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Home > Ocean > Ocean Zones > Sunlight Zone

Understanding Ocean Zones: The Sunlight Zone

This ocean zone is from the surface of the sea down to a depth of about 200 meters. As you begin your descent you see that the ocean is absolutely teeming with life forms of every sort. From the microscopic plankton and diatoms that give the ocean its murky color and limits visibility, to bony fishes of every shape and size, to starfishes, and warm-blooded, oxygen-breathing mammals. You can see the most fantastic array of colors; reds, pinks, purples, bright yellows, oranges, blues, greens. Everywhere you look the ocean is brimming with brightly colored life forms, much more so than anything on land. As you swim down deeper, it seems that the colors begin to fade and the palette of life is becoming more monochromatic. In fact, the colors are not disappearing, you simply are not able to see them. Water, especially murky, turbid water, scatters and absorbs some of the sun's light, filtering out colors below certain depths. kelp forestThe first colors to go are the reds, then oranges, yellows, greens, and then finally blues. You may reach a certain depth, say about 60 feet, and it appears that everything around you is shades of brown, black, gray and white. But lucky for you – you brought a bright dive light. You turn on the light and once again marvel at the incredible variety of colors, even at that depth. The sunlight zone of the blue reefsea contains some of the most plentiful, but tiny organisms in the ocean, krill (a type of crustacean). Their existence is responsible for sustaining the largest living creatures on earth – the Blue Whales.

coralsAs you dive deeper in the sunlight life zone of the sea, you will also quickly notice the effects of hydrostatic pressure on your body. The deeper you dive the more water is over the top of you. The more gallons of water you put between you and the surface of the ocean, the greater the pressure is on your body because of the weight of the water over the top of you. You can really get a sense of hydrostatic pressure as you dive deeper because you'll feel the pressure against your ear drums, like they're being squeezed, or pushed in. You'll need to equalize the pressure against your eardrums to avoid rupturing them, so you descend slowly to prevent them from being damaged. Once you get down to about a depth of 100 feet you will feel the pressure against every square inch of your body. It really becomes noticeable as you breathe. At a depth of 100 feet, the size and volume of your lungs has been reduced to 1/3rd their capacity at sea level. You will also notice that it is much darker at 100 feet and COLD. The lack of sunlight at that depth also means the ocean is not getting warmed by sunlight, either. At a depth of about 180 feet you've pretty much reached the limit of safe diving for a human breathing compressed air. Because of the possible hazards of nitrogen narcosis, hypothermia, fatigue, and the need for decompression after a deep dive, you'll need to return to the surface and put on a special suit to dive deeper into the next zone...the twilight zone.

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