Dead Zones: Sucking the Life from the World’s Oceans
By Marisa Bauxbaum
The difference between hypoxic water (murky green) and well-oxygenated water (clear blue). Photo: Nancy Rabalais/Louisiana Universities Marine Consortium
Understandably, recent environmental media coverage on the Gulf of Mexico has focused on the Deepwater Horizon oil spill of 2010. Further threatening the Gulf and other aquatic systems around the globe is a different, chronic affliction. Since the 1960s, both the incidence and severity of dead zones have increased in coastal and estuarine ecosystems. Dead zones are incapable of supporting most aquatic life due to a lack of dissolved oxygen in the water. This is a condition known as hypoxia, or in extreme cases, anoxia (no dissolved oxygen). The emergence of dead zones is linked with human-driven eutrophication, or the stimulation of excessive algal growth through nutrient overloading. For an animation of the process see: www.gulfhypoxia.net.
How do humans influence the formation of dead zones? Fertilizers, which are used to meet modern agriculture’s high output demands, are the main offender. Although these fertilizers are laden with two essential plant nutrients – phosphorus and nitrogen – the Gulf of Mexico is receiving too much of a good thing. Runoff from the farms contains the excess nitrogen and phosphorus, draining into streams and rivers that feed into the Mississippi. Once deposited in the Gulf, algae make quick use of the abundant nutrients and multiply into enormous blooms. When the algae die, they sink to the bottom and decompose in a process that uses up available oxygen. Mobile animals, such as fish, can flee these suffocating conditions; those that cannot are doomed to die. The organisms that are most likely to perish constitute the base of the Gulf’s food web, with ecosystem damage and a loss of biodiversity radiating upward. Another animated graphic of the process can be viewed here: www.smm.org
The dead zones that result from nitrogen and phosphorus over-loading can be persistent, irregularly present, or occur at seasonal intervals – the Gulf produces a roughly 8,000-square mile swath of dead ocean every summer. Annually, the amount of nitrogen and phosphorus washed into the Gulf totals at 1.6 million metric tons and 100,000 metric tons, respectively.
Hypoxic areas are highlighted in color, with the most severe hypoxia shown in red. NASA/Goddard Image.
But the problem is international in scope. By 2008, over 400 dead zones were identified worldwide.
The loss of aquatic life incurred by dead zones can have economic consequences for the fisheries in which they tend to manifest. In 1979, fishing of Norway lobster was eliminated in the Baltic as a result of its hypoxic waters. Because of the stressful conditions, overharvested Baltic Cod are failing to reproduce effectively (see National Geographic article: www.news.nationalgeographic.com). In China, harmful algal blooms in the South and Bohai seas have triggered massive fish kills, and roughly 80% of fish stocks were destroyed by a 1998 bloom near Hong Kong. Canada, too, has experienced fish kills and even human poisonings from affected estuaries. While the Gulf has yet to be impacted significantly, one cannot rule out an eventual collapse for the $2.8 billion per year seafood industries based there. It may only be a matter of time before valuable shrimp and fish species, with their habitats choked of oxygen, cannot recover their numbers. This year’s heavy rainfall and flooding means more runoff and nutrient input for the Gulf, and the summer’s forecast looks grim. 2011 will most likely see the worst Gulf dead zone yet. (tamunews.tamu.edu)
Dead zones tend to spring up wherever human development has significantly altered the landscape. Automobile exhaust and improperly treated sewage can both contribute to their emergence, and impervious surfaces like roads and parking lots allow for more nutrient-polluted runoff. Unless nutrient sources are properly managed, this aquatic plague will only worsen. Regulations must target fertilizer overuse and application methods to minimize runoff into coastal and estuarine systems. Reductions in fossil fuel burning and improved wastewater treatment are also essential to mitigation. Commercially valuable fish species, already imperiled by overharvest, cannot afford to lose remaining habitat to hypoxia. Restoration of the affected water bodies is sure to be a long-term challenge, but unless action is taken, a cascade of ecological destruction may await.
See also: "Chemical Fertilizers feed the world for the Short Term - but what about the Future?" inspirationgreen.org/chemical-fertilizers
- 1) Robert J. Diaz and Rutger Rosenberg, 2008: “Spreading Dead Zones and Consequences for Marine Ecosystems.” Originally published in the journal Science, but available here in PDF: water.epa.gov/type/watersheds/named/msbasin/upload/2008_08_15_msbasin_diaz_article.pdf
- 2) NY Times article that covers the above study in more digestible format: www.washingtonpost.com/wp-dyn/content/article/2008/08/14/AR2008081401910.html
- 3) A Washington Post article that includes commentary from Diaz and Rosenberg’s study: www.washingtonpost.com/wp-dyn/content/article/2008/08/14/AR2008081401910.html
- 4) Extensive Greenpeace publication on dead zones, also PDF, with information on various global sites: www.greenpeace.to/publications/dead-zones.pdf
- 5) Stephanie Joyce, 2000: “The Dead Zones: Oxygen Starved Coastal Waters.” Covers the science of dead zone formation in-depth, as well as the causes and consequences. www.ncbi.nlm.nih.gov/pmc/articles/PMC1637951/?page=1
- 6) The World Resources Institute has an interactive map tracking hypoxic zones around the globe here: www.wri.org/project/eutrophication/map
- 7) CBS business network article on how the Gulf dead zone can inhibit fish reproduction and hurt fisheries: findarticles.com/p/articles/mi_m1200/is_20_166/ai_n8588876/
- 8) J Henry Fair is an artist drawing inspiration from environmental issues, and an enlightening slideshow on the detriments of fertilizer production can be found at his website: www.jhenryfair.com/aerial/fertilizer/index2.html
- 9) Biofuel has its downsides – how ethanol production in the U.S. may worsen dead zones: www.livescience.com/4869-ethanol-production-worsen-dead-zone.html
10) A San Francisco Gate article on how global warming may be contributing: articles.sfgate.com/2008-02-20/news/17142648_1_dead-zone-oxygen-starved-waters-low-oxygen
11) More detailed information on fertilizer production waste can be found on the EPA’s website here: www.epa.gov/rpdweb00/tenorm/fertilizer.html