Striped dolphins (Stenella coeruleoalba) observed in emulsified oil on April 29, 2010 in the Gulf of Mexico, a few days after the Deepwater Horizon oil spill. It's going to take years to understand how the oil spill will impact marine mammals because they are so mobile.

Samantha Joye, a marine biochemist at the University of Georgia, says that fewer marine mammals have been observed in the area in the two years since the spill, but no one knows why. "Are they avoiding the water because it doesn't have food supplies anymore? It's hard to say," she said. "But if the upper food web is showing impacts, that means the lower food web has been impacted."

More about the Gulf oil spill can be found in our Gulf oil spill featured story.