CASE STUDY
GULF OF MEXICO DEAD ZONE: Currently, the Gulf of Mexico’s dead
zone, off the coast of Louisiana and Texas, is the largest hypoxic (low
oxygen) zone in the United States (about the size of the state of Connecticut).
The hypoxic zone in the Gulf of Mexico forms every summer and is a result
of excess nutrients from the Mississippi River (which is the drainage area for
about 40% of the continental United States) and seasonal stratification
(layering) of waters in the Gulf. Nutrient-laden freshwater from the
Mississippi River flows into the Gulf of Mexico. This freshwater is less dense
and remains above the more dense saline Gulf water. In addition to the saline
gradient caused where the freshwater and saline water meet, the freshwater is
warmer than the deeper ocean water, further contributing to the stratification.
This stratification prevents the mixing of oxygen-rich surface water with
oxygen-poor water on the bottom of the Gulf. Without mixing, oxygen in the
bottom water is limited and the hypoxic condition continues.THERMAL SOURCES
Heat that is produced by industry and power plants and discharged into
waterways reduces the water’s ability to hold oxygen and kills organisms that
cannot tolerate heat and/or low oxygen levels.
Global warming is also imparting additional heat to the oceans, rivers, and
streams. Marine species that are especially affected by climate change include
plankton, which form the base of most marine food chains.
CASE STUDIES
MINAMATA DISEASE: Twenty-seven tons of mercury-containing
compounds from industrial processes were dumped into Minamata Bay in
Japan between 1932 and 1968. The mercury collected in fish and shellfish
caught from the bay. Symptoms included blurred vision, hearing loss, loss of
muscular coordination, and reproductive disorders.