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Why are green plants not found beyond a certain depth in the ocean?

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Hint:Availability of light of optimum wavelength is an important factor for the growth of plants. Under great depths, plants do not get enough sunlight. It is mostly dark on the ocean floor.

Complete answer: Green plants are not found beyond a certain depth in the ocean because only about 1 percent of sunlight penetrates to this depth. This 1 percent of sunlight is not sufficient for the plants to photosynthesize and thus grow and survive.
Visible radiation, or light, from the Sun, is important for the world's ocean systems for several reasons. It provides the energy which is necessary for ocean currents and wind-driven waves. Conversion of some of that energy into heat helps form the thin layer of warm water near the ocean's surface that supports the majority of marine life. The visible part of the spectrum of light is captured by chlorophyll-bearing marine plants, which then make their own food by the process of photosynthesis. Similarly, the transmission of light is a key factor in the ecology of lakes and streams.

Additional information: The plants which are able to grow at great depths in the ocean floor are –
Kelps – Kelp forests are found in oceans and seas throughout the world, including the Antarctic and Arctic circles.
Poseidonia – It grows in large clumps on the ocean floor, creating a virtual meadow in the sand.
Seagrasses – These are a type of flowering plant with roots. They reproduce via rhizomes, a type of plant node that shoots out roots and shoots. They look similar to regular land grass, with long, blade-like green leaves, except that they thrive in underwater and rocky or sandy soil.

Note: Optimum amount of sunlight is very important for the growth of plants because green plants survive solely on sunlight for performing photosynthesis. Darkness on the ocean floor will automatically lead to no photosynthesis. Hence, no green plants are found on the ocean floor.