Fertilizer runoff and the Gulf Dead Zone

Dead Zone graphic by NOAA.

Kristopher Hite, a post-doctoral fellow in biology at Emory, wrote about the “Dead Zone” in a guest blog post for Scientific American. Below is an excerpt:

“Each summer, after the famers of the American Midwest spread manure or spray anhydrous ammonia over their emerging crops, summer rains (usually) come and carry much of that fertilizer down a massive web of tributaries into the mighty Mississippi River. The annual spike in nutrient (mostly nitrogen, phosphorous, and potassium – NPK) causes massive algal blooms. As the algae decompose bacteria feast on the detritus only to die when there is no more food taking with them dissolved oxygen. The resultant area of low oxygen or hypoxia is eerily named the ‘Dead Zone.’ This is a slight misnomer as the area is not completely dead although the lower oxygen levels do threaten large portions of the aquatic food web. In addition to oxygen deprivation a small percentage of the blooming algae also produce lethal toxins to fish, birds, and mammals. The size of the Dead Zone varies summer to summer from about the size of Delaware to New Hampshire depending on the amount of rainfall. …

“It varies. The American Midwest experienced two straight years of drought in 2011 and 2012. Less rain meant less nutrient run-off. Though the Dead Zone was smaller than predicted by the National Oceanic and Atmospheric Administration (NOAA) in 2012, the increased rains throughout the Mississippi watershed in 2013 resulted in a Dead Zone twice as big as last year’s. Fertilizer accumulated during the drought was released with vengeance during the heavy summer rainfall this year. I am curious to see if the horrific flooding we’ve seen recently in Colorado will have any latent effect on the Gulf’s Dead Zone this fall.”

Read the whole article at Scientific American.

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