Unless otherwise instructed by a public body a depository may pledge assets to secure public deposits on a pooled basis in order that the total amount of assets pledged is at least equal to the total amount of such assets required to secure all the public deposits of the depository. It shall be the duty of each depository to maintain continuously the total amount of assets pledged in the required amount and to carry in its accounting records at all times a general ledger or other appropriate account of the total amount of public deposits required to be secured and the total amount of assets pledged to secure such public deposits. The total amount of public deposits required to be secured shall be determined as of the opening of business each day on the basis of the public deposits collected by the depository.