Page 650 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

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C oastal ocean waters are often highly productive ecosystems teeming with marine life. Yet they can become dead zones where organisms die because of low-oxygen conditions (GIA 19.1). Dead zones result from a process that begins with agricultural runoff of fertilizers and farm-animal
wastes (GIA 19.2, 19.3, and 19.4). The size of dead zones can vary from year to year (GIA 19.5). Freshwater lakes are also subject to “blooms” of the algae that form dead zones (GIA 19.6).
19.1
19.2
19.3
19.3a
19.3b
Baltic Sea dead zone:
Gulf of Mexico dead zone:
SWEDEN
BALTIC SEA
LATVIA
Visit the Study Area in MasteringGeographyTM to explore dead zones.
Visualize: Study the video of phytoplankton. Assess: Demonstrate understanding of dead zones (if assigned by instructor).
FORMATION OF A DEAD ZONE
1
Agricultural runoff enters rivers, which then moves downstream to the ocean or to a lake.
Algal bloom
2
The nutrients, mainly nitrogen and phosphorus, cause algal blooms.
In the water’s surface layer, agricul- tural runoff delivers nitrogen and phosphorus, nutrients that greatly boost the growth of algae, producing an algal bloom. When the algae die, they sink into the bottom layer. Bacteria feed on the dead algae and deplete the water of oxygen, forming a dead zone. Marine organisms that cannot leave the dead zone will die.
Explain: How does the water in a dead zone become depleted of oxygen?
DEAD ZONES: A GLOBAL PROBLEM
Generally, dead zones correlate with large human populations, mouths
of rivers that deliver nitrogen-rich pollutants, and semi-enclosed bodies of water such as seas,
bays, and estuaries. [NASA.]
Reading the map: The amount of organic matter
in the ocean surface layer
is shown in shades of blue. Dead zones, which form where organic matter is high, are shown as red circles.
Infer: What do you think explains the large number of dead zones along the east coast of the United States?
VIEWS OF DEAD ZONES
Saltwater
3
The algae die, sink into the bottom layer, and are decomposed by bacteria, using up the oxygen in the water.
Oxygen-deprived
saltwater
4
4 A dead zone (defined as water
with less than 2 mg · l –1 dissolved oxygen) is formed, killing organisms that cannot flee.
River water
1
2
3
Satellite images reveal the effects of nutrient-rich water reaching the ocean (GIA 19.3a and 19.3b). Dead zone formation is seasonal: algal blooms occur in the spring, and dead zones can last through the summer. [NASA.]
Channels in the Mississippi River’s delta bring nutrients that fuel the green algal blooms seen in this image and help to form the Gulf’s large, and expanding, dead zone.
Swirling, green plumes of algae are visible in the Baltic Sea between Sweden and Latvia. Countries around the sea have reduced nitrogen fertilizer runoff and the duration of the dead zone.
geosystems in action 19 COasTal DEaD ZONEs