Section 6. The department may lease for a period not exceeding twenty years, under such covenants and conditions as it prescribes, storage facilities, wharves, piers, bulkheads, docks, sheds, warehouses, foreshores and industrial locations in Boston harbor within its charge. It may lease sites for warehouses and industrial locations for a longer period, but every lease of such sites for a period exceeding twenty years shall provide for a readjustment of the rental at intervals of not more than twenty years. No lease for a term exceeding five years shall be valid until approved by the governor and council. The income from all wharfage and storage rates, use of cranes, lighterage, dockage and other charges, and from the leases of lands, storage structures, wharves, piers, docks, sheds, warehouses and industrial sites, all moneys received by the commonwealth under section twenty-one for tide water displacements in Boston harbor, and all moneys hereafter received which on May twenty-eighth, nineteen hundred and twelve, were required to be paid into the Commonwealth's Flats Improvement Fund, shall be collected by the department and paid to the commonwealth, to the credit of the Port of Boston Fund. Said fund may be invested by the state treasurer at his discretion from time to time as provided for the investment of the commonwealth's funds; and all income from such investments shall be added to the fund. The department may expend the Port of Boston Fund to operate, maintain, repair and preserve the property in Boston harbor in the control of the department, and such sums as the legislature may appropriate annually for salaries, office expenses, and general engineering expenses in connection with the work of the department in Boston harbor shall be paid so far as possible from said fund; any balance remaining from said fund after the aforementioned expenses have been paid shall be applied to the payment of interest and to the annual payments on account of principal of any securities which may have been or may be issued to raise money to be expended by the department for the development of the port of Boston.