Many organisms depend on the land as a food source. seasons can affect the amount of food that is available to these organisms. how would a long, cold winter most likely affect a population of deer

The cold weather may weaken their senses and overall health and well-being, therefore it will be much harder to seek food. Due to the possible snow or frost, most berries and such may be limited due to them being frozen. Their young may also die or become seriously ill due to the conditions stated and it may be difficult for them to continue living in their habitat. The prederators such as wolves, boars etc. are stronger and more resistant creatures, therefore they may be killed and consumed. 

However, white deer may take this as an advantage.