SOUTH PORTLAND, Maine — Director of Economic Development and former Assistant City Manager Erik Carson submitted his immediate resignation from city government Wednesday.

Don Brewer, the city’s human resources director, said Carson “decided to pursue other interests.”

Carson had been on paid administrative leave since Aug. 14, for reasons Brewer said he could not disclose.

Before being placed on leave, Carson, who had worked for the city since March 2008, was quietly demoted last year by City Manager Jim Gailey. Brewer said Gailey removed Carson as assistant city manager on Oct. 31, 2011, for reasons Brewer said Gailey never specified.

The demotion reduced Carson’s pay from about $78,000 annually to $69,500. Carson had earned $40.14 per hour with a 37.5-hour work week and his salary was reduced to $35.68 per hour.

City councilors were made aware of the demotion, but the city website this week — 10 months later — continued to identify Carson as assistant city manager, and councilors never publicly acknowledged the administrative change.

Councilor Rosemarie De Angelis on Wednesday said she did not recall any explanation from Gailey, who is out of the office until Sept. 4 and could not be reached.

No replacement as assistant city manager was ever named, although Finance Director Greg L’Heureux has substituted for Gailey during recent council workshops, including the one held Monday.

Efforts to reach Carson this week were unsuccessful.