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Geology LibreTexts

4.3: Light in the ocean

[This page needs more information on each of its main points (light in each zone, photosynthesis). Photosynthesis is important when regarding light in the ocean and so it should be discussed in more depth. This page also needs some visual aids, maybe of each of the zones of the ocean showing the difference amount of light that is distributed though the ocean. The affect of light on the marine life in each zone could probably be included as well.]

 

 

Light does not penetrate very far down into the vast depths of the ocean but can be detected about 1,000 meters in the ocean.Only the uppermost 200 meters of the ocean, called the epipelagic zone or sunlight zone, is suitable for photosynthesis: deeper than this, too little light is available for significant amounts of light-driven primary production to occur. From the base of the epipelagic zone to a depth of about 850 meters, there is still enough light for a human to see. The second zone between 200 meters and 1,000 meters is known as the "twilight zone". Some light penetrates as far as 1000 meters down into the ocean. No sunlight reaches a depth greater than 1000 meters, this part of the ocean is referred to as the aphotic zone or know as "midnight" zone. Seawater preferentially absorbs shorter wavelengths of light, leaving mainly bluish light to continue traveling to greater depths. This is why much of the ocean appears blue. 

The disadvantage of having bright light in the ocean is that bright light can cause living organisms to leave part of the ocean that light is the brightest because those organisms are light-sensitive organisms.

 

Many organisms take advantage of this differential absorption of wavelengths of light.  Because most red light is absorbed higher up in the water column, many animals living in the twilight zone are colored with red pigments: without red light available, these animals appear black and are nearly invisible against the dark backdrop.

 

Although photosynthesis is impossible below 200 meters, most organisms living in the twilight and aphotic zones are still reliant on primary producers living in the surface zone.