If the mayor is unable to perform the duties of office by reason of temporary absence or temporary incapacitation, the board must, by a majority vote of all the commissioners, appoint one of its members to act as mayor until the temporary absence or temporary incapacitation ends. The appointed commissioner's official designation is mayor pro tempore. The mayor pro tempore is invested with all the powers and shall perform all the duties of the mayor during the mayor's temporary absence or temporary incapacitation. The mayor pro tempore has only one vote as a commissioner and is not entitled to vote as mayor pro tempore. The temporary absence or temporary incapacitation of the mayor is not a vacancy in the office of the mayor. If the office of mayor is vacated, the board of commissioners must, by a majority vote of all commissioners, appoint one of its commissioners as acting mayor. The acting mayor is invested with all the powers and shall perform all the duties of the mayor, until the vacancy is filled by election for the unexpired term at the next annual election or by special election as provided in §