To help the creatures that live in and around your pond survive the cold weather it is important to have a range of wildlife-friendly areas in your garden that will provide them with shelter in the water and on land. Areas of longer grass, overgrown corners, deep leaf litter or piles of wood or stones can provide a vital refuge for many creatures. Allow submerged plants to grow throughout a large area of the pond to provide cover, release oxygen into the water, and keep the pond healthy.

Source: How to help amphibians through the winter