Prime contaminant of lake for eutrophication is

The correct answer is Option (3) → Nitrate and phosphate

Eutrophication is the ageing of a lake as its water is enriched with nutrients. Human wastes make it happen much faster; this is called cultural or accelerated eutrophication.

Option (1) Dissolved oxygen — Incorrect. Dissolved oxygen is not a contaminant. In fact eutrophication robs the water of its dissolved oxygen, and fish die.

Option (2) Algal bloom — Incorrect. An algal bloom is the result of eutrophication, not its cause. The extra nutrients make the algae grow too much.

Option (3) Nitrate and phosphate — Correct. These come in with sewage and with farm and industrial wastes. They act as plant nutrients and overstimulate the growth of algae.

Option (4) Fungi and Bacteria — Incorrect. Decomposers multiply when the algae die, but they are not what enriches the lake.

Note: This question is from Chapter 16, Environmental Issues, of the pre-rationalised NCERT book. This chapter is not in the present NCERT book (2026-27). The extract below is from the pre-rationalised edition.

NCERT Extract (Pre-rationalised edition, Chapter 16, Environmental Issues):
Domestic Sewage and Industrial Effluents – “… Eutrophication is the natural aging of a lake by nutrient enrichment of its water. In a young lake the water is cold and clear, supporting little life. With time, streams draining into the lake introduce nutrients such as nitrogen and phosphorus, which encourage the growth of aquatic organisms. As the lake’s fertility increases, plant and animal life burgeons, and organic remains begin to be deposited on the lake bottom. Over the centuries, as silt and organic debris pile up, the lake grows shallower and warmer, with warm-water organisms supplanting those that thrive in a cold environment. Marsh plants take root in the shallows and begin to fill in the original lake basin. Eventually, the lake gives way to large masses of floating plants (bog), finally converting into land. Depending on climate, size of the lake and other factors, the natural aging of a lake may span thousands of years. However, pollutants from man’s activities like effluents from the industries and homes can radically accelerate the aging process. This phenomenon has been called Cultural or Accelerated Eutrophication. During the past century, lakes in many parts of the earth have been severely eutrophied by sewage and agricultural and industrial wastes. The prime contaminants are nitrates and phosphates, which act as plant nutrients.”