Characteristics of Oceans Help (page 3)

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By — McGraw-Hill Professional
Updated on Sep 4, 2011


Most of the organisms in the ocean depend on sunlight. Plants and bacteria, such as kelp , sea grass , and plankton , use sunlight to make energy by photosynthesis . These organisms provide food for fish and some whales. Fish are eaten by larger fish, sharks, and other predators.

This is the food chain of the sea. Sunlight is the energy and heat source for the ocean’s food chain. Surface heat and thermal currents that bring in nutrients make it possible for animals to live in warmed ocean waters.

As sunlight penetrates the ocean, it gets absorbed. Like the other ocean gradients we have seen in temperature, salinity, and density, ocean waters can be divided into three vertical regions based on the amount of sunlight that penetrates.

The first zone, or euphotic zone , starts at the water’s surface and dips downward to about 50m in depth. This depends on the time of year, the time of day, the water’s transparency, and whether or not it is a cloudy day. This is the ocean region where there is still enough light to allow plants to carry on photosynthesis. All plankton, kelp forests, and sea grass beds are found in the euphotic zone. Figure 13-4 shows how these optical regions stack up.


Fig. 13-4. Sunlight depth creates different zones in ocean waters.

The next zone is the dysphotic zone , which reaches from around 50 m, or the edge of the euphotic zone, to about 1000m in depth. In this zone, there is enough light for an organism to see, but it is too dim for photosynthesis to take place. When divers go deeper into the dysphotic zone, they experience less and less light with depth.

When you get to the aphotic zone , there is no light. The aphotic zone extends from about 1000m of depth or the lower edge of the dysphotic zone to the sea floor. For many years, scientists didn’t think there were any animals that lived in this zone, but with deep diving scientific submarines, they have found that several specialized species do exist.

In 1977, geologists who had been exploring ocean fractures discovered booming thermal volcanic vent communities living without sunlight on the barren sea floor. These big, alien-looking creatures used a previously unknown energy process that doesn’t include solar heat.

Scientists discovered that the food chain depends on sulfur for energy, not sunlight in vent communities. Deep ocean bacteria transform the chemicals they get from this high-sulfur environment to energy. This energy transformation process is called chemosynthesis .

Other dark-living animals eat bacteria, shelter bacteria in their bodies, or consume bacteria-eaters in the chain. Vent worms, for example, have no mouth or digestive tract. Instead, they maintain a symbiotic relationship with these chemosynthetic bacteria. The bacteria live in their tissues and provide them with food. Stranger still, scientists found that hemoglobin, which transports hydrogen sulfide to the bacteria, gives vent worms a red color.

Black smokers , the hottest deep ocean hot springs, have been known to reach temperatures of 380°C. This extremely hot water, mixed with hydrogen sulfide and other leached basaltic trace minerals, is emitted from fractures in the earth’s crust. White smokers have a different composition and lower temperatures.


Some animals in the aphotic zone create their own light through a chemical reaction. This is called bioluminescence . It is a lot like the reaction that fireflies display in their warm summer evening dances on land.

These microscopic organisms, floating on the surface, produce their own light through bioluminescence. Disturbances by boats, ships, and swimmers can all cause these organisms to glow. This is an awesome sight at night! Because of these glowing party animals of the sea, the wakes of passing ships have been seen to last for over 10km!

Practice problems of this concept can be found at: Oceans Practice Test

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