Marine pollution takes many forms, from the millions of gallons of oil that run off our highways each year to the Great Pacific Garbage Patch, a massive gyre of floating plastic trash. But the most devastating pollutants are the nitrogen and phosphorus found in our fertilizer and sewage. When too much of either washes downstream, coastal waters become choked with heavily fertilized algae, which then dies and decomposes, consuming the oxygen in the water and asphyxiating animal life. This process, called eutrophication, has created at least 405 "dead zones" worldwide. Human sewage is the top source of eutrophication in the developing world, but in the U.S., Europe and China, animal waste and fertilizer are the main culprits. Every year in the U.S. alone, some 10 billion chickens, 80 million cows and 149 million pigs produce 500 million tons of manure, much of which is sprayed on fields. Farmers also add another 55 million tons of synthetic fertilizer to their cropland, much of which washes back into the watershed.
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