Page 131 - Environment: The Science Behind the Stories

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Figure 5.8 A world map of net primary production based on satellite data shows that on land, net
primary production varies geographically with temperature and precipitation. In the world’s oceans,
net primary production (shown here as grams of carbon fixed per square meter per year) is highest around the
margins of continents, where nutrients (of both natural and human origin) run off from land. Data from Field, C.B.,
et al., 1998. Primary production of the biosphere: Integrating terrestrial and oceanic components. Science 281: 237–240. Reprinted
with permission from AAAS.
levels typical for lakes in the region (Figure 5.9). This dif- of Michigan. Scientists calculate that the amount of marine
ference held until shortly after they stopped fertilizing seven life missing from the oceans as a result of dead zones likely
years later. At that point, algae decreased to normal levels exceeds the total amount of shellfish harvested each year from
in the half that had previously received phosphate. Such the entire United States—a harvest worth over $2 billion.
experiments showed clearly that phosphorus addition can The good news is that in locations where people have
markedly increase primary productivity in freshwater lakes. reduced nutrient runoff, dead zones have begun to disappear.
Similar experiments in coastal ocean waters show nitrogen In New York City, hypoxic zones at the mouths of the Hudson
to be the more important limiting factor for primary productiv-
ity. In experiments in the 1980s and 1990s, Swedish ecologist
Edna Granéli took samples of ocean water from the Baltic Sea Figure 5.9 The upper portion of this lake in Ontario was
and added phosphate, nitrate, or nothing. Chlorophyll and phyto- experimentally treated with the addition of phosphate. This
plankton increased greatly in the flasks with nitrate, whereas those treated portion experienced an immediate, dramatic, and pro-
with phosphate did not differ from the controls. Experiments in longed algal bloom, identifiable by its opaque waters.
Long Island Sound by other researchers show similar results. For
open ocean waters far from shore, research indicates that iron is
a highly effective nutrient for stimulating phytoplankton growth.
Increased nutrient pollution from farms, cities, and indus-
tries has led to the development of over 500 documented
hypoxic dead zones globally as of 2010 (Figure 5.10), includ-
ing that of the Chesapeake Bay as well as a large dead zone
that forms each year in the Gulf of Mexico off the Louisiana
coast near the mouth of the Mississippi River (pp. 430–431).
Some are seasonal (like the Chesapeake Bay’s), some occur
irregularly, and others are permanent. The increase in the num-
ber of dead zones—there were 162 documented in the 1980s
and 49 in the 1960s—reflects how the activities of people are
changing nutrient concentrations in waters around the world.
If one were to add up all the world’s marine and coastal
130 dead zones, they would cover an area the size of the state

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