By simply looking at the surface, sitting in a boat or standing on the shore, it’s very difficult to see the amazing diversity of life that exists in the ocean.
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.
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 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 fish, the whale shark, lives off plankton alone!
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.
We don’t know a lot about some of the unusual creatures that live in the deep. One such creature is the giant squid, which can grow to be over 60 feet (18 meters) long! Even more unusual is the oarfish, a long thin fish, which has only been seen by humans a few times. Anglerfish dwell far down in the darkness and have adapted to their environment by forming a glowing lure to attract smaller fish within reach of their huge toothy mouths. Lantern fish, gulpers (so named because of their gaping mouths), skates and shrimp are also found at great depths. The rare coelecanth, nearly unchanged from fossil remains millions of years old, was discovered by scientists off the southern coast of Africa in the late 1930s and has since been found in other corners of the Indian Ocean.
People have had a significant impact on the health of the ocean. Runoff from the land carries silt and harmful chemicals, such as pesticides, through rivers into the ocean. Dumping trash directly in the ocean or in other water bodies (streams, rivers, bays, etc.) is another way humans disturb the ocean environment.
Overfishing is also a concern. Some areas that were once filled with a particular species are now off-limits to fishing, so that the fish populations can recover and repopulate. Along the Pacific Coast of South America, the native sardine populations have been wiped out, most likely because of agressive fishing and El Nino’s warming effect on those coastal waters. Salmon catches along the North American coast are now closely monitored by both the Canadian and United States governments to make sure that enough of these fish can return to their river of birth to spawn and populate the next generation.