Question

ASSERTION :Oceans are a low productivity ecosystem despite occupying about 70% of the earth’s surface.
REASON :In aquatic ecosystems, productivity is limited by light which decreases with increasing water depth.
A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C) Assertion is correct but Reason is incorrect.
D) Both Assertion and Reason are incorrect.

Answer 1

Hint:In oceans, almost all of the photosynthesis is performed by algae, with a small amount done by vascular plants and other groups. Eubacteria are important photosynthesizers in both oceanic and terrestrial ecosystems, and while some archaea are phototrophic, none are known to utilize oxygen-evolving photosynthesis. Several eukaryotes are significant contributors to primary production in the ocean, including green algae, brown algae, and red algae, and a diverse group of unicellular groups. Groups such as the seagrasses are vascular plants.

Complete answer:
The availability of light and mineral nutrients play a vital role in regulating primary production in the ocean.
Light : The photic or the euphotic zone of the ocean gets the maximum amount of light. This layer is relatively thin (10- 100m). Here, the light is sufficient for photosynthesis to occur. For practical purposes, the thickness of the photic zone is defined as the depth at which light reaches 1% of its surface value. Light is weakened down the water column due to absorption or scattering by the water itself, and by dissolved or particulate material within it (including phytoplankton).
Net photosynthesis in the water column is determined by the interaction between the photic zone and the mixed layer. When the layer is much deeper than the photic zone, it results in phytoplankton more time in the dark for net growth to occur
As long as there are adequate nutrients available, net primary production occurs whenever the mixed layer is shallower than the critical depth.

Nutrients : Mixing of nutrients plays an important role in primary production. Inorganic nutrients, such as nitrate, phosphate, and silicic acid are important for phytoplankton to synthesize their cells and cellular machinery. Because of the gravitational sinking of particulate material, such as plankton, nutrients are constantly lost from the photic zone and are only replenished by mixing of deeper water. However, as long as the photic zone is deep enough, primary production may continue below the mixed layer where light-limited growth rates mean that nutrients are usually more abundant.

Hence, the correct option is A, “Both Assertion and Reason are correct and Reason is the correct explanation for assertion”. The deeper the layer of the ocean, the lesser will be the light and thereby decreasing productivity.

Note:Unlike terrestrial ecosystems, the majority of primary production in the ocean is performed by free-living microscopic organisms called phytoplankton. Larger autotrophs, such as the seagrasses and macroalgae are mostly confined to the littoral zone and adjacent shallow waters, where they can attach to the underlying substrate but still be within the photic zone. There are exceptions, such as Sargassum, but the vast majority of free-floating production takes place within microscopic organisms.