While oysters are known to be one of nature's best natural filters, new research suggests there are limits to how much pollution they can clean up.

In fact, some of the nation's estuaries are so overwhelmed with excess fertilizer that it would take more oysters than the bays can hold in order to purify the water.

Adult oysters are known to filter about 50 gallons of water daily. But existing research had never fully addressed how much pollution was removed from the water filtered by an oyster, versus how much pollution passed through the animal's body back into the water.

In particular, scientists sought to understand how much nitrogen oysters remove from the water. Nitrogen is one of the key nutrients responsible for the creation of the Gulf of Mexico's dead zone off Louisiana, and a similar dead zone that builds in Mobile Bay each year. Sewage treatment plants, industries such as the ThyssenKrupp steel mill, and farm fertilizers all contribute nitrogen to waterways. In the water, nitrogen fuels the growth of phytoplankton, which are tiny plants.

via Oysters can handle some, but not all pollution we dump into the nation's estuaries | al.com.