in the winter time ice forms a barrier between the water and the air. Snowfall can cover the surface of the water and block out sunlight. Without sunlight little plants called phytoplankton stop producing oxygen in the water. As the phytoplankton die they start to decompose, this is a process that uses up oxygen. If this continues other plants and fish will start to die due to a lack of oxygen. This illustration shows how a winterkill happens.