If a lake improvement district has been established by order of the commissioner of natural resources under section 103B.531, voting by county boards on joint actions of the lake improvement district shall be based on proportional representation for each county according to the proportion of the population of the lake improvement district residing within each county and not on the basis of one vote per county or one vote per county board member unless each county or each board member represents substantially the same number of persons residing within the lake improvement district.