Lesson Objectives

• Explain the significance of the oceans.
• Describe the composition of ocean water.
• Define the parts of the water column and oceanic divisions.

Vocabulary

• aphotic zone
• biomass
• intertidal zone
• neritic zone
• oceanic zone
• photic zone
• salinity
• water column

Introduction

As terrestrial creatures, humans think of the importance of the planet’s land surfaces. But Earth is mostly a water planet. From space, the dominance of water is obvious (Figure below). Most of Earth’s water is in the oceans.

Because all of Earth’s oceans are somehow connected, should this chapter be titled “Earth’s Ocean” or “Earth’s Oceans?” Try to decide by the end of the chapter.

An animation will help you see Earth’s one, three, four, or five oceans: http://en.wikipedia.org/wiki/File:World_ocean_map.gif.

Significance of the Oceans

Earth would not be the same planet without its oceans.

Moderates Climate

The oceans, along with the atmosphere, keep temperatures fairly constant worldwide. While some places on Earth get as cold as -70^oC and others as hot as 55^oC, the range is only 125^oC. On Mercury temperatures go from -180^oC to 430^oC, a range of 610^oC.

The oceans, along with the atmosphere, distribute heat around the planet. The oceans absorb heat near the equator and then move that solar energy to more polar regions. The oceans also moderate climate within a region. At the same latitude, the temperature range is smaller in lands nearer the oceans than away from the oceans. Summer temperatures are not as hot, and winter temperatures are not as cold, because water takes a long time to heat up or cool down.

Water Cycle

The oceans are an essential part of Earth’s water cycle. Since they cover so much of the planet, most evaporation comes from the ocean and most precipitation falls on the oceans.

Biologically Rich

The oceans are home to an enormous amount of life. That is, they have tremendous biodiversity (Figure below). Tiny ocean plants create the base of a food web that supports all sorts of life forms. Marine life makes up the majority of all biomass on Earth. (Biomass is the total mass of living organisms in a given area.) These organisms supply us with food and even the oxygen created by marine plants.

Continental Margin

Recall from the Plate Tectonics chapter that the ocean floor is not flat: mid-ocean ridges, deep sea trenches, and other features all rise sharply above or plunge deeply below the abyssal plains. In fact, Earth’s tallest mountain is Mauna Kea volcano, which rises 10,203 m (33,476 ft.)meters) from the Pacific Ocean floor to become one of the volcanic mountains of Hawaii. The deepest canyon is also on the ocean floor, the Challenger Deep in the Marianas Trench, 10,916 m (35,814 ft).

The continental margin is the transition from the land to the deep sea or, geologically speaking, from continental crust to oceanic crust. More than one-quarter of the ocean basin is continental margin. (Figure below).

Composition of Ocean Water

Remember from the Mineral’s chapter that H₂O is a polar molecule so it can dissolve many substances (Figure below). Salts, sugars, acids, bases, and organic molecules can all dissolve in water.

Where does the salt in seawater come from? As water moves through rock and soil on land it picks up ions. This is the flip side of weathering. Salts comprise about 3.5% of the mass of ocean water, but the salt content or salinity is different in different locations.

What would the salinity be like in an estuary? Where seawater mixes with fresh water, salinity is lower than average.

What would the salinity be like where there is lots of evaporation? Where there is lots of evaporation but little circulation of water, salinity can be much higher. The Dead Sea has 30% salinity—nearly nine times the average salinity of ocean water (Figure below). Why do you think this water body is called the Dead Sea?

Interactive ocean maps can show salinity, temperature, nutrients, and other characteristics: http://earthguide.ucsd.edu/earthguide/diagrams/levitus/index.html.

With so many dissolved substances mixed in seawater, what is the density (mass per volume) of seawater relative to fresh water?

Water density increases as:

• salinity increases
• temperature decreases
• pressure increases

Differences in water density are responsible for deep ocean currents, as will be discussed in the Ocean Movements lesson.

The Water Column

In 1960, two men in a specially designed submarine called the Trieste descended into a submarine trench called the Challenger Deep (10,910 meters) (Figure below).

The average depth of the ocean is 3,790 m, a lot more shallow than the deep trenches but still an incredible depth for sea creatures to live in. What makes it so hard to live at the bottom of the ocean? The three major factors that make the deep ocean hard to inhabit are the absence of light, low temperature, and extremely high pressure.

Vertical Divisions

To better understand regions of the ocean, scientists define the water column by depth. They divide the entire ocean into two zones vertically, based on light level. Large lakes are divided into similar regions.

• Sunlight only penetrates the sea surface to a depth of about 200 m, creating the photic zone (photic means light). Organisms that photosynthesize depend on sunlight for food and so are restricted to the photic zone. Since tiny photosynthetic organisms, known as phytoplankton, supply nearly all of the energy and nutrients to the rest of the marine food web, most other marine organisms live in or at least visit the photic zone.
• In the aphotic zone there is not enough light for photosynthesis. The aphotic zone makes up the majority of the ocean, but has a relatively small amount of its life, both in diversity of type and in numbers. The aphotic zone is subdivided based on depth (Figure below).

Horizontal Divisions

The seabed is divided into the zones described above, but ocean itself is also divided horizontally by distance from the shore.

• Nearest to the shore lies the intertidal (littoral) zone, the region between the high and low tidal marks. This hallmark of the intertidal is change: water is in constant motions in waves, tides, and currents. The land is sometimes under water and sometimes is exposed.
• The neritic zone is from low tide mark and slopes gradually downward to the edge of the seaward side of the continental shelf. Some sunlight penetrates to the seabed here.
• The oceanic zone is the entire rest of the ocean from the bottom edge of the neritic zone, where sunlight does not reach the bottom. The sea bed and water column are subdivided further, as seen in the above).

Lesson Summary

• The oceans help to moderate Earth’s temperatures.
• The main elements in seawater are chlorine, sodium, magnesium, sulfate, and calcium.
• The average salinity of the oceans is about 3.5%.
• In seawater, if evaporation is high, salinity is high. If fresh water mixes in, salinity is low.
• In the photic zone there is enough available light for photosynthesis.
• The vast majority of the ocean lies in the aphotic zone, where there is not enough light for photosynthesis.
• The ocean floor averages about 3,790 m but ocean trenches are as deep as 10,910 m.
• The neritic zones are nearshore areas, including the intertidal zone. The oceanic zones are offshore regions of the ocean.

Review Questions

1. What percent of the Earth’s surface is covered by water?

2. How do the oceans help to moderate Earth’s temperatures?

3. What is the most common substance that is dissolved in ocean water?

4. Define density. Why is density important to the water column?

5. Compare and contrast the photic and aphotic zones.

6. Briefly describe the types of organisms found in the intertidal, neritic, and oceanic zones.

Points to Consider

• How do water motions such as tides and waves affect living creatures in and near the sea?
• Is it possible to have a river in the middle of the ocean?
• What factors affect the movement of ocean water? How do these factors affect the world’s climate and the ocean’s ecosystem?