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What is "Eutrophication" and "Algal bloom" ?

news-details Image Source Mar 03, 2021 20:13 IST , Updated: Jun 09, 2021 20:13 IST · 3 min read

What is "Eutrophication" ?

Eutrophication is an enrichment of water by nutrient salts that causes structural changes to the ecosystem such as: increased production of algae and aquatic plants, depletion of fish species, general deterioration of water quality and other effects that reduce and preclude use.

When algae start to grow in an uncontrolled manner, to decompose all the dead algae, an excessive consumption of oxygen (almost total) is required by microorganisms. This creates an anoxic (oxygen-free) environment on the lake bottom, with the growth of organisms capable of living in the absence of oxygen (anaerobic).

The absence of oxygen reduces biodiversity causing, in certain cases, even the death of animal and plant species.

All this happens when the rate of degradation of the algae by microorganisms is greater than that of oxygen regeneration, which in summer is already present in low concentrations.

The two most acute phenomena of eutrophication are hypoxia in the deep part of the lake (or lack of oxygen) and algal blooms that produce harmful toxins, processes that can destroy aquatic life in the affected areas.

Eutrophication occurs naturally over centuries as lakes age and are filled in with sediments, however, human activities have accelerated the rate and extent.

The cultural eutrophication process consists of a continuous increase in the contribution of nutrients, mainly nitrogen and phosphorus until it exceeds the capacity of the water body, structural changes mainly depend on 3 factors:

1) The use of fertilisers in the soil contribute to the accumulation of nutrients. When these nutrients reach high concentration levels and the ground is no longer able to assimilate them, they are carried by rain into rivers and groundwater that flow into lakes or seas.

2) Waste water is discharged directly into water bodies, the result of this is the release of a high quantity of nutrients which stimulates the disproportionate growth of algae.

3) Over the years, lakes accumulate large quantities of solid material transported by the water (sediments). These sediments reduce the self purification capacity of water body by absorbing large amounts of nutrients and pollutants.

What is "Algal bloom" ?

An algal bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems, and is often recognized by the discoloration (green, blue-green, red, or brown) in the water from their pigments.

The term algae encompasses many types of aquatic photosynthetic organisms, both macroscopic, multicellular organisms like seaweed and microscopic, unicellular organisms like cyanobacteria.

Algal blooms are the result of a nutrient, like nitrogen or phosphorus from fertilizer runoff, entering the aquatic system and causing excessive growth of algae.

Excessive growth of algae can block sunlight and stunt the growth of other plants, which may provide important habitats for aquatic animals.

A harmful algal bloom (HAB) occurs when toxin-producing algae grow excessively in a body of water.

As more algae and plants grow, others die, this dead organic matter becomes food for bacteria that decompose it. With more food available, the bacteria increase in number and use up the dissolved oxygen in the water.

When the dissolved oxygen content decreases, many fish and aquatic insects cannot survive - this results in a dead area.

Phytoplankton (microalgae)

Phytoplankton, also known as microalgae, are similar to terrestrial plants in that they contain chlorophyll and require sunlight in order to live and grow, obtain their energy through photosynthesis.

Phytoplankton also require inorganic nutrients such as nitrates, phosphates, and sulfur which they convert into proteins, fats, and carbohydrates.

In a balanced ecosystem, phytoplankton provide food for a wide range of sea creatures including shrimp, snails, and jellyfish.

When too many nutrients are available, phytoplankton may grow out of control and form harmful algal blooms (HABs). These blooms can produce extremely toxic compounds that have harmful effects on fish, shellfish, mammals, birds, and even people.

Phytoplankton are extremely diverse, varying from photosynthesising bacteria (cyanobacteria), to plant-like diatoms, to armour-plated coccolithophores.

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