Consider the following statements regarding the eutrophication of the lake:

Question

Consider the following statements regarding the eutrophication of the lake:

i. It is an anthropogenic process and does not occur naturally.

ii. Nitrate and phosphate are the main contaminants for eutrophication.

The correct answer/s is/are:

1. A. i only
2. B. ii only (Correct answer)
3. C. Both i and ii
4. D. Neither i nor ii

Correct Answer

Option B — ii only

Detailed Solution & Explanation

The correct answer is ii only.

Key Points

• Statement i is incorrect. Eutrophication is not exclusively anthropogenic. Natural eutrophication is the slow ageing of a lake over centuries or millennia, as nutrients and sediment accumulate and the waterbody gradually fills in and becomes a marsh. What human activity has done is to accelerate this enormously — cultural eutrophication.
• Statement ii is correct. Nitrates and phosphates are the principal nutrients driving eutrophication, arriving from fertiliser runoff, untreated sewage and detergents.

Additional Information

• The sequence of events in eutrophication:
  • Nutrient enrichment triggers an algal bloom, forming a dense mat over the surface
  • The mat blocks sunlight, killing submerged plants that would otherwise oxygenate the water
  • The bloom dies and is decomposed by aerobic bacteria, which consume dissolved oxygen
  • BOD rises and dissolved oxygen falls, producing hypoxia and fish kills, and creating dead zones
  • Anaerobic decomposition follows, releasing hydrogen sulphide and methane
• Harmful algal blooms of cyanobacteria can release toxins — microcystins, anatoxins — that poison livestock, wildlife and people, and make water unfit for supply.
• Phosphorus is usually the limiting nutrient in freshwater, and nitrogen in marine systems, which is why phosphate detergent bans have been effective for lakes.
• India's affected waterbodies: Dal Lake, Loktak Lake, Chilika, the Hussain Sagar and the Ulsoor and Bellandur lakes of Bengaluru, whose foaming and periodic fires became emblematic of urban sewage overload.
• Related concepts: BOD (Biochemical Oxygen Demand) measures the oxygen required by microorganisms to decompose organic matter — a high BOD indicates heavy organic pollution; COD (Chemical Oxygen Demand) measures the oxygen needed for chemical oxidation, and is always higher than BOD for the same sample.
• The Gulf of Mexico dead zone, fed by fertiliser runoff from the Mississippi basin, is the largest and most studied example worldwide.