AP Environmental Science Study Guide
Chapter 15
Key Terms
Aquifers
dam
fresh water
point source
secondary treatment
unconfined aquifer
water table
artesian aquifer
desalination
groundwater
pollution
septic systems
wastewater
wetlands
confined aquifer
flooding
nonconsumptive use
primary treatment
sinkholes
water pollution
consumptive use
floodplain
nonpoint source
reservoir
surface water
watershed
I. Central Case Study: Starving the Louisiana Coast of Sediment
A. Since the 1930s alone, Louisiana has lost nearly 4900 km2 (1900 mi2) of coastal wetlands—an area
roughly the size of Delaware.
B. The salt marshes in the river’s delta naturally compact over time. This compaction lowers the level of the
marsh bottom and submerges vegetation under increasingly deeper waters.
C. So why are Louisiana’s wetlands being swallowed by the sea? It’s because people have modified the
Mississippi River so extensively that much of its sediments no longer reach the wetlands that need them.
D. Also, levees prevent the river from fanning out into its delta and turn the lower Mississippi into a “barrel”
that shoots sediments off the continental shelf into the deep waters of the Gulf.
E. And although Louisiana’s economy has benefited from oil and gas extraction, these activities have also
promoted wetland losses.
F. Proposed solutions for coastal erosion center on restoring the system to its natural state by diverting large
quantities of water from the Mississippi River into coastal wetlands instead of shot gunning it out into the
Gulf in the river’s main channel.
II. Freshwater Systems
Of all the water on Earth, only 2.5% is considered fresh water—or water that is relatively pure, with few
dissolved salts. Most of that is tied up in glaciers, ice caps, and aquifers. Water is renewed and recycled as it
moves through the water cycle.
A. Groundwater plays key roles in the hydrologic cycle.
1. Surface water is water located atop Earth’s surface and groundwater is water beneath the surface
held within pores in soil
or rock.
2. Groundwater is contained within aquifers: porous, spongelike formations of rock, sand, or gravel
that hold water. An aquifer’s upper layer, or zone of aeration, contains pore spaces partly filled with
water. In the lower layer, or zone of saturation, the spaces are completely filled with water. The
boundary between these two zones is the water table.
3. When a porous, water-bearing layer of rock, sand, or gravel is trapped between upper and lower
layers of less permeable substrate (often clay), we have a confined aquifer, or artesian aquifer. In
such a situation, the water is under great pressure. In contrast, an unconfined aquifer has no
impermeable upper layer to confine it, so its water is under less pressure and can be readily
recharged by surface water.
B. Surface water converges in river and stream ecosystems.
1. Once water falls from the sky as rain, emerges from springs, or melts from snow or glaciers, it may
soak into the ground or it may flow downhill over land. Water that flows over land is called runoff.
2. The area of land drained by a river system—a river and all its tributaries— is that river’s
watershed, also called a drainage basin.
3. Groundwater and surface water interact extensively.
4. Landscapes determine where rivers flow, but rivers shape the landscapes through which they run.
5. Areas nearest a river’s course that are flooded periodically are said to be within the river’s
floodplain.
6. Rivers and streams host diverse ecological communities.
C. Lakes and ponds are ecologically diverse systems.
1. Lakes and ponds are bodies of standing surface water.
2. There are four zones in lakes or ponds.
a. The region ringing the edge of a water body is the littoral zone.
b. The bottom of a lake or pond is the benthic zone.
c. The shallow waters away from shore are termed the limnetic zone, where light enters and
supports phytoplankton, which in turn support zooplankton.
d. Deeper in open water, where the sunlight does not reach, is the profundal zone, which
lacks plant life.
3. Ponds and lakes change over time as streams and runoff bring them sediment and nutrients.
4. The largest lakes are sometimes known as inland seas.
D. Freshwater wetlands include marshes, swamps, bogs, and vernal pools.
1. Wetlands are systems in which the soil is saturated with water and that generally feature shallow
standing water with ample vegetation.
2. Some wetlands are seasonal, being wet only at some times of the year.
3. Wetlands are extremely valuable as habitat for wildlife.
4. Despite the vital roles played by wetlands, people have drained and filled them extensively for
agriculture.
5. Wetlands, like other aquatic systems, are affected by people when we withdraw water for human
use, build dams and levees, and introduce pollutants that alter water’s chemical, biological, and
physical properties.
III. Human Activities Affect Waterways
A. Fresh water and human populations are unevenly distributed across Earth.
B. Water supplies households, industry, and especially agriculture.
1. Globally, we allot about 70% of our annual fresh water use to agriculture. Industry accounts for
roughly 20%, and residential and municipal uses for only 10%.
2. When we remove water from an aquifer or surface water body and do not return it, this is called
consumptive use. In contrast, nonconsumptive use of water does not remove, or only temporarily
removes, water from an aquifer or surface water body.
C. Excessive water withdrawals can drain rivers and lakes.
D. Groundwater can also be depleted.
1. Groundwater is more easily depleted than surface water because most aquifers recharge very
slowly.
2. As aquifers are mined, water tables drop.
3. When groundwater is over pumped in coastal areas, salt water from the ocean can intrude into
aquifers, making water undrinkable.
4. Sometimes land subsides suddenly, creating sinkholes—areas where the ground gives way with
little warning, occasionally swallowing homes and businesses.
5. Falling water tables also do vast ecological harm.
E. Groundwater supplies our bottled water.
1. Bottled water also exerts substantial ecological impact because it is
because we transport it long distances using fossil fuels.
heavily packaged and
2. After use, at least three out of four bottles in the United States are
recycled.
thrown away and not
F. People build dikes and levees to control floods.
1. Flooding is a normal, natural process that occurs when snowmelt or heavy rain swells the volume
of water in a river so that water spills over the river’s banks.
2. When we engineer rivers to stay in their channels, we often end up increasing the frequency of
floods in downriver areas.
G. We divert surface water to suit our needs.
H. We have erected thousands of dams.
1. A dam is any obstruction placed in a river or stream to block its flow. Dams create reservoirs,
artificial lakes that store water for human use.
I. Some dams are being removed.
1. By removing dams and letting rivers flow free, we can restore riparian ecosystems, reestablish
economically valuable fisheries, and revive river recreation.
J. Wetlands are affected by human manipulations of waterways.
1. As wetlands disappear, we lose the many ecosystem services they provide us, such as filtering
pollutants, harboring wildlife, controlling floods, and helping to maintain drinking water supplies.
2. Fortunately, many people today see the value in wetlands and are trying to protect and restore
them.
IV. Solutions to Depletion of Fresh Water
A. Solutions can address supply or demand.
B. Desalination “makes” more fresh water.
1. A supply strategy with some potential for sustainability is to generate fresh water by desalination,
or desalinization, the removal of salt from seawater or other water of marginal quality.
a. One method of desalination mimics the hydrologic cycle by evaporating allotments of
ocean water with heat and then condensing the vapor—essentially distilling fresh water.
b. Another method forces water through membranes to filter out salts; the most common
such process is reverse osmosis.
2. Desalination is expensive, requires large inputs of fossil fuel energy, kills aquatic life at water
intakes, and generate concentrated salty waste.
C. Agricultural demand can be reduced.
1. Farmers can improve efficiency by lining irrigation canals to prevent leaks, leveling fields to
minimize runoff, and adopting efficient irrigation methods.
2. Choosing crops to match the land and climate in which they are farmed can save huge amounts of water.
3. Selective breeding and genetic modification can produce crop varieties that produce high yields with less
water.
D. We can lower residential and industrial water use.
1. Each of us can cut our daily water use dozens, hundreds, or even thousands of gallons a day by
reexamining aspects of our daily lives.
2. Industry and municipalities can take water-saving steps as well.
E. Market-based approaches to water conservation are being debated.
1. Economists who want to use market-based strategies to achieve sustainable water use have
suggested ending government subsidies of inefficient practices and instead letting water become a
commodity whose price reflects the true costs of its extraction.
a. Others worry that making water a fully priced commodity would make it less available to
the world’s poor and increase the gap between rich and poor.
b. Because industrial use of water can be 70 times more profitable than agricultural use,
market forces alone could favor uses that would benefit wealthy and industrialized people,
companies, and nations at the expense of the rural poor.
2. The privatization of water supplies has been tried in hope of increasing efficiency, but many people
worry that firms have little incentive to allow equitable access to water for rich and poor alike.
3. Decentralization of control over water, from the national level to the local level, can help conserve water.
4. Regardless of how demand is addressed, the shift from supply-side to demand-side solutions is paying
dividends.
F. Nations often cooperate to resolve water disputes.
V. Freshwater Pollution and Its Control
A. Water pollution comes from point and non-point sources.
1. Pollution is the release into the environment of matter or energy that causes undesirable impacts
on the health or wellbeing of humans or other organisms. Water pollution comes in many forms and
can cause diverse impacts on aquatic ecosystems and human health.
2. Some water pollution is emitted from point sources—discrete locations, such as a factory or sewer
pipe. In contrast, pollution from non-point sources is cumulative, arising from multiple inputs over
larger areas such as farms, city streets, and residential neighborhoods.
B. Water pollution takes many forms.
1. Toxic chemicals
2. Pathogens and waterborne diseases
3. Nutrient pollution
4. Biodegradable wastes
a. Wastewater is water affected by human activities and can be a source of biodegradable
wastes.
5. Sediment
6. Thermal pollution
C. Groundwater pollution is a difficult problem.
D. There are many sources of groundwater pollution.
1. A variety of chemicals that are toxic at high concentrations, including aluminum, fluoride, nitrates,
and sulfates, occur naturally in groundwater.
2. Industrial, agricultural, and urban wastes—from heavy metals to petroleum products to industrial
solvents to pesticides—can leach through soil and seep into aquifers. Pathogens and other pollutants
can also enter groundwater through improperly designed wells and from the pumping of liquid
hazardous waste below ground.
3. Leakage of carcinogenic pollutants (such as chlorinated solvents and gasoline) from underground
tanks of oil and industrial chemicals also poses a threat to groundwater.
4. The leaking of radioactive compounds from underground tanks is also a source of groundwater
pollution.
5. Agriculture also contributes to groundwater pollution.
a. Pesticides
b. Nitrate from fertilizers
c. Pathogens
E. Legislative and regulatory efforts have helped to reduce pollution.
F. We treat our drinking water.
G. We treat our wastewater.
1. Wastewater includes water that carries sewage; water from
showers, sinks, washing machines, and dishwashers; water used in manufacturing or
industrial cleaning processes; and storm water runoff.
2. In rural areas, septic systems are the most popular method of wastewater disposal.
In a septic system, wastewater runs from the house to an underground septic tank,
inside which solids and oils separate from water. The clarified water proceeds
downhill to adrain field of perforated pipes laid horizontally in gravel-filled trenches
underground. Microbes decompose pollutants in the wastewater that these pipes
emit. Periodically, solid waste from the septic tank is pumped out and taken to a
landfill.
3. In more densely populated areas, municipal sewer systems carry wastewater from
homes and businesses to centralized treatment locations. There, pollutants are
removed by physical, chemical, and biological means.
a. At a treatment facility, primary treatment, the physical removal of
contaminants in settling tanks or clarifiers, removes about 60% of suspended
solids.
b. Wastewater then proceeds to secondary treatment, in which water is stirred
and aerated so that aerobic bacteria degrade organic pollutants. Roughly 90%
of suspended solids may be removed after secondary treatment.
c. Finally, the clarified water is treated with chlorine and sometimes ultraviolet
light to kill bacteria.
H. Constructed wetlands can aid treatment.
VI. Conclusion
A. Citizen action, government legislation and regulation, new technologies, economic incentives, and public
education are all helping us to confront a rising challenge of our new century: ensuring adequate quantity
and quality of fresh water for ourselves and for the planet’s ecosystems.
B. Accessible fresh water comprises a minuscule percentage of the hydrosphere, but we generally take it for
granted.
C. There is reason to hope that we may yet attain sustainability in our water use.
Chapter 16
Key Terms
benthic
continental shelves
currents
El Niño
El Niño–Southern Oscillationv(ENSO)
harmful algal blooms
kelp
La Niña
mangroves
marine protected areas (MPAs)
pelagic
photic zone
red tides
thermohaline circulation
tides
upwelling
esturaries
downdwelling
harmful algal blooms
littoral
ocean acidification
saltmarshes
I. Central Case Study: Collapse of the Cod Fisheries
A. No fish has had more impact on human civilization than the Atlantic cod.
B. Cod fishing has been the economic engine for hundreds of communities in coastal New England and
eastern Canada.
C. One stock inhabits the Grand Banks off Newfoundland and another lives on Georges Bank off
Massachusetts.
D. The Grand Banks provided ample fish for centuries.
E. Then came the crash.
F. It became illegal for Canadians in these areas to catch even one cod for their family’s dinner. Fishers
challenging the ban were arrested, fined, and jailed.
G. In 2009, a portion of the Grand Banks off the southeastern coast of Newfoundland was reopened to cod
fishing after data showed the stock was recovering slightly.
H. Across the border in U.S. waters, cod stocks had collapsed in the Gulf of Maine and on Georges Bank.
I. There is good news in the cod fishery, however. Recoveries in no-fishing areas are showing scientists,
fishers, and policymakers that protecting areas of ocean can help save dwindling marine populations and
restore fisheries.
II. The Oceans
A. Oceans cover most of Earth’s surface.
1. The world’s five oceans—Pacific, Atlantic, Indian, Arctic, and Southern—are all connected,
comprising a single vast body of water. This one “world ocean” covers 71% of Earth’s surface and
contains 97.5% of its water.
B. Seafloor topography can be rugged.
1. To comprehend underwater geographic features, we can examine a stylized map that reflects
bathymetry (the measurement of ocean depths) and topography (physical geography, or the shape
and arrangement of landforms).
a. In bathymetric profile, gently sloping continental shelves underlie the shallow waters
bordering the continents.
2. Wherever reefs, volcanism, or other processes create physical structure underwater, life thrives.
C. Ocean water contains high concentrations of dissolved salts.
1. Ocean water contains approximately 96.5% H2O by mass. Most of the remainder consists of ions
from dissolved salts.
a. Ocean water is salty primarily because the ocean basins are the final repositories for water
that runs off the land.
b. The water in the ocean evaporates, the salts do not, and they accumulate in ocean basins.
2. The salinity of ocean water generally ranges from 33,000 to 37,000 parts per million (ppm), varying
from place to place because of differences in evaporation, precipitation, and freshwater runoff from
land and glaciers.
3. Besides dissolved salts, nutrients such as nitrogen and phosphorus occur in seawater in trace
amounts (well under 1 part per million) and play essential roles in nutrient cycling in marine
ecosystems.
D. Solar energy structures ocean water from surface to bottom.
1. Water density increases as salinity rises and as temperature falls, iving rise to different layers of
water.
a. The waters of the surface zone are heated by sunlight each day and are stirred by wind.
b. The pycnocline is the region below the surface zone in which density increases rapidly with
depth.
c. The deep zone of the ocean lies beneath the pycnocline and is not affected by wind and
sunlight.
2. By absorbing heat and releasing it to the atmosphere, the oceans help regulate Earth’s climate.
E. Surface water flows horizontally in currents.
1. Earth’s ocean is composed of currents—vast, riverlike flows driven by density differences, heating
and cooling, gravity, and wind.
2. Besides influencing climate, ocean currents have aided navigation and have shaped human history.
F. Vertical movement of water affects marine ecosystems.
1. Where horizontal surface currents diverge from one another, cold deep waters are pulled to the
surface in a process called upwelling.
2. In areas where surface currents converge, or come together, surface water sinks—a process called
downwelling.
G. Ocean currents affect Earth’s climate.
1. The thermohaline circulation is a worldwide current system in which warmer, fresher water moves
along the surface and colder, saltier water (which is denser) moves deep beneath the surface.
2. Scientists hypothesize that interrupting the thermohaline circulation could trigger rapid climate
change.
3. Another interaction between ocean currents and the atmosphere that influences climate is the El
Niño–Southern Oscillation (ENSO), a systematic shift in atmospheric pressure, sea surface
temperature, and ocean circulation in the tropical Pacific Ocean.
4. El Niño conditions are triggered when air pressure decreases in the eastern Pacific and increases in
the western Pacific, weakening the equatorial winds and allowing the warm water to flow eastward.
5. La Niña events are the opposite of El Niño events; in a La Niña event, unusually cold waters rise to
the surface and extend westward in the equatorial Pacific when winds blowing to the west
strengthen, and weather patterns are affected in opposite ways.
H. Climate change is altering ocean chemistry.
1. The ocean’s surface water may soon become saturated with as much CO2 as it can hold.
2. As ocean water soaks up CO2 it becomes more acidic. As ocean acidification proceeds, many sea
creatures have difficulty forming shells because the chemicals they need to create them are less
available.
III. Marine and Coastal Ecosystems Regions of ocean water differ greatly, and some zones support more life than
others.
a. The uppermost 10 m (33 ft) of water absorbs 80% of solar energy, so nearly all of the oceans’
primary productivity occurs in the top layer, or photic zone.
b. Habitats and ecosystems occurring between the ocean’s surface and floor are termed pelagic.
c. Those that occur on the ocean floor are called benthic.
A. Intertidal zones undergo constant change.
1. Where the ocean meets the land, intertidal, or littoral, ecosystems spread between the uppermost
reach of the high tide and the lowest limit of the low tide.
a. Tides are the periodic rising and falling of the ocean’s height at a given location, caused by
the gravitational pull of the moon and sun.
2. The intertidal environment is a tough place to make a living, but it is home to a remarkable
diversity of organisms.
3. The rocky intertidal zone is so diverse because environmental conditions change dramatically from
the high to low reaches.
B. Salt marshes line temperate shorelines.
1. Along many of the world’s coasts at temperate latitudes, salt marshes occur where the tides wash
over gently sloping sandy or silty substrates.
2. Salt marshes boast very high primary productivity and provide critical habitat for shorebirds,
waterfowl, and many commercially important fish and shellfish species.
C. Mangrove forests line coasts in the tropics and subtropics.
1. Mangroves are some of the few types of trees that are salt-tolerant. They have unique types of
roots that curve upward like snorkels to attain oxygen lacking in the mud or that curve downward
like stilts to support the tree in changing water levels.
2. When mangroves are removed, coastal areas lose the ability to slow runoff, filter pollutants, and
retain soil.
D. Fresh water meets salt water in estuaries.
1. Many salt marshes and mangrove forests occur in or near estuaries—water bodies where rivers
flow into the ocean, mixing fresh water with salt water.
2. Estuaries everywhere have been affected by coastal development, water pollution, habitat
alteration, and overfishing.
E. Kelp forests harbor many organisms.
1. Kelp is a large, brown algae.
F. Coral reefs are treasure troves of biodiversity.
1. A coral reef is a mass of calcium carbonate composed of the skeletons of tiny marine animals
known as corals.
2. Corals are tiny invertebrate animals related to sea anemones and jellyfish.
3. Coral reefs protect shorelines by absorbing wave energy.
4. Coral reefs are experiencing alarming declines worldwide, however.
G. Open-ocean ecosystems vary in their biodiversity.
1. Primary production and animal life near the surface are concentrated in regions of nutrient-rich
upwelling.
2. In the little-known deep-water ecosystems, animals have adapted to tolerate extreme water
pressures and to live in the dark without food from autotrophs.
3. Ecosystems form around hydrothermal vents in the ocean, where heated water spurts from the
seafloor, carrying minerals that precipitate to form rocky structures.
IV. Marine Pollution
A. Plastic debris endangers marine life.
1. Organisms can become entangled in such debris and drown, or they may die as a result of
ingesting material they cannot digest or expel.
2. Because plastic is designed not to break down, it can drift for decades before washing up on
beaches.
3. Floating plastics are particularly harmful to marine life.
4. Plastics can have toxic effects on organisms.
5. Floating pieces of plastic also serve as rafts and transport species over long distances they could
not travel on their own.
6. Because plastics take 500–1,000 years to degrade at sea and there is no viable way to collect the
small bits of plastics that litter the oceans, preventing their entry into the oceans is key to remedying
oceanic plastic pollution.
B. Oil pollution comes from spills of all sizes.
1. Most oil pollution in the oceans accumulates from innumerable, widely spread, small non-point
sources, including leakage from small boats and runoff from human activities on land.
2. Over the past three decades, the amount of oil spilled by tankers worldwide has decreased, in part
because of an increased emphasis on spill prevention and response.
C. Toxic pollutants can contaminate seafood.
1. Mercury is a toxic heavy metal released into the environment from coal combustion, mine tailings,
and other sources.
2. Because seafood is an important part of a healthy diet, nutritionists do not advocate avoiding
seafood entirely. However, people in at-risk groups should avoid fish high in mercury (such as
swordfish, shark, and albacore tuna), while continuing to eat seafood low in mercury (such as catfish,
salmon, and canned light tuna).
D. Excess nutrients cause algal blooms.
1. Excessive nutrient concentrations sometimes give rise to population explosions among some
species of marine algae. These dinoflagellate algae produce powerful toxins that attack the nervous
systems of vertebrates. Blooms of these algae are known as harmful algal blooms.
a. Some dinoflagellate species produce reddish pigments that discolor surface waters, and
blooms of these species are nicknamed red tides.
V. Emptying the Oceans
A. We have long overfished.
B. Fishing has industrialized.
1. Today’s industrialized commercial fishing fleets employ fossil fuels, huge vessels, and powerful
new technologies to capture fish in great volumes.
2. The modern fishing industry uses several methods to capture fish at sea that are highly efficient
but also environmentally damaging.
a. Some vessels set out long driftnets that span large expanses of water.
3. Longline fishing involves setting out extremely long lines (up to 80 km [50 mi] long) with up to
several thousand baited hooks spaced along their lengths.
4. Trawling entails dragging immense cone-shaped nets through the water, with weights at the
bottom and floats at the top.
C. Fishing practices kill nontarget animals and damage ecosystems.
1. Bycatch, the accidental capture of animals, accounts for the deaths of millions of fish, sharks,
marine mammals, and birds each year.
D. Modern fishing fleets deplete marine life rapidly.
1. When animals at high trophic levels are removed from a food web, the proliferation of their prey
can alter the nature of the entire community.
E. Several factors mask declines.
1. Although industrialized fishing has depleted fish stocks in region after region, the overall global
catch has remained roughly stable for two decades.
2. More powerful technology helps explain large catches despite declining stocks.
F. We are “fishing down the food chain.”
1. Analyses of fisheries data reveal that as fishing increases, the size and age of fish caught decline.
2. In addition, as particular species become too rare to fish profitably, fleets begin targeting other
species that are more abundant.
3. Some of these species were undesirable species that fishermen formerly threw back when fishing
for more marketable species, but underwent “image makeovers” to aid their sale to consumers.
G. Marine biodiversity loss erodes ecosystem services.
VI. Marine Conservation
A. Fisheries management has been based on maximum sustainable yield.
1. The goal of this strategy is to allow for maximal harvests of particular populations while keeping
fish available for the future.
2. Despite efforts to restrict fish harvests to sustainable levels, a number of fish and shellfish stocks
have plummeted.
B. We can protect areas in the ocean.
1. Hundreds of marine protected areas (MPAs) have been established, mostly along coastlines of
developed countries. Nearly all MPAs allow fishing and other extractive activities.
2. Because of the lack of true refuges from fishing pressure, many scientists want to establish areas
where fishing is prohibited. Such “no-take” areas have come to be called marine reserves.
C. Reserves can work for both fish and fishers.
1. Data indicate that marine reserves do work, boosting fish biomass and total catch while decreasing
habitat destruction.
D. How should reserves be designed?
1. Studies are modeling how to optimize the size and spacing of reserves so that ecosystems are
protected, fisheries are sustained, and people are not overly excluded from marine areas.
VII. Conclusion
A. Oceans cover most of our planet, and we are still exploring their diverse topography and ecosystems. As
we learn more about marine and coastal environments, we continue to intensify our use of their resources
and to exert more severe impacts.
B. Yet scientists are demonstrating that setting aside protected areas of the ocean can serve to maintain and
restore natural systems and also to enhance fisheries.
C. In the meantime, we can all contribute through consumer choices that can help move us toward
sustainable fishing practices.