85
ranks highly with respect to total species richness and total number of endemic
species (WWF 2013).
With regard to water quality, the Mississippi River drainage is in equally bad
shape. Levels of more than 80 pesticides and pesticide metabolites tested for by the
U.S. Geological Survey routinely exceed the guidelines for sustaining aquatic life,
particularly in rivers draining urban areas. Other pollutants include additional
organochlorine compounds (e.g., PCBs), trace elements (e.g., arsenic, mercury,
cadmium), volatile organic compounds (e.g., toluene, MTBE), and pathogens
(Kleiss et al. 2000). These are among the factors underlying the informal designation of the lower Mississippi River region between Baton Rouge and New Orleans,
Louisiana as ‘cancer alley.’
Like an unfortunately large number of other rivers around the world, the
Mississippi is now infamous for the presence of an extensive hypoxic or dead zone
around its mouth. Hypoxia occurs when dissolved oxygen levels are ≤2 mg/L
(Rabalais et al. 2007). This results from excess nutrients that lead to high primary
production and then decay of organic matter. The Mississippi and the nearby
Atchafalaya River deliver 91% of the annual nitrogen load and 88% of the annual
phosphorus load to the northern Gulf of Mexico (Dunn 1996). The northern Gulf
water is stratified as a result of salinity and thermal differences and this stratification
intensifies during summer months with thermal warming of surface waters (Wiseman
et al. 1997). Nutrient delivery from the rivers fuels high organic production in the
surface waters, creating a flux of carbon to the bottom waters in the form of senescent plankton and fecal pellets. Decomposition of this material by aerobic bacteria
consumes dissolved oxygen faster than it is resupplied from stratified surface
waters, causing hypoxia that persists for months in bottom waters (CENR 2000;
Rabalais et al. 2007). Few marine animals can survive in hypoxic conditions, which
can extend upward into the water column (Rabalais and Turner 2001). The first
systematic mapping and monitoring of dissolved oxygen levels around the mouth of
the Mississippi began in 1985 and revealed the existence of a hypoxic zone in the
Gulf of Mexico. Between 1985 and 1992 the extent of this zone averaged 8200 km
2
,
but the average extent increased to 15,900 km
2
between 1992 and 2007, reaching a
historic maximum extent of 22,000 km
2
in 2002 (Rabalais et al. 2007).
Declining sediment delivery to the mouth of the Mississippi River has caused
delta and coastal erosion. The Mississippi reaches the Gulf of Mexico at the border
of the State of Louisiana. This state has been losing coastal wetlands at a rate that
starts at 17 km
2
each year in 1917 and rises to 117 km
2
a year during the 1960s
before leveling off to the present rate of 69 km
2
pear year (Boesch et al. 1994). In
addition to declining sediment delivery by the river, oil and gas pipelines and access
canals within the coastal wetlands create pollution and salinity intrusions that kill
wetland vegetation. When present, this vegetation stabilizes barrier islands off the
river’s mouth (Turner 1997). Vegetation die offs exacerbate coastal erosion.
Vegetation in inland coastal wetlands is also stressed by lack of flooding and associated nutrient delivery as a result of levees and flow regulation, as well as herbivory
by nutria (Myocastor coypus), a large rodent introduced from South America. Sea
level at the Mississippi delta is rising by 1.2–4.3 cm per year. This rise results from
3.3 Cumulative Effects
ranks highly with respect to total species richness and total number of endemic
species (WWF 2013).
With regard to water quality, the Mississippi River drainage is in equally bad
shape. Levels of more than 80 pesticides and pesticide metabolites tested for by the
U.S. Geological Survey routinely exceed the guidelines for sustaining aquatic life,
particularly in rivers draining urban areas. Other pollutants include additional
organochlorine compounds (e.g., PCBs), trace elements (e.g., arsenic, mercury,
cadmium), volatile organic compounds (e.g., toluene, MTBE), and pathogens
(Kleiss et al. 2000). These are among the factors underlying the informal designation of the lower Mississippi River region between Baton Rouge and New Orleans,
Louisiana as ‘cancer alley.’
Like an unfortunately large number of other rivers around the world, the
Mississippi is now infamous for the presence of an extensive hypoxic or dead zone
around its mouth. Hypoxia occurs when dissolved oxygen levels are ≤2 mg/L
(Rabalais et al. 2007). This results from excess nutrients that lead to high primary
production and then decay of organic matter. The Mississippi and the nearby
Atchafalaya River deliver 91% of the annual nitrogen load and 88% of the annual
phosphorus load to the northern Gulf of Mexico (Dunn 1996). The northern Gulf
water is stratified as a result of salinity and thermal differences and this stratification
intensifies during summer months with thermal warming of surface waters (Wiseman
et al. 1997). Nutrient delivery from the rivers fuels high organic production in the
surface waters, creating a flux of carbon to the bottom waters in the form of senescent plankton and fecal pellets. Decomposition of this material by aerobic bacteria
consumes dissolved oxygen faster than it is resupplied from stratified surface
waters, causing hypoxia that persists for months in bottom waters (CENR 2000;
Rabalais et al. 2007). Few marine animals can survive in hypoxic conditions, which
can extend upward into the water column (Rabalais and Turner 2001). The first
systematic mapping and monitoring of dissolved oxygen levels around the mouth of
the Mississippi began in 1985 and revealed the existence of a hypoxic zone in the
Gulf of Mexico. Between 1985 and 1992 the extent of this zone averaged 8200 km
2
,
but the average extent increased to 15,900 km
2
between 1992 and 2007, reaching a
historic maximum extent of 22,000 km
2
in 2002 (Rabalais et al. 2007).
Declining sediment delivery to the mouth of the Mississippi River has caused
delta and coastal erosion. The Mississippi reaches the Gulf of Mexico at the border
of the State of Louisiana. This state has been losing coastal wetlands at a rate that
starts at 17 km
2
each year in 1917 and rises to 117 km
2
a year during the 1960s
before leveling off to the present rate of 69 km
2
pear year (Boesch et al. 1994). In
addition to declining sediment delivery by the river, oil and gas pipelines and access
canals within the coastal wetlands create pollution and salinity intrusions that kill
wetland vegetation. When present, this vegetation stabilizes barrier islands off the
river’s mouth (Turner 1997). Vegetation die offs exacerbate coastal erosion.
Vegetation in inland coastal wetlands is also stressed by lack of flooding and associated nutrient delivery as a result of levees and flow regulation, as well as herbivory
by nutria (Myocastor coypus), a large rodent introduced from South America. Sea
level at the Mississippi delta is rising by 1.2–4.3 cm per year. This rise results from
3.3 Cumulative Effects