Each spring, the area in the Gulf of Mexico just off the Louisiana and Texas coasts turns into a "dead zone." Excessive amounts of nitrogen and phosphorus--which make their way to the Gulf from the atmosphere and via rivers polluted with agricultural runoff and municipal and industrial waste--trigger algal blooms. The algae use up available oxygen, killing bottom-dwellers such as oysters, clams, and snails, and driving away fish, shrimp, and crabs.

And it isn't just the Gulf area that is affected by an overabundance of nitrogen and phosphorus. All of our coasts are being damaged. Of 139 U.S. coastal areas assessed recently, 44 were identified as severely affected by high levels of these nutrients. Excess nitrogen is particularly harmful for marine ecosystems and can be linked to everything from increased outbreaks of red tides to the deaths of marine mammals and the loss of biodiversity. What's more, many scientists predict...

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