Environmental Engineering Reference
In-Depth Information
valuable commercial and recreational Gulf fisheries. The size
of the 2012 dead zone was the fourth smallest since monitor-
ing began in 1985, at about 2,889 square miles. This was due
to the drought that reduced the amount of runoff. In contrast,
in 2011, flood conditions, carrying large amounts of nutrients,
resulted in a dead zone measuring 6,770 square miles, the size
of the state of New Jersey. In 2013 it was up to 5,840 square
miles—a bit bigger than Connecticut.
Dead zones now affect more than 400 systems, and cover
vast areas of the ocean—more than 475,000 square kilometers
(183.4 sq miles). While trends show increases worldwide, some
localized areas are improving. Of the 415 areas around the
world identified as experiencing some form of eutrophication,
169 are hypoxic and only 13 are classified as being in recov-
ery. In Chesapeake Bay the dead zone affects the distribution
and abundance of fishes including croaker, white perch, spot,
striped bass, and summer flounder, which are key parts of the
ecosystem and support commercial and recreational fisher-
ies. Scientists saw a drastic decline in species richness, species
diversity, and catch rate under low dissolved oxygen condi-
tions, suggesting that the fishes begin to avoid an area when
levels of DO drop below about 4 milligrams per liter, as they
start to suffer physiological stress. The response at this value
is interesting because it is greater than the 2 mg/l that is the
formal definition of hypoxia. Efforts to reduce inputs of fertil-
izers, animal waste, and other pollutants into Chesapeake Bay
appear to be helping. The size of summer dead zones in deep
channels of this bay has been declining.
Can excess nutrients damage salt marshes?
Coastal wetlands support fisheries, protect coasts from storms,
and provide habitat for wildlife. They are also able to absorb
nutrients from runoff, thereby protecting the nearby estuar-
ies. Salt marshes have been disintegrating and dying over the
past two decades along the US Eastern seaboard and other
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