“Paul’s leadership at DEP has had a positive effect on the agency and the state,” Gov. Paul LePage said in a statement. “Paul has worked collaboratively with businesses and advocates, as well as state and federal agencies, to protect Maine’s natural resources while growing our economy. His innovative and balanced approach has helped to move our state forward while ensuring our environment is protected. I thank him for his service to the people of Maine.”

Mercer is an engineer who specialized in energy and waste issues in the private sector as well as a former professor and administrator at Maine Maritime Academy. He was the third DEP commissioner during LePage’s nearly eight years in office and took over leadership of the agency after a tumultuous, four-year term by predecessor Patricia Aho.

While environmental groups continued to criticize LePage administration actions and stances during his term, Mercer has kept a lower profile than Aho and department staff appeared to have good relations with lawmakers overseeing the agency.

Mercer is departing the DEP at a busy time for the agency, however. In addition to several major rule-making processes and petitions, the DEP is gearing up to begin its review of Central Maine Power’s controversial proposal for a 145-mile high-voltage transmission line through Maine to allow HydroQuebec to supply electricity to Massachusetts.

Additionally, Mercer is the latest member of the governor’s controversial Wind Energy Advisory Commission to leave office just as the group begins its controversial review of the wind power industry’s impacts in Maine.

It is common for top administration officials to depart the executive branch in the months prior to an election because the next governor will select his or her own Cabinet members. In addition to Mercer, the commissioners of the Maine Department of Health and Human Services and the Department of Economic and Community Development, as well as the director of the Governor’s Energy Office, have left in recent months.

The new governor will take the oath of office in January. Three candidates are vying for the job: Republican Shawn Moody, Democrat Janet Mills and independent Terry Hayes.