The three members appointed by the Mayor shall be appointed for staggered terms, with no more than two members having their appointment end in a single year. All appointments shall be for a period of three years. In the event of any vacancy, a successor shall be appointed by the Mayor to fill the unexpired term for which the member was originally appointed. As for the three members from Penn Township, who shall be duly elected trustees, their term shall be as provided for under the Code of Iowa, and any successor that is necessary to be appointed shall be appointed as required by the Code of Iowa for the replacement of trustees.