Hypoxic Dead Zones: How Fertilizers Are Drowning the Seas
By the middle of 2013, there were some 500 places in the world's oceans, seas, lakes, and estuaries where even a fish could drown. These so-called dead zones, hypoxic areas where too little oxygen is dissolved in the water to support conventional levels of marine life, present a growing problem to ecosystems and economies all over the planet.
Water is considered hypoxic when it contains less than two parts per million of oxygen, rather than the normal five or more parts per million. Because hypoxic water is unable to support any appreciable amount of marine life, large areas of hypoxic water can threaten entire aquatic populations--as well as the land animals and fishers who depend on these populations for their respective food supplies and livelihoods. Hypoxic water can occur naturally, but dead zones are becoming an increasingly common phenomenon owing in large part to the excesses of modern agriculture.
In nature, dead zones most often form as a result of eutrophication, a process by which an explosion in an algae population exhausts the oxygen supply of the local water, thereby killing or driving off all other marine life. Eutrophication is caused, ironically, by too much of a good thing: nutrients. Algae take advantage of the excess of nutrients, which subsequently leads to a massive algae bloom. This alone poses problems, as floating algae can block sunlight from reaching lower levels of water, impeding photosynthesis for aquatic plants. When the algae die, however, the problem is compounded, because their decomposition leaches oxygen from the water, further threatening marine life.
Although eutrophication can occur on its own, the runoff from agricultural fertilizer is largely responsible for exacerbating the occurrence of hypoxic dead zones. Groundwater runoff from heavily fertilized farms dumps staggering amounts of nitrogen- and phosphorus -rich nutrients into various bodies of water. These nutrients spur mass eutrophication, in turn creating large hypoxic dead zones.
Perhaps the most infamous dead zone recurs each summer in the Gulf of Mexico. The Runoff from Seasonal rains over farms in the American Midwest carries more than 1.5 million tons of nitrogen from fertilizer into the Mississippi River, which eventually deposits the elemental nutrient into the Gulf. The ensuing eutrophication creates a hypoxic zone that often reaches more than 7,000 square miles (18,100 sq km) in size, making it the second largest such zone in the world.
While the increase in the area and frequency of dead zones can be directly correlated with increases in the use of nitrogen- and phosphorus-rich fertilizers, several other factors-such as the draining of wetlands, which absorb fertilizer runoff before it reaches major water bodies-complicate the issue. There is no universal solution to the dead zone problem, but there have been some successes. The Baltic Sea dead zone, the largest recurring hypoxic zone in the world, has been appreciably reduced since the 1980s, when the Danish government began controlling phosphorus content in wastewater, regulating fertilizer use, and restoring wetlands. Even greater results were achieved in the Black Sea, where extreme reduction in nutrient runoff into the Danube River basin saw a hypoxic zone that had reached a size of 7,700 sq miles (19,940 sq km) in the 1980s disappear entirely in 1996.
Today, several "ecotechnology" ventures are under way to create natural wetlandlike "filters" between fertilized farmlands and water supplies in order to mitigate eutrophication downstream. Although the rate of dead zone recurrence has slowed in recent years, it is still rising, and it remains to be seen whether a combination of runoff controls and ecotechnology can counter the growing phenomenon. Unless more rigorous investigation and regulation of nutrient runoff is undertaken on a global scale, however, the fertilizer-enriched bounties of the land could very easily lead to a global famine at sea.
Water is considered hypoxic when it contains less than two parts per million of oxygen, rather than the normal five or more parts per million. Because hypoxic water is unable to support any appreciable amount of marine life, large areas of hypoxic water can threaten entire aquatic populations--as well as the land animals and fishers who depend on these populations for their respective food supplies and livelihoods. Hypoxic water can occur naturally, but dead zones are becoming an increasingly common phenomenon owing in large part to the excesses of modern agriculture.
In nature, dead zones most often form as a result of eutrophication, a process by which an explosion in an algae population exhausts the oxygen supply of the local water, thereby killing or driving off all other marine life. Eutrophication is caused, ironically, by too much of a good thing: nutrients. Algae take advantage of the excess of nutrients, which subsequently leads to a massive algae bloom. This alone poses problems, as floating algae can block sunlight from reaching lower levels of water, impeding photosynthesis for aquatic plants. When the algae die, however, the problem is compounded, because their decomposition leaches oxygen from the water, further threatening marine life.
Although eutrophication can occur on its own, the runoff from agricultural fertilizer is largely responsible for exacerbating the occurrence of hypoxic dead zones. Groundwater runoff from heavily fertilized farms dumps staggering amounts of nitrogen- and phosphorus -rich nutrients into various bodies of water. These nutrients spur mass eutrophication, in turn creating large hypoxic dead zones.
Perhaps the most infamous dead zone recurs each summer in the Gulf of Mexico. The Runoff from Seasonal rains over farms in the American Midwest carries more than 1.5 million tons of nitrogen from fertilizer into the Mississippi River, which eventually deposits the elemental nutrient into the Gulf. The ensuing eutrophication creates a hypoxic zone that often reaches more than 7,000 square miles (18,100 sq km) in size, making it the second largest such zone in the world.
While the increase in the area and frequency of dead zones can be directly correlated with increases in the use of nitrogen- and phosphorus-rich fertilizers, several other factors-such as the draining of wetlands, which absorb fertilizer runoff before it reaches major water bodies-complicate the issue. There is no universal solution to the dead zone problem, but there have been some successes. The Baltic Sea dead zone, the largest recurring hypoxic zone in the world, has been appreciably reduced since the 1980s, when the Danish government began controlling phosphorus content in wastewater, regulating fertilizer use, and restoring wetlands. Even greater results were achieved in the Black Sea, where extreme reduction in nutrient runoff into the Danube River basin saw a hypoxic zone that had reached a size of 7,700 sq miles (19,940 sq km) in the 1980s disappear entirely in 1996.
Today, several "ecotechnology" ventures are under way to create natural wetlandlike "filters" between fertilized farmlands and water supplies in order to mitigate eutrophication downstream. Although the rate of dead zone recurrence has slowed in recent years, it is still rising, and it remains to be seen whether a combination of runoff controls and ecotechnology can counter the growing phenomenon. Unless more rigorous investigation and regulation of nutrient runoff is undertaken on a global scale, however, the fertilizer-enriched bounties of the land could very easily lead to a global famine at sea.
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