Coastal Ocean The coastal ocean extends from the low-tide mark to the outer edge of the continental shelf—the relatively shallow border that surrounds the continents. Water here is brightly lit, and is often supplied with nutrients by freshwater runoff from land. As a result, coastal oceans tend to be highly productive. Kelp forests and coral reefs are two exceptionally important coastal communities.

Open Ocean The open ocean begins at the edge of the continental shelf and extends outward. More than 90 percent of the world's ocean area is considered open ocean. Depth ranges from about 500 meters along continental slopes to more than 10,000 meters in deep ocean trenches. The open ocean can be divided into two main zones according to light penetration: the photic zone and the aphotic zone.

▸ The Open Ocean Photic Zone The open ocean typically has low nutrient levels and supports only the smallest species of phytoplankton. Still, because of its enormous area, most photosynthesis on Earth occurs in the sunlit top 100 meters of the open ocean.

▸ The Open Ocean Aphotic Zone The permanently dark aphotic zone includes the deepest parts of the ocean. Food webs here are based either on organisms that fall from the photic zone above, or on chemosynthetic organisms. Deep ocean organisms, like the fish in Figure 4–23, are exposed to high pressure, frigid temperatures, and total darkness. Benthic environments in the deep sea were once thought to be nearly devoid of life but are now known to have islands of high productivity. Deep-sea vents, where superheated water boils out of cracks on the ocean floor, support chemosynthetic primary producers.

FIGURE 4–23 Creature From the Deep This silver hatchetfish lives in the aphotic zone of the Gulf of Mexico. Apply Concepts What kinds of adaptations do you think this fish has that enable it to live in the harsh deep-ocean environment?

4.5 Assessment

Review Key Concepts

1. 1. Review What are the primary abiotic factors that affect life underwater?

   2. Compare and Contrast What are some ways in which life in an aphotic zone might differ from life in a photic zone?

2. 1. Review What are the major categories of freshwater ecosystems?

   2. Apply Concepts What is a wetland? Why are wetlands important?

3. 1. Review Where are estuaries found? Why is it important to protect estuaries?

   2. Predict How might a dam upriver affect an estuary at the river's mouth?

4. 1. Review List the three major marine ecological zones. Give two abiotic factors for each zone.

   2. Apply Concepts Using Figure 4–22 as a guide, draw a cross section of the ocean starting with a beach and ending with an ocean trench. Label the intertidal zone, coastal ocean, and open ocean. Subdivide the open ocean into photic and aphotic zones.

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Explanation

5. Choose three different aquatic ecosystems. For each of these ecosystems, select a plant and an animal, and explain how the organisms have adapted to their environment.

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