As the lotus leaves begin to turn brown, the lotus is beginning to go dormant for the winter. Do not cut off the leaves; allow them to die back naturally. After they die back at this point, it is safe to cut them off above the water line. Its important to leave a portion of the stem above the water. You do not want water getting down the hollow green stem and having it get into the air channels of the tuber, if this happens you risk drowning the tuber and cause it to rot. Make sure the potted lotus is below the ice. Allowing the tuber to freeze will kill the plant.
Floating plants such as Water Hyacinth and Water Lettuce are considered tropical and will not survive temperatures below freezing. After the first hard frost, remove from the pond to prevent them from decaying and adding unwanted debris to the pond.