Unless its a life and death situation, I would suggest not to drink water from a lake. If you can wait to get home, or you can find a source of fresh water, you should drink from there. Millions of tiny living organisms live in lake waters, including fish waste, urine, fecal matter from local wildlife. If you study germs and cells, you'd know that even bacteria defecates. In that sense, even in the coldest environments, bacteria will find a way to grow in extreme conditions. Rest assured, if there are animals in the area where the lake is located (penguins, polar bears and fish) there is also fecal matter in the waters.