Page 140 - Environment: The Science Behind the Stories

Precipitation, runoff, and surface water Water returns fossil fuels, from plastics to pharmaceuticals, carbon (C) atoms
from the atmosphere to Earth’s surface as precipitation when are everywhere. The carbon cycle describes the routes that car-
water vapor condenses and falls as rain or snow. This moisture bon atoms take through the environment (Figure 5.17).
may be taken up by plants and used by animals, but much of it
flows as runoff into streams, rivers, lakes, ponds, and oceans. Photosynthesis, respiration, and food webs Pro-
Amounts of precipitation vary greatly from region to region, ducers—including plants, algae, and cyanobacteria—pull car-
helping give rise to our planet’s variety of biomes (pp. 111–117). bon dioxide out of the atmosphere and out of surface water to
use in photosynthesis. Photosynthesis (p. 50) breaks the bonds
in carbon dioxide (CO ) and water (H O) to produce oxygen
Groundwater Some precipitation and surface water soaks (O ) and carbohydrates (e.g., glucose, C H O ). Autotrophs
2
2
down through soil and rock and becomes groundwater, water use some of the carbohydrates to fuel cellular respiration,
2
6
6
12
found beneath layers of soil. Groundwater recharges aquifers, thereby releasing some of the carbon back into the atmos-
spongelike regions of rock and soil that are underground reser- phere and oceans as CO . When producers are eaten by pri-
voirs of water. The upper limit of groundwater held in an aquifer mary consumers, which in turn are eaten by secondary and
2
is referred to as the water table. (See Figure 15.4, p. 410, for tertiary consumers, more carbohydrates are broken down in
an illustration of these features.) Aquifers can hold groundwater cellular respiration, producing carbon dioxide and water. The
for long periods of time, so the water may be quite ancient. In same process occurs as decomposers consume waste and dead
some cases groundwater can take hundreds or even thousands organic matter. Cellular respiration from all these organisms
of years to recharge fully after being depleted. Groundwater releases carbon back into the atmosphere and oceans.
becomes surface water when it emerges from springs or flows Organisms use carbon for structural growth, so a por-
into streams, rivers, lakes, or the ocean from the soil (p. 411).
tion of the carbon an organism takes in becomes incorporated
into its tissues (such as net primary production in plants;
Our impacts on the water cycle p. 129). The abundance of plants and the fact that they take
are extensive in so much carbon dioxide for photosynthesis makes plants a
major reservoir for carbon. Because CO is a greenhouse gas
2
Human activity affects every aspect of the water cycle. By of primary concern (p. 502), much research on global climate
damming rivers, we slow the movement of water from the change is directed toward measuring the amount of CO that
2
land to the sea, and we increase evaporation by holding plants store. Scientists are working hard to better understand
water in reservoirs. We remove natural vegetation by clear- exactly how this portion of the carbon cycle influences Earth’s
cutting and developing land, which increases surface runoff, climate (see The Science behind The STory, pp. 142–143).
decreases infiltration and transpiration, and promotes soil
erosion. Our withdrawals of surface water and groundwater Sediment storage of carbon As aquatic organisms
for agriculture, industry, and domestic uses deplete rivers, die, their remains may settle in sediments in ocean basins or
lakes, and streams and lower water tables. And by emitting freshwater wetlands. As sediment accumulates, older layers
into the atmosphere pollutants that dissolve in water droplets, are buried more deeply, experiencing high pressure over long
we change the chemical nature of precipitation, in effect sab- periods of time. These conditions can convert soft tissues into
otaging the natural distillation process that evaporation and fossil fuels—coal, oil, and natural gas (p. 539)—and can turn
transpiration provide. Water shortages have already given rise shells and skeletons into sedimentary rock, such as limestone.
to conflicts worldwide, from the Middle East to the American Sedimentary rock (p. 55) comprises the largest reservoir in the
West (pp. 425–426). (We will revisit the water cycle, water carbon cycle. Although any given carbon atom spends a rela-
resources, and human impacts in more detail in Chapter 15). tively short time in the atmosphere, carbon trapped in sedimen-
tary rock may reside there for hundreds of millions of years.
Carbon trapped in sediments and fossil fuel deposits may
WEIGhING ThE ISSUES eventually be released into the oceans or atmosphere by geo-
logic processes such as uplift, erosion, and volcanic eruptions CHAPTER 5 • Envi R onm E n TA l S y STE m S A nd E C o S y STE m E C ology
yOUr WaTEr Has your region faced any water shortages or
conflicts over water use? If not, can you describe how such (pp. 58–60). It also reenters the atmosphere when we extract
problems affect some other region? What is the quality of and burn fossil fuels.
your region’s water, and what pollution threats does it face? The oceans The world’s oceans are the second-largest
Given your knowledge of the water cycle, what solutions reservoir in the carbon cycle. They absorb carbon-containing
would you propose for water shortages and/or water pollu- compounds from the atmosphere, from terrestrial runoff, from
tion in your region?
undersea volcanoes, and from the waste products and detri-
tus of marine organisms. Some carbon atoms absorbed by the
2−
oceans—in the form of carbon dioxide, carbonate ions (CO ),
3
The carbon cycle circulates and bicarbonate ions (HCO )—combine with calcium ions
−
3
2+
a vital organic nutrient (Ca ) to form calcium carbonate (CaCO ), an essential ingredi-
3
ent in the skeletons and shells of microscopic marine organ-
As the definitive component of organic molecules (p. 46), carbon isms. As these organisms die, their calcium carbonate shells
is an ingredient in carbohydrates, fats, and proteins and occurs in sink to the ocean floor and begin to form sedimentary rock. The
the bones, cartilage, and shells of all living things. From DNA to rates at which the oceans absorb and release carbon depend on 139

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