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Ocean Dead Zones - Interaction Among Earth's Sub-systems |
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A prime example of the interconnectedness of ocean, land, and impacts of human activity in the Earth system is the increase in number and intensity of "dead zones." Dead zones are ocean areas where dissolved oxygen in bottom and near-bottom waters declines to deadly proportions. Such areas of the seafloor with too little oxygen for most marine life are produced when excess nutrients, especially nitrogen and phosphorus compounds, enter coastal surface waters and spur algal blooms. When the algae die, they sink to the seafloor. Their decomposition consumes the oxygen dissolved in the bottom waters, leaving a "hypoxic" (low oxygen) or "anoxic" (no oxygen) environment that is lethal to many marine species.
Ocean dead zones are primarily a coastal phenomenon. It has been estimated that over 400 such areas exist worldwide (see http://www.eurekalert.org/pub_releases/2008-08/viom-ssc081108.php). A dead zone was first reported in the Chesapeake Bay in the 1930s. Most are seasonal, typified by the largest dead zone in the United States which expands off the coast of Louisiana and Texas in late spring and summer. It results from the Mississippi River system which transports huge quantities of nutrients originating as farm fertilizers and organic wastes to the Gulf of Mexico. Figure 1 displays the drainage system of the Mississippi River and its tributaries that drain about 40% of the coterminous US. Consequently, Iowa farmers fertilizing their land to increase corn crop yield are contributing to a key stressor on marine ecosystems over a thousand miles away that ranks with over-fishing, habitat loss, and harmful algal blooms as global environmental problems. This demonstrates clearly that human activity far from the ocean [(can) (cannot)] have dramatic effect on the ocean.
Figure 1. The Mississippi River and its tributaries that drain into the Gulf of Mexico. [Goddard Space Flight Center/NASA]
Image 1 graphically depicts how the Gulf of Mexico dead zone forms. During the spring, relatively warm freshwater discharge from the Mississippi River system flows into the Gulf and floats over the [(more) (less)] dense seawater. This surface water layer acts as a barrier that prevents the replenishment of dissolved oxygen in the seawater from the overlying atmosphere. [Image 1 is from The Times-Picayune, http://blog.nola.com/graphics/deadzone_how061007.gif ]
The nutrients, including those from fertilizer runoff and from sewage discharged into the Mississippi River system, ignite algal blooms in the warmed surface water subjected to the increasingly intense sunlight during spring that remains strong into summer. The dead algae then sink and begin to decompose as they settle through the underlying seawater to the ocean bottom. The decomposition process results in the [(loss) (gain)] of dissolved oxygen in the deeper seawater.
Dissolved oxygen is an essential ingredient for sustaining life in the marine food chain. Without oxygen, commercially important fish and shellfish (e.g., crabs and oysters) die or are driven from their habitat. The result is the creation of a dead zone in bottom and near-bottom waters. The Gulf of Mexico dead zone typically persists until late summer and into early autumn when passing storms, including hurricanes, and cooler temperatures act to stir and break up the layered water structure.
Figure 2 depicts the bottom dissolved oxygen concentration in milligrams per liter (mg/L) along the Gulf of Mexico coast extending from Louisiana to eastern Texas as observed 21-27 July 2008. The color coding indicates that the red core of the dead zone had dissolved oxygen values generally lower than [(2) (4) (6)] mg/L.
Figure 2. Gulf of Mexico Dead Zone Bottom Dissolved Oxygen, 21-27 July 2008. [Louisiana Universities Marine Consortium (LUMCON)]
The size of the 2008 low-oxygen Gulf of Mexico dead zone as measured from the Louisiana Universities Marine Consortium's research vessel Pelican by a group of scientists led by Nancy Rabalais measured over 20,720 square kilometers (8,000 square miles). The 2008 dead zone ranked second in size, equal to that of 2001, for the area of hypoxia since mapping began in 1985. The maximum size was recorded in 2002. (The 2008 dead zone was expected to set a new record for size until growth was slowed by the mixing associated with Hurricane Dolly in July.)
The phenomenon of dead zones, an example of cultural eutrophication (accelerated process of nutrient and sediment concentration in an aquatic system due to human activity), clearly evidence growing human impacts on the ocean environment. It, along with other observational evidence, demonstrates that we live in and are part of an Earth system. It also shows that no matter where we live, our actions can impact all the sub-systems of the Earth, including the ocean.
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