The Environmental Protection Agency says states that make up the Chesapeake Bay Watershed have fallen behind in reducing harmful nitrogen levels in the country's largest estuary.

The plan is to lower the amount of nitrogen in the bay by 60 percent by 2017. But the agency says it expects only a 46 percent drop by that time.

The EPA says Pennsylvania's farms pose one of the biggest challenges as they fail to adequately stop animal waste and other runoff from flowing into the watershed. The agency says other states, such as Virginia, also must do more to address problems with stormwater runoff in urban and suburban areas.

Increased nitrogen levels have led to oxygen-deprived "dead zones" in the bay that kill fish and other wildlife.