With reference to Eutrophication, which of the following statements is/are correct ?
1. It is a phenomenon of nutrient enrichment of a water body.
2. It depletes dissolved oxygen of the water.
Select

Question

With reference to Eutrophication, which of the following statements is/are correct ?
1. It is a phenomenon of nutrient enrichment of a water body.
2. It depletes dissolved oxygen of the water.
Select the correct answer using the codes given below :

UnlockIAS · Accepted Answer

Correct — C, Both 1 and 2. Statement 1 is the textbook definition: eutrophication is the enrichment of a water body with plant nutrients, chiefly nitrogen (nitrates) and phosphorus (phosphates), reaching it from untreated sewage, phosphate detergents and fertiliser run-off. Statement 2 states its defining consequence: the extra nutrients trigger a dense algal bloom; the bloom cuts off light to submerged plants and then dies; aerobic bacteria decomposing that mass of dead organic matter consume the dissolved oxygen of the water (which is what a high BOD measures). Oxygen falls to hypoxic levels, fish and other aerobic organisms suffocate, and the water body degrades. Cause and effect are both correctly stated, so the answer is 'Both 1 and 2'.

Why the other options are wrong — (a) Wrong — statement 1 is correct, but so is statement 2. Oxygen depletion is not an incidental side effect of eutrophication; it is the change that makes eutrophication an ecological problem at all.  (b) Wrong — statement 2 is correct, but statement 1 cannot be excluded: nutrient enrichment IS the definition of eutrophication, not merely one of its causes.  (d) Wrong — both statements are correct. This option would only work if eutrophication were something other than nutrient enrichment and left dissolved oxygen untouched, which is the opposite of what happens.

Concept: Eutrophication is the nutrient enrichment of a water body and the chain of changes that follows. Nitrogen and phosphorus loading sets off an explosion of algae and phytoplankton; the bloom blankets the surface, shades out rooted and submerged vegetation, and then dies; aerobic decomposers break down the dead biomass and in doing so strip the water of dissolved oxygen. The result is hypoxia, fish kills, loss of species and eventually foul-smelling anaerobic water. Lakes age and fill in naturally over geological time, but the human-accelerated version — driven by sewage, detergents and fertiliser run-off — is called cultural eutrophication.

State PCS 2019 Q21 — Environment & Ecology PYQ with Answer

State PCS 2019 Q21 — Environment & Ecology PYQ with Answer