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A Closer Look

Health of Chesapeake Bay
ubmerged aquatic vegetation—SAV for grasses came the decline of the aquatic spe- manure. The manure is used as fertilizer, and
Sshort—is vital to the Chesapeake Bay cies living in the bay. Shad, haddock, and significant amounts of nitrogen and phos-
ecosystem and a key measuring stick for the sturgeon once supported large fisheries but phates wash from these fields. These nutri-
Chesapeake’s overall health. Eel and wigeon are now scarce. The rockfish (striped bass) ents accelerate algae growth in the bay, which
grasses grow in the shallows around the bay as well as the prized blue crab were once blocks sunlight. The grass dies and the loss
and across the bottom, providing food and abundant but seem to decline and recover results in muddying of the water, making it
shelter for baby blue crabs and fish, filter- over the years. The decline of the blue crab impossible for new grasses to begin growing.
ing pollutants, and providing life-sustaining population has shocked watermen because There is some evidence to support this
oxygen for the water. So the health of the the crab is amazingly fertile. The crabs idea since the 600,000 acres of underwater
grasses provides an indication of the overall reach sexual maturity in about a year, and grasses died back to a low of 40,000 acres in
health of the Chesapeake. one female bears millions of eggs. Part of 1984, then began to rebound when sewage
Chesapeake Bay once had an estimated the problem is the loss of the underwater treatment plants were modernized and there
600,000 acres of grasses and provided an grasses, which shelter the baby crabs from were fewer pollutants from industry and farms.
abundance of oysters, blue crab, shad, had- predators. In addition to the loss of habitat, The grasses recovered to some 63,500 acres
dock, sturgeon, rockfish, and other prized some believe the crab is being overfished. by 1999 but are evidently very sensitive to
sport fish and seafood. At this time, the bay What happened to the underwater even slight changes in water quality. Thus,
water was clear, and watermen reported grasses of the Chesapeake? Scientists believe they may be expected to die back with un-
they could clearly see grasses on the bottom it is a combination of natural erosion and usual weather conditions that might bring
some 6 m (about 20 ft) below their boats. pollutants that wash from farms in the water- more pollutants or muddy conditions but
Then the water clouded, the grasses began shed. The pollutants include nutrient-rich continue to rebound when the conditions are
to die, and the ecosystem of the bay began to fertilizers, chemical residues, and overflow right. The abundance of blue crabs and other
decline. The low point was reached in 1984, from sewage treatment plants. There are aquatic species can be expected to fluctuate
when fewer than 40,000 acres of grasses some 6,000 chicken houses around the Ches- with the health of the grasses. The trends of
could be found in the now murky waters apeake, raising more than 600 million chick- underwater grass growth do indeed provide a
of the Chesapeake. With the decline of the ens a year and producing 750,000 tons of key measure for the health of Chesapeake Bay.

deep water continues on at the same velocity. The slowing at the After the waves reach water that is less than one-half
shoreward side refracts, or bends, the wave so it is more parallel the wavelength, friction between the bottom and the circular
to the shore. Thus, waves always appear to approach the shore motion of the water particles progressively slows the bottom
head-on, arriving at the same time on all parts of the shore. part of the wave. The wave front becomes steeper and steeper
as the top overruns the bottom part of the wave. When the wave
front becomes too steep, the top part breaks forward, and the
One wavelength wave is now called a breaker (Figure 24.18). In general, this oc-
curs where the water depth is about one and one-third times the
Wave wave height. The zone where the breakers occur is called surf
height (Figure 24.19).
One wavelength One-half wavelength Wave base Deep-water wave Crest Breaker Beach

Wavelength decreases
Surf
Wave height increases
zone

No movement
below wave base Trough
1 wave-
2 length
Sea bottom
FIGURE 24.17 Water particles are moved in a circular motion
by a wave passing in the open ocean. On the surface, a water FIGURE 24.18 As a pattern of swell approaches a gently
particle traces out a circle with a diameter that is equal to the wave sloping beach, friction between the circular motion of the water
height. The diameters of the circles traced out by water particles particles and the bottom slows the wave, and the wave front
decrease with depth to a depth that is equal to one-half the wave- becomes steeper and steeper. When the depth is about one and
length of the ocean wave. Bottom sediment cannot be moved by one-third times the wave height, the wave breaks forward, moving
waves when the sediment is below the wave base. water toward the beach.

612 CHAPTER 24 Earth’s Waters 24-16