In addition to the salary paid to each of the circuit judges of the 33rd Judicial Circuit by the state, there shall also be paid to each of the judges a supplemental salary set out as follows: Beginning on October 1, 1989, an amount equal to 25 percent of the base state salary paid to circuit court judges. Beginning on October 1, 1990, an amount equal to 30 percent of the base state salary paid to circuit court judges. Beginning on October 1, 1991, an amount equal to 35 percent of the base state salary paid to circuit court judges. For the purposes of this section, the base state salary of circuit court judges shall be that base state salary upon which percentage supplements for other circuit court judges is figured. Such supplemental salary shall be paid out of the general funds of each of the counties comprising such circuit in a pro rata amount based on the most recent official decennial census of the United States and shall be paid in equal monthly installments. Such supplements shall be in lieu of any other supplemental salary or expense allowance heretofore provided for by law to be paid by the counties in such circuit to circuit court judges. All previous supplements and expense allowances paid to the circuit judges of the 33rd Judicial Circuit are abolished effective September 30, 1989.