Within 60 days after an optional unified form of county government containing alternate A becomes effective, the board of county commissioners by a majority vote of all members elected and serving shall appoint a county manager. The manager shall be the administrative head of the county government and shall be responsible for the overall supervision of all county departments not headed by elected officers. He shall be appointed on the basis of merit only. He need not be a resident of the county at the time of his appointment but shall assume and maintain residence in the county following appointment except in counties of 1,000,000 or more he shall also be a resident at the time of his appointment. He shall be paid a compensation as the board determines. A member of the board during his term of office and for 1 year thereafter is not eligible for appointment as county manager. The county manager shall hold office at the pleasure of the board and may be removed by a majority vote of all members elected and serving.