Subject to the exclusions, adjustments, exemptions, and deductions herein provided, an annual tax of 1% on corporations and resident individuals and of 1/2% on nonresident individuals for general revenue purposes and the purposes provided for in section 11a and 11b is hereby imposed as an excise on income earned and received on and after the effective date of this ordinance. However, if the governing body of the city adopts a resolution to impose the tax at a lower rate, the tax is hereby imposed at that lower rate. If the tax is imposed at a lower rate, the rate on nonresident individuals shall not exceed 1/2 of the rate on corporations and resident individuals.