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A Closer Look

Estuary Pollution
ollution is usually understood to mean mixes with the saline water from the oceans aquatic life. The algae die and eventually
Psomething that is not naturally occur- is called a coastal estuary. Estuary waters decompose, and this depletes the available
ring and contaminates air, soil, or water to include bays and tidal rivers that serve as oxygen supply, leading to further fish and
interfere with human health, well-being, or nursery areas for many fish and shellfish shellfish kills.
quality of the environment. An important populations, including shrimp, oysters, The second most common pollutant
factor in understanding pollution is the size crabs, and scallops. Unfortunately, the riv- was the presence of bacteria, which pollute
of the human population and the amount ers carry pollution from their watersheds 16 percent of all the estuary waters sur-
of material that might become a pollutant. and adjacent wetlands to the estuary, where veyed. Because Escherichia coli is a bacte-
When the human population was small it affects the fish and shellfish industry, rium commonly found in the intestines of
and produced few biological wastes, there swimming, and recreation. humans and other warm-blooded animals,
was no pollution problem. The decompos- In 1996, the U.S. Environmental Pro- the presence of E. coli is evidence that sew-
ers broke down the material into simpler tection Agency asked the coastal states age is polluting the water. Bacteria interfere
nonpolluting substances such as water and to rate the general water quality in their with recreational activities of people and
carbon dioxide, and no harm was done. estuaries. The states reported that pollut- can contaminate fish and shellfish.
For example, suppose one person empties ants affect aquatic life in 31 percent of the The states also reported that toxic or-
the tea leaves remaining from a cup of tea area surveyed, violate shellfish harvesting ganic chemicals pollute 15 percent of sur-
into a nearby river once a week. In this case, criteria in 27 percent of the area surveyed, veyed waters, oxygen-depleting chemicals
decomposer organisms in the water would and violate swimming-use criteria in 16 per- pollute 12 percent, and petroleum products
break down the tea leaves almost as fast as cent of the area surveyed. pollute another 8 percent of surveyed wa-
they were added to the river. But imagine The most common pollutants affect- ters. These pollutants impact the fish and
100,000 people doing this every day. In this ing the surveyed estuaries were excessive shellfish industry, swimming, and recreational
case, the tea leaves are released faster than nutrients, which were found in 22 percent activities that require contact with the water.
they decompose, and the leaves become of all the estuaries surveyed. Excessive nu- The leading sources of the pollutants
pollutants. trients stimulate population explosions of were identified as agriculture runoff, ur-
The part of the wide lower course of algae. Fast-growing masses of algae block ban runoff, industrial discharges, municipal
a river where the freshwater of the river light from the habitat below, stressing the wastewater, and wastes from landfills.

There must be a terrific amount of stirring in such an enormous in some places and down in other places. Finally, there are enor-
amount of seawater to produce the well-mixed, uniform chemi- mous deep ocean currents that move tremendous volumes of
cal composition that is found in seawater throughout the world. seawater. The overall movement of many of the currents on the
The amount of mixing required is more easily imagined if you surface and their relationship to the deep ocean currents are not
consider the long history of the ocean, the very long period of yet fully mapped or understood. The surface waves are better un-
time over which the mixing has occurred. Based on investiga- derstood. The general trend and cause of permanent, worldwide
tions of the movement of seawater, it has been estimated that currents in the ocean can also be explained.
there is a complete mixing of all Earth’s seawater about every
2,000 years or so. With an assumed age of 3 billion years, this
means that Earth’s seawater has been mixed 3,000,000,000 ÷ Waves
2,000, or 1.5 million, times. With this much mixing, you would Any slight disturbance will create ripples that move across a
be surprised if seawater were not identical all around Earth. water surface. For example, if you gently blow on the surface of
How does seawater move to accomplish such a complete water in a glass, you will see a regular succession of small rip-
mixing? Seawater is in a constant state of motion, both on the sur- ples moving across the surface. These ripples, which look like
face and below the surface. The surface has two types of motion: small, moving wrinkles, are produced by the friction of the
(1) waves, which have been produced by some disturbance, such air moving across the water surface. The surface of the ocean
as the wind, and (2) currents, which move water from one place is much larger, but a gentle wind produces patches of ripples
to another. Waves travel across the surface as a series of wrin- in a similar way. These patches appear, then disappear as the
kles that range from a few centimeters high to more than 30 m wind begins to blow over calm water. If the wind continues
(100 ft) high. Waves crash on the shore as booming breakers to blow, larger and longer-lasting ripples are made, and the
and make the surf. This produces local currents as water moves moving air can now push directly on the side of the ripples. A
along the shore and back out to sea. There are also permanent, ripple may eventually grow into an ocean wave, a moving dis-
worldwide currents that move 10,000 times more water across turbance that travels across the surface of the ocean. In its sim-
the ocean than all the water moving in all the large rivers on the plest form, each wave has a ridge, or mound, of water called a
land. Beneath the surface, there are currents that move water up crest, which is followed by a depression called a trough. Ocean

610 CHAPTER 24 Earth’s Waters 24-14