NDA (II) 2018 General Ability Test · Question 77 of 146

Correct — B, the halocline. The word is built from the Greek hals, salt, and klinein, to slope, and it names the layer in a water column where salinity changes sharply with depth. That is the definition the question asks for, word for word. The three ocean 'clines' are named for whichever property is doing the sloping — heat, salt or density — so once you know that the prefix carries the property, the item answers itself.

• (a)Thermocline — The thermocline is the layer of sharp temperature change with depth — thermo is heat. It separates the warm, wind-mixed surface water from the cold deep water, and it is usually the strongest of the three transitions in the open tropical ocean, which is why it is the most tempting wrong answer.
• (c)Photic zone — The photic or euphotic zone is defined by light, not by any property of the water itself — it is the sunlit upper layer, roughly the top 200 m, in which enough light penetrates for photosynthesis. It is a zone, not a cline, and salinity plays no part in its definition.
• (d)Pycnocline — The pycnocline is the layer of sharp density change — pyknos is dense. It is a real feature and it is genuinely related to salinity, because density depends on both salt and temperature, so a halocline and a thermocline together often produce a pycnocline. But it measures the combined result, not salinity by itself.

Ocean water is not uniform from top to bottom; it settles into layers, and the boundaries between them are called clines. Each cline is named for the property that changes steeply across it: the thermocline for temperature, the halocline for salinity, the pycnocline for density. Because seawater density rises with salinity and falls with temperature, a pycnocline is generally produced by a thermocline, a halocline, or both acting together. These layers matter because a strong cline resists vertical mixing, keeping surface nutrients from being replenished from below.

This is a vocabulary question dressed as a geography question, and the fastest route is the prefix. Therm- for heat, halo- for salt, pycno- for density, photo- for light. A student who has met only 'thermocline' in class will reach for it because it is the familiar word, but the stem specifies salinity, and only one option carries the salt prefix. Note also that a halocline is strongest where fresh water meets salt water — near a river mouth, in the Baltic, or in the Arctic where melting ice caps a saltier layer below.

• A halocline is a layer in a water column across which salinity changes sharply with depth; the name comes from the Greek for salt and to slope.
• A thermocline marks a sharp change in temperature with depth, and a pycnocline a sharp change in density.
• Seawater density depends on both temperature and salinity, so a thermocline and a halocline can combine to form a pycnocline.
• The photic zone is defined by light penetration — the sunlit upper layer, about the top 200 m, where photosynthesis is possible.

• Reaching for 'thermocline' because it is the word most often taught, when the stem clearly specifies salinity.
• Treating the pycnocline as interchangeable with the halocline — density is the outcome, salinity is one of its two causes.
• Classing the photic zone with the clines; it is defined by light from outside the water, not by a gradient within it.

NDA asks either for the meaning of one of the ocean clines, or for the factors that control salinity and its distribution across ocean basins.

No directly related past PYQ was found.

• practice — not a real PYQ

  The layer of an ocean across which temperature falls rapidly with increasing depth is called the

  • (a)halocline
  • (b)thermocline
  • (c)photic zone
  • (d)benthic zone

  Answer(b) thermocline — it separates the warm, wind-mixed surface water from the cold water below.

• practice — not a real PYQ

  The density of seawater is determined mainly by

  • (a)temperature and salinity
  • (b)depth and light penetration
  • (c)wave height and tidal range
  • (d)dissolved oxygen alone

  Answer(a) temperature and salinity — which is why a thermocline and a halocline together produce a pycnocline.