Ocean Zones

The ocean covers about 70 percent of Earth's surface and has an average depth of 3.7 km (2.3 mi). The deepest parts of the ocean are about 11 km (6.8 mi) deep. The water contains about 3 percent salt, mostly sodium chloride, a factor that strongly affects the biology of the organisms that live there.

Another important factor affecting marine organisms is the availability of light. Most of the ocean is cold and dark. This zone, where sunlight cannot penetrate and photosynthesis cannot occur, is called the aphotic (ay-FOHT-ik) zone. Because water absorbs light, sunlight penetrates only the upper few hundred meters of the ocean. The part of the ocean that receives sunlight is the photic (FOHT-ik) zone. The depth of the photic zone varies.

Ecologists recognize three zones relative to the ocean's edges, as illustrated in Figure 21-8. The intertidal zone is the area of shoreline that is twice daily covered by water during high tide and exposed to air during low tide. Farther out is the neritic (nee-RIT-ik) zone, which extends from the intertidal zone over the continental shelf and to relatively shallow water depths of about 180 m.

objectives

• Identify the major ocean zones.

• Compare the aphotic zone with the photic zone.

• Compare the neritic zone with the oceanic zone.

• Describe estuaries.

• Compare eutrophic lakes with oligotrophic lakes.

• Explain the significance of gradient in rivers and streams.

• Describe freshwater wetlands.

vocabulary aphotic zone photic zone intertidal zone neritic zone oceanic zone pelagic zone benthic zone plankton estuary eutrophic lake oligotrophic lake freshwater wetland figure 21-8

This diagram shows the major ecological zones of the ocean.

Oceanic zone

Intertidal figure 21-8

This diagram shows the major ecological zones of the ocean.

Oceanic zone

Intertidal

Grades Ocean For Kids

figure 21-9

The intertidal zone shown here is similar to intertidal zones all over the planet. A sandy beach is part of the intertidal zone, but it is usually distinguished from the rocky intertidal zones and not as rich in species as the rocky intertidal zones are.

figure 21-9

The intertidal zone shown here is similar to intertidal zones all over the planet. A sandy beach is part of the intertidal zone, but it is usually distinguished from the rocky intertidal zones and not as rich in species as the rocky intertidal zones are.

figure 21-10

Coral reefs, such as the one shown here, are among the most diverse ecosystems on Earth. Coral reefs are extremely sensitive to pollution. Many reefs around the world are threatened, and many have been damaged or destroyed as a result of human activities.

Beyond the continental shelf is the oceanic zone, which is the deep water of the open sea. The neritic and oceanic zones are further divided. The open ocean is known as the pelagic (pi-LAJ-ik) zone, and the ocean bottom is known as the benthic zone.

The Intertidal Zone

Organisms in the intertidal zone are adapted to periodic exposure to air during low tide. Crabs avoid dehydration by burrowing into the sand or mud. Clams, mussels, and oysters retreat into their shells at low tide. Organisms living in the intertidal zone must be able to withstand the force of crashing waves. Sea anemones cling to rocks with a muscular disk, and sea stars use tube feet to adhere to surfaces. Figure 21-9 shows some of the organisms living in the rocky intertidal zone of the California coast.

The Neritic Zone

The neritic zone is the most productive zone in the ocean, supporting more species and numbers of organisms than any other zone. The water throughout most of the neritic zone is shallow enough for photosynthesis to occur. Strong currents called upwellings carry nutrients from the ocean bottom and mix them with nutrients contained in runoff from land. These waters are rich in plankton, communities of small organisms that drift with the ocean currents. Plankton is consumed by many larger animals. Numerous fishes, sea turtles, and other animals also live in these waters.

Coral reefs form in the neritic zone of tropical areas. Like tropical rain forests, coral reefs are very productive and rich in species. A coral reef is built by coral animals over a long time period. They construct external skeletons of calcium carbonate. As the animals grow and die, the skeletons accumulate to form the base of a reef, as shown in Figure 21-10. A reef is home to many species of fishes, crustaceans, mollusks, and other animals. Most species of reef-forming coral have a mutualistic relationship with photosynthetic protists, from which they receive food.

The Oceanic Zone

The oceanic zone encompasses the deepest parts of the ocean and contains fewer species than the neritic zone does. Even in photic areas, nutrient levels are too low to support as much life. Although the productivity per square meter of open ocean is very low, the total productivity of the oceanic zone is high because the ocean covers such a vast area. About half of the photosynthesis that occurs on Earth takes place in the oceanic zone. The producers of the upper parts of the oceanic zone are microscopic protists, bacteria, plants, and invertebrates in the plankton. Animals include fishes, mammals such as whales, and many large invertebrates.

In the aphotic zone, animals feed primarily on sinking plankton and dead organisms. Organisms living deep in the ocean must cope with near-freezing temperatures and crushing pressure. Deep-sea organisms, such as the squid shown in Figure 21-11, have slow metabolic rates and reduced skeletal systems. Fishes in these depths have large jaws and teeth and expandable stomachs that can accommodate the rare prey that they can catch.

In the 1970s, scientists found diverse communities living near volcanic vents 2,500 m (8,200 ft) below the surface. These vents release water that is rich in minerals and often exceeds 750°C. The producers for this ecosystem are chemosynthetic bacteria that use energy contained in hydrogen sulfide, H2S, from the vents. Unique clams, crabs, and worms feed on these bacteria. Tube worms living near the vents have lost their digestive system over evolutionary time and receive all of their food directly from chemosynthetic bacteria living in their bodies.

Estuaries

An estuary (ES-tyoo-er-ee) occurs where freshwater rivers and streams flow into the sea. Examples of estuary communities include bays, mud flats, mangrove swamp forests, and salt marshes. The shallow water receives plenty of light, and rivers deposit large amounts of mineral nutrients. However, the interaction between fresh water and salt water causes great variation in temperature and salinity In addition, like the surface of the intertidal zone, the surface of an estuary is partly exposed during low tide. Inhabitants of estuaries are adapted for frequent change. For example, some kinds of mangrove trees have special glands on their leaves that eliminate excess salt water taken up by the roots. Softshell clams lie buried in mud with only their long siphons protruding above the surface. The siphon filters plankton from the salt water at high tide and detects predators at low tide, contracting whenever it senses danger. Figure 21-12 shows an example of an estuary.

figure 21-11

Organisms in the deep ocean have many adaptations to their environment. The squid in the photograph, Vampyroteuthis sp., is adapted to eating a large quantity of food at once because prey is hard to find.

figure 21-11

Organisms in the deep ocean have many adaptations to their environment. The squid in the photograph, Vampyroteuthis sp., is adapted to eating a large quantity of food at once because prey is hard to find.

figure 21-12

Estuaries are almost as rich in species as tropical rain forests are. Estuaries serve as stopovers and nurseries for both terrestrial and marine organisms. Organisms that spend at least part of their lives in estuaries include shrimp, oysters, mullet, redfish, anchovies, striped bass, and other animals that people consume as food.

Sirens Sleep Solution

Sirens Sleep Solution

Discover How To Sleep In Peace And Harmony In A World Full Of Uncertainty And Dramatically Improve Your Quality Of Life Today! Finally You Can Fully Equip Yourself With These “Must Have” Tools For Achieving Peace And Calmness And Live A Life Of Comfort That You Deserve!

Get My Free Ebook


Post a comment