The members of the County Commission of Mobile County shall devote their full time to the discharge of their official duties. Effective for the term of office commencing on the first Monday after the second Tuesday in January 1969, and thereafter, the president of the commission shall receive an annual salary of sixteen thousand dollars ($16,000) and each associate commissioner shall receive an annual salary of fifteen thousand dollars ($15,000). The salaries of the president and associate commissioners shall be paid in equal monthly installments from any funds in the county treasury available for that purpose, as provided by law.