Page 128 - Environment: The Science Behind the Stories

Atmospheric Figure 5.4 The
deposition— Chesapeake Bay
agriculture receives inputs of
(8%) Natural nitrogen (a) and
Agriculture—fertilizer Agriculture—fertilizer sources
and manure and manure (3%) phosphorus (b) from
(32%) (45%) many sources in its
Atmospheric watershed. Data from
deposition— Municipal
mobile, utilities and industrial Chesapeake Bay Program
Septic and industries wastewater Watershed Model Phase 4.3
systems (24%) Urban/suburban (21%) (Chesapeake Bay Program
(4%) Office, 2009).
fertilizer runoff and
Municipal and transported sediments
Urban/suburban industrial wastewater (31%)
fertilizer runoff (19%) Atmospheric
(10%) deposition—natural
(1%)

(a) Sources of nitrogen entering the Chesapeake Bay (b) Sources of phosphorus entering the Chesapeake Bay

We may perceive Earth’s systems surrounding our planet. The hydrosphere (p. 409) encompasses
in various ways all water—salt or fresh, liquid, ice, or vapor—in surface bod-
ies, underground, and in the atmosphere. The biosphere (p. 78)
There are many ways to delineate natural systems. Categorizing consists of all the planet’s organisms and the abiotic (nonliving)
environmental systems can help make Earth’s dazzling complex- portions of the environment with which they interact.
ity comprehensible to the human brain and accessible to prob- Picture a robin plucking an earthworm from the ground
lem solving. For instance, scientists sometimes divide Earth’s after a rain. You are witnessing an organism (the robin) con-
components into structural spheres. The lithosphere (p. 52) is suming another organism (the earthworm) by removing it
the rock and sediment beneath our feet, the planet’s uppermost from part of the lithosphere (the soil) that the earthworm had
mantle and crust. The atmosphere (p. 468) is composed of the air been modifying, after rain (from the hydrosphere) moistened

Freshwater river 1 Nitrogen and 2 Phytoplankton
phosphorus flourish at the
input surface

1 2

Warmer, less
dense, fresh-
water layer
(oxygenated)

Colder, denser
ocean water
layer
(hypoxic) 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

3 Dead phytoplankton 4 Microbial 5 Insufficient oxygen suffocates
and their waste drift decomposer oysters and grasses, fish and shrimp
to the bottom, population grows at the bottom; dead zone (hypoxic
providing more food and consumes zone) forms
for bacteria to more oxygen
decompose
Figure 5.5 Excess nitrogen and phosphorus causes eutrophication in aquatic systems such as the
Chesapeake Bay. Coupled with stratification (layering) of water, eutrophication can severely deplete dissolved

oxygen. Nutrients from river water 1 boost growth of phytoplankton 2 , which die and are decomposed
at the bottom by bacteria 3 . Stability of the surface layer prevents deeper water from absorbing oxygen to
replace oxygen consumed by decomposers 4 , and the oxygen depletion suffocates or drives away bottom-
dwelling marine life 5 . This process gives rise to hypoxic zones like those in the bay. 127

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