Question

Profundal zone is
A. Aphotic zone in deep water
B. Euphotic zone in open water
C. Disphotic zone in deep water
D. Dark bottom of deep-water body

Answer 1

The deep zone of an inland body of free standing water, such as a lake or pond which is located below the range of effective light penetration is known as profundal zone. This zone is typically below the thermocline which is the vertical zone in water by which temperature drops down rapidly. In some seasons, the temperature difference might be large enough to make it difficult to mix with the littoral zone which can cause a decrease in oxygen concentrations.

Complete answer:
Option A) Aphotic zone in deep water: The portion of a lake or ocean where there is little or no sunlight is termed as aphotic zone. Profundal zone is often a part of the aphotic zone. The sediments of the profundal zone comprises primarily silt and mud. Profundal is also defined as the deepest, vegetation free, and muddy zone of lacustrine benthal.
Hence option A is the correct answer.
Option B) Euphotic zone in open water: The layer closer to the surface that receives enough light for photosynthesis to occur is known as the euphotic zone and is also known as photic zone. But the profundal zone does not have any kind of light penetration and is deep down the water body.
Hence option B is not correct.
Option C) Disphotic zone in deep water: Also known as the twilight zone, Disphotic zone is the layer of water with enough light for the predators to see but not enough light for the rate of photosynthesis to be greater than rate of respiration. There is a small percentage of light penetration in the Disphotic zone.
Hence option C is not correct.
Option D) Dark bottom of deep-water body: Dark bottom of deep-water body is the midnight zone or bathypelagic zone, which extends from $1000$ meters to the bottom deep-water benthic zone. Profundal zone is not the dark bottom of the deep-water body.
Hence option D is not correct.

Hence, Option A) Aphotic zone in deep water is the correct answer.

Note:
The type of biological community that can live in profundal zone is determined by the lack of light and oxygen in this region. This is distinctly different from the community in overlying waters. The profundal macrofauna is hence characterized by the physiological and behavioral adaptations to lower oxygen concentrations. Organisms rely on detritus sinking from the photic zone due to the low productivity of the profundal zone.