WASHINGTON –

Interior Secretary Ken Salazar announced Wednesday that he intends to resign, leaving the White House in exactly the same place it was four years ago: looking to fill the three most important environmental posts in the federal government.

The Interior Department, the Environmental Protection Agency and the National Oceanic and Atmospheric Administration all have openings at the top, and the Obama administration has only a limited amount of time to press a second-term environmental agenda before the next election cycle begins.

Activists say they are optimistic that President Barack Obama can pursue some ambitious environmental policy goals.

"This clearly doesn't reach the level of gun control or immigration, but I feel it's on the table in a way it wasn't in the first term," said William Meadows, a counselor to and former president of The Wilderness Society.

By contrast, American Petroleum Institute President Jack Gerard said a new slate of agency heads could "adjust some of the more extreme positions taken early on" during the administration. "The potential is there for a reset button, to turn the focus to job growth and economic recovery," he said.

Obama's environmental legacy is likely to be defined by how he handles climate change issues — including whether to permit the Keystone XL pipeline extension or to impose carbon limits on existing power plants.

His choices to head the three agencies also will affect decisions ranging from what kind of offshore drilling will take place in the Arctic to whether gold mining can proceed in Alaska's Bristol Bay watershed.

The top contenders to replace Salazar include former Washington Gov. Chris Gregoire, a Democrat; David Hayes, deputy secretary at the Interior Department; former Democratic Sen. Byron Dorgan; and Office of Personnel Management Director John Berry, who was an assistant secretary at Interior in the Clinton administration and has also led the National Fish and Wildlife Foundation and the National Zoo.

Several green groups are backing Democratic Rep. Ral Grijalva from Arizona for the job.

Gregoire is also under consideration for the EPA slot being vacated by Lisa Jackson, along with the agency's deputy secretary, Bob Perciasepe, and the head of its air and radiation office, Gina McCarthy. Rohit "Rit" Aggarwala, a former top aide to New York Mayor Michael Bloomberg, an independent, is another potential candidate.