Download 5. The biggest causes of topsoil erosion are

Science 7 – 1/12/2016
Name___________________________
Per____
Topsoil erosion
1. Topsoil is the uppermost layer of soil, composed of _______________, _______________, and
____________________.
2. More than ______ percent of the world’s food comes from topsoil.
3. It often takes over ________ years to create an ____________ of topsoil.
4. Scientists estimate that ___________________ of the world’s topsoil has been lost or degraded. If this
trend continues, it is estimated that all of the world’s topsoil will be gone in ______ years.
5. The biggest causes of topsoil erosion are _____________________________, _______________________,
and ______________________. These practices mainly cause erosion through ________________ and
____________.
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Gulf of Mexico “Dead Zone”
Based on the article, describe how agricultural runoff has led to a “dead zone” in the Gulf of Mexico.
Dartmouth Undergraduate Journal of Science
Eutrophication in the Gulf of Mexico: How Midwestern farming practices are creating a
‘Dead Zone’
Human activity in the Midwestern United States impacts not only the surrounding land but also the water in
rivers and streams, which run all the way into the Gulf of Mexico. Scientists have identified several human
actions, both historical and ongoing, that have contributed to major changes in the Gulf’s waters and
ecosystems. Each of these manmade environmental changes has resulted in unnaturally high levels of specific
nutrients, particularly nitrogen and phosphorus. This excess of nutrients in a body of water as a result of runoff
is called eutrophication, and can often lead to hypoxia—the subsequent depletion of dissolved oxygen in the
water.
A report written in 2000 by the National Science and Technology Council Committee on Environment and
Natural Resources outlines three historical trends that have contributed to eutrophication. The first event was
river channelization for navigation and flood control, which occurred prior to the 1950’s. Next was landscape
alteration by humans. This includes deforestation and the expansion of agricultural drainage, both historically
and currently. The problem with changing the landscape in these ways is that natural buffers for runoff are
removed, which results in more nutrients entering rivers and streams. Vegetation acts as a filter for pollutants
before they enter rivers or streams, and deforestation removes these plants and destroys the soil which would
also help filter out harmful nutrients. The final event that the report lists, which is most important because it is
ongoing and accelerating in rate, is the recent increase of nitrogen input to the Mississippi-Atchafalaya River
Basin, primarily from the application of agricultural fertilizers (1).
Agriculture has caused an increase in the flow of nutrients from chemical fertilizers into bodies of water. The
excess nutrients change the chemical composition of the water, impacting biological life forms in the affected
areas. Sewage is another major source of nutrient flow to the Gulf. There are sections of lakes and oceans all
over the world affected by eutrophication and hypoxia, and this has not only biological effects but also
economic and social problems. The largest hypoxic area in the western Atlantic Ocean is found in the Gulf of
Mexico (1). The biological repercussions of eutrophication, in the form of habitat alteration and entire trophic
structure disintegration are devastating to the Gulf; remediation, though costly, must be put into effect in order
to bring life back into the area.
The ‘Dead Zone’
The Gulf of Mexico acts as a drainage zone for all the major rivers and tributaries in the Midwestern United
States. This network is called the Mississippi-Atchafalaya River Basin (MARB), and includes major rivers such
as the Mississippi, the Missouri, and their tributaries, which all eventually drain into the Gulf. As shown in Fig.
1, the MARB contains water from 31 states and is the third-largest river basin in the world, behind the Amazon
and the Congo basins (2). Several nutrients, the most harmful of which is nitrogen, enter streams as runoff
partly from sewage but mainly as runoff from agricultural fertilizers used in the 31 states in the MARB. The
exact location of the area suffering from eutrophication stretches westward from the Mississippi River delta to
the upper Texas coast. This zone’s area varies between 6,000 and 7,000 square miles—nearly the size of New
Jersey (3).
Why is excess nitrogen so harmful to a body of water? The main reason is that it is no longer a limiting factor to
growth, particularly to algal growth. Immense blooms of algae are produced from the nitrogen surplus, and
these blooms are problematic. In addition to lowering the water quality, they also lead to hypoxia; when
bacteria in the water eat these algae, much of the water column’s oxygen supply is used up so that the overall
amount of oxygen dissolved in the water is depleted. It should also be noted that, though nitrogen is present in
the MARB in a few different forms such as dissolved organic or particulate organic nitrogen, its most common
form is as dissolved inorganic nitrogen, or nitrate. The National Science and Technology Council reports, “The
most significant trend in nutrient loads has been an increase in nitrate load, which has almost tripled from 0.33
million metric tons per year during 1955-70 to 0.95 million metric tons per year during 1980-96” (1).
Nitrogen in the form of nitrates is a main component of fertilizers that are used in massive quantities in the
Midwest. In 2008, over 12.5 million tons of nitrogen were applied as part of fertilizers throughout the United
States (4). Because farming induces it, the zone of eutrophication in the Gulf fluctuates seasonally, decreasing
in size during the winter months. The nutrient flux through the MARB also varies with weather – hurricanes,
flooding, or heavier rainfall increase flux by resulting in more runoff. These nutrients travel from farmlands all
the way down to the Mississippi River basin, where they are emptied into the Gulf of Mexico and wreak havoc
on the aquatic ecosystems.
Hypoxic waters are specifically defined as those with oxygen levels below 2 milligrams of dissolved oxygen per
liter. Hypoxic areas in the Gulf are normally between 5 and 30 meters deep, but can be as deep as 60 meters or
as shallow as 4 near the shore (1). Hypoxia in the area creates what is known as a ”dead zone,” or an area in the
water where there are little to no living organisms. In severe cases, ecosystems can completely collapse (5).
While the growth of organisms is inhibited when oxygen levels dip below 5 mg O2/L, the natural processing of
nutrients, pollutants, and organic matter is also disrupted (1).