Dead zones, otherwise known as hypoxic zones, are areas that
have little to no oxygen concentrations. This means that life cannot thrive as
it would under normal oxygen concentrations. These “dead zones” have the potential
to occur naturally, such as the churning of ocean waters, but are more often
than not the culprit of human activity.
This human activity consists mainly of nutrient pollution
into the water. The rain will wash nutrients, namely nitrogen and phosphorus, from
the fertilizer runoff. Nitrogen and phosphorus will stimulate algae production
and feed marine phytoplankton. This algae will grow in excess and the plankton
will practice respiration and both will absorb the oxygen in the water leaving little
oxygen for marine life; the marine life will eventually suffocate and die.
Climate change may be a cause of dead zones because there is
a lesser concentration of oxygen in warmer waters. Airborne nitrogen from cars,
trucks, and power plants emit nitrogen that settle into waterways that make
their way to the ocean. This is a problem in Long Island Sound and the
Chesapeake Bay area. For the past 20 years Long Island Sound has suffered from
dead zones. Chesapeake Bay is host to many dead zones that all originate from a
different river. Seventy-five percent of the dead zones in the Chesapeake Bay
area are caused by agricultural runoff while the other twenty-five percent is
caused by airborne nitrogen.
The second largest dead zone in the world is located in the
Gulf of Mexico in the United States. It is estimated that this dead zone is
about the size of the state of Connecticut and is caused by nutrients in
agricultural runoff. While the Gulf dead
zone does not appear to be getting any bigger, nutrients may still be stored in
the sediment and the dead zone no longer thrives only with runoff. There are
few dead zones on the west coast compared to the east coast because there just
simply aren’t rivers draining into the ocean on the west coast. Coastal waters
are where most dead zones exist, but waters inland are not an exception to the
phenomena. There are an estimated 166 inland dead zones in the United States
alone, Lake Erie being home to one of them.
Worldwide, there are more than 400 dead zones. In Africa and
South America, sewage is the majority cause of dead zones. In India, it is
human waste and pesticide pollution; in China it is polluted rivers and
aquifers.
Dead zones have an impact on the aquaculture industry. In
the U.S. this includes coastal fisheries. The marine life will move because
they cannot tolerate the low oxygen concentrations. Economically, this is not
ideal for the fishing industry and the nation’s food supply. Hypoxic conditions
also have an adverse effect on the reproductive processes of fish. The marine
animals face severe stress and stop reproducing and laying eggs. This is in
part because of the hibernation-like state experienced by many species when the
oxygen supply becomes limited. A study done in Texas observed female fish
becoming masculinized with sperm appearing in the ovaries.
Dead zones have the ability to recover. Minimization of
fertilizer, the burning of fossil fuels, and using surfaces such as concrete
would play a large role in the convalescence of the ocean’s dead zones.
Sources:
The Atlas of Water, Part 5: Damaged Water
