The ocean is vast, covering a little more than 71 percent of Earth's surface. Of that, 65 percent is considered blue water (open ocean)--waters that lie beyond the coastal shallows (coastal ocean). The world of people, trees and birds is relatively flat, never extending too far above or below the ground. Oceans are different; they have an average depth of more than 2 miles and contain life nearly everywhere, even on the deepest bottoms!

Oceanic life is divided into two major categories: the benthic environment (the sea floor) and the pelagic environment (the ocean waters). The pelagic environment is further divided based on water depth.

The neritic zone is the first 200 meters (656 feet) of ocean water, which includes the seashore and most of the continental shelf. Most photosynthetic life (life that uses light enery to convert carbon dioxide and water into food), such as phytoplankton and floating sargassum, is found in this region. Zooplankton, which is the floating creatures ranging from microscopic diatoms to small fish and shrimp, also live here. Many species of whales, like the gigantic blue and humpback whales, feed almost entirely on the tiny zooplankton. These whales force seawater through baleen plates (combs of bony material that form in the place of teeth) to filter out the tiny sea creatures. The largest of all ocean dwellers, the whale shark, lives off plankton alone!

Although fish are found everywhere in the ocean, the abundance of small organisms in the neritic zone provides a plentiful source of food for larger animals. Great schools of tuna and mackerel feed on squid, krill and small fish that gather where warm waters meet nutrient-rich cooler waters. The largest example, the Northern Bluefin Tuna, can grow to be more than 10 feet long and weigh over 1500lbs! Most sharks are common near the surface as well, some feeding on schools of fish, while others, including the basking and whale sharks, eat plankton.

The oceanic zone extends from 200 meters (656 feet) deep all the way down to the bottom of the ocean, which can be thousands of meters deep.


Another way to describe ocean zones is by the amount of light they receive. The topmost layer is called the euphotic zone and is defined by how deeply photosynthetic life can be found. Below this is the dysphotic zone, where light can be measured, sometimes as deeply as a kilometer down, but is too faint to support photosynthesis. From the lower boundary of this zone and extending all the way to the bottom is the aphotic zone, where no light ever passes, and animals have evolved to take advantage of other sources of food. One such environment is hydrothermal vent communities.

Information provided by: http://www.onr.navy.mil