organisms increases BOD (more oxygen is required), which reduces dissolved oxygen available for aquatic life. HABs can cause human illnesses (respiratory issues, food poisoning via shellfish, skin rashes). They are rarely fatal, but can be harmful.
HABs discolour water, produce smelly piles on beaches and coastal regions, and contaminate drinking water.

Frequently Asked Questions (FAQs)

Q1. Why do algal blooms often occur near agricultural areas?

A. Fertilisers present in the agricultural areas are rich in nitrogen and phosphorus. They are washed into water bodies by rainfall or irrigation. This fuels algal growth.

Q2. Are all algal blooms harmful?

A. Many are not, and they even support ecosystems. The harmful algal blooms (HABs) release toxins or deplete oxygen, and this is how they damage aquatic life.

Q3. How does an algal bloom affect humans?

A. HABs can contaminate drinking water, cause respiratory issues, and lead to food poisoning through seafood consumption.