(01.04 mc)water at about 4°c (39°f) is more dense than water at 0°c (32°f). why would the living organisms in the lake die if the water at the freezing point was denser than the water at 4°c?

Didn't you say that the water at 39 degrees is more dense than water at its freezing point? I think the reason organisms would die is because the water already starting to structure in a crystal form, when it starts to freeze into ice. This probably why it kills the organisms, is because the water ceases to allow the organism to function at those temperatures.