The natural ageing of a lake through nutrient enrichment of its water, leading to excessive growth of algae, is known as:

Eutrophication is the process by which a water body, such as a lake or pond, becomes enriched with nutrients, especially nitrates and phosphates. The word comes from the Greek "eutrophos", meaning "well-nourished".

Natural eutrophication happens slowly over hundreds or thousands of years, as nutrients and sediments are carried into a lake by streams. Over time, the lake becomes shallower and richer in nutrients, and plants grow along its edges. Eventually, the lake may fill up and become a marsh, and then dry land. This is called the natural ageing of a lake.

Cultural or accelerated eutrophication is caused by human activities and happens much faster. Its main sources are:

The stages of eutrophication:

Excess nutrients cause a rapid growth of algae and aquatic plants, called an "algal bloom". The water turns green, and a thick layer of algae covers the surface.

The algal layer blocks sunlight, so plants growing below it cannot photosynthesise and die.

When the algae and plants die, they are decomposed by bacteria, which use up large amounts of dissolved oxygen in the water. This increases the Biochemical Oxygen Demand (BOD).

The oxygen level in the water falls sharply (hypoxia), causing fish and other aquatic animals to die. Large areas with very low oxygen are called "dead zones", as in the Gulf of Mexico.

Some algal blooms, especially of cyanobacteria, also produce toxins harmful to animals and humans.

Water hyacinth (Eichhornia crassipes), an invasive plant called the "terror of Bengal", spreads rapidly in nutrient-rich waters and chokes lakes and rivers.

Indian lakes affected by eutrophication include Dal Lake (Srinagar), Hussain Sagar (Hyderabad) and Bellandur Lake (Bengaluru), which has caught fire and produced toxic foam.

Why not the other options?

• ✗Biomagnification is the increase in concentration of toxic substances, such as DDT and mercury, at higher levels of a food chain.
• ✗Bioremediation uses microorganisms to clean up pollutants.
• ✗Stratification is the layering of water in a lake by temperature.