UPPSC 2022 Prelims GS Paper-I · Question 131 of 150

Correct — B, Both (A) and (R) are true and (R) is the correct explanation of (A). Take the two propositions in turn. The Reason is a definition and is plainly true: in the standard zonation of a lake, the limnetic zone is the open-water zone — the water lying beyond the shallow, rooted-plant fringe, out where nothing is anchored to the bottom. The Assertion is also true: phytoplankton, the free-floating microscopic algae and cyanobacteria of the water column, are at their most abundant exactly there, and the limnetic zone is where a lake's primary production is dominated by them rather than by rooted plants. The whole question then turns on whether the second sentence explains the first, and it does. 'Open water' is not an incidental label; it is the same thing as 'the sunlit, well-illuminated layer'. Because the limnetic zone is open and free of shading, light penetrates it — it is also called the photic zone — and sunlight there supports photosynthetic algae and the organisms that feed on them. Being unattached, phytoplankton must stay suspended in that lit layer to survive, which they do by forming gas vesicles and vacuoles or by changing shape to increase drag. Openness gives light, light drives photosynthesis, photosynthesis lets free-floating producers multiply: the causal chain runs straight from R to A. That is the definition of 'R is the correct explanation of (A)', which on this booklet is printed at option (b).

• (a)(A) is true but (R) is false — The Reason is not false — it is simply the textbook definition of the term. Lake zonation runs littoral (the shallow near-shore belt with rooted plants), limnetic (the open water beyond it), profundal (the deep water below light penetration) and benthic (the bottom sediment). 'Limnetic zone is open water zone' restates the second of these correctly, so this option fails on its own terms.
• (c)(A) is false but (R) is true — The Assertion is not false either. Phytoplankton are precisely the producers of the open-water zone: the littoral belt's production comes largely from rooted and emergent plants, the profundal zone lies below usable light and runs on detritus sinking from above, and only the limnetic zone is both lit and unobstructed enough for free-floating algae to dominate. Marking (A) false usually comes from confusing 'abundance' with 'visible vegetation' — phytoplankton are microscopic, so an open lake surface looks empty while being the most productive part of it.
• (d)Both (A) and (R) are true but (R) is not the correct explanation of (A) — This is the option the question is really built to catch, and the one that needs a deliberate decision rather than a reflex. Both statements are indeed true, so the first half of this option is right; it fails on the second half. Openness is not a separate fact that merely happens to sit alongside phytoplankton abundance — it is the cause of it, because an open water column is an illuminated water column, and illumination is what free-floating photosynthesisers need. If the Reason had instead said something true but inert, such as 'the limnetic zone lies above the profundal zone', then (d) would have been correct. Judge the link, not just the two truth values.

A standing freshwater body such as a lake is divided horizontally and vertically into four life zones, and each is defined by how much light reaches it and whether organisms are attached to a substrate. The LITTORAL zone is the shallow water near the shore, well lit and shallow enough for rooted and emergent plants. The LIMNETIC zone is the open water beyond the rooted plants, down to the depth light can usefully penetrate; because it is also the photic zone, its producers are the free-floating phytoplankton, and the whole open-water food web — zooplankton, then small fish, then larger fish — rests on them. The PROFUNDAL zone is the deeper water into which sunlight does not effectively reach, so no photosynthesis sustains it and its food web is based on detritus falling in from the littoral and photic zones above. The BENTHIC zone is the bottom sediment itself, the domain of decomposers and burrowers. The single organising variable across all four is light.

Assertion-Reason items are not answered by checking two truth values and stopping. They are answered by asking whether the Reason supplies the mechanism the Assertion needs. Here the Assertion states an observation (phytoplankton are abundant in a particular zone) and the Reason states a property of that zone (it is open water). Ask yourself: does 'open water' explain 'phytoplankton thrive'? It does, once you notice that in limnology 'open water zone' and 'sunlit photic zone' describe the same layer — openness is what admits the light, and light is what phytoplankton run on. Being unattached organisms, they cannot exploit the shaded profundal depths or compete with rooted macrophytes in the littoral fringe; the open, lit layer is their niche by construction. Two exam-specific cautions. First, do not let the vagueness of the Reason's wording push you to (d) — a short definitional Reason can still be the correct explanation. Second, this 2022 paper scrambles the assertion-reason code: the meaning candidates habitually look for at (a) is printed at (b) here, and no option keeps its usual sense. Always read the four printed lines before marking.

• Lake zonation: littoral = shallow near-shore water with rooted plants; limnetic = the open water beyond them; profundal = deep water below effective light penetration; benthic = the bottom sediment.
• The limnetic zone is also called the photic zone — sunlight penetrating it supports photosynthetic algae and the species that feed on them, and the rate of photosynthesis falls with depth as light is attenuated.
• Phytoplankton are free-floating; they stay suspended in the lit layer by forming gas vesicles and vacuoles or by changing shape to increase drag, rather than by attaching to anything.
• The profundal zone receives no usable sunlight, so its food web is based on detritus entering from the littoral and photic zones rather than on local photosynthesis.
• This paper's assertion-reason code, in full: (a) = '(A) is true but (R) is false'; (b) = 'Both true and (R) IS the correct explanation'; (c) = '(A) is false but (R) is true'; (d) = 'Both true but (R) is NOT the correct explanation'. Not one of the four carries its usual UPSC meaning.

• Treating an Assertion-Reason item as two independent true/false checks and never testing the causal link between them
• Choosing 'both true but R does not explain A' as the cautious answer when the Reason is short or definitional — brevity is not irrelevance
• Assuming the four assertion-reason options carry their usual UPSC meanings; in this 2022 paper all four are moved

UPPSC likes to name one lake zone and ask what lives there, or to pair the zone with its defining property in an Assertion-Reason frame, as here. UPSC prefers to test the same ecology functionally — what phytoplankton do for the carbon sink and the food chain, or what would follow if they disappeared. Either way the anchor is the same: phytoplankton are the free-floating producers of the sunlit open water.

• practice — not a real PYQ

  In the zonation of a lake, the region that lies below the depth of effective light penetration and whose food web depends on detritus sinking from above is the

  • (a)Littoral zone
  • (b)Limnetic zone
  • (c)Profundal zone
  • (d)Benthic zone

  Answer(c) Profundal zone — it is the deep water lying below effective light penetration, so no photosynthesis sustains it and it runs on detritus from the lit zones above. The benthic zone is the bottom sediment itself, not the deep water column.

• practice — not a real PYQ

  Which one of the following best explains why free-floating phytoplankton, rather than rooted plants, dominate primary production in the open-water zone of a lake?

  • (a)The open water is richer in dissolved oxygen than the shore
  • (b)The open water is too deep for rooting yet still receives sunlight
  • (c)The open water is warmer than the shallow margins throughout the year
  • (d)The open water has a higher salt concentration than the margins

  Answer(b) The open water is too deep for rooting yet still receives sunlight — depth rules out anchored macrophytes while light still permits photosynthesis, so the producers must be organisms that can stay suspended in the lit layer.