I apologize in advance if this particular variation of this question has been asked, but my searches find nothing but the normal "My Water Froze!" type of posts.
I also freely admit that I haven't explored this scenario in-game (yet), but might later in the life of my current fort. It'll just take many dwarf-years/lives to test out, and there are higher priority projects (such as digging a borehole to the Magma Sea).
The question is simple: If, through liberal use of drawbridges, walls, and pumps I manage to construct a multi-z-level lake over sourced water (brook, stream, river, etc) in a biome subject to freezing, will the entire body of water freeze? Or just the upper layers? More specifically: can I keep my water source active by doing this (without using a 150-200 z-level Magma pump)?
Of course the water source would still be "above-ground/light," but I've observed cases where only the upper parts of an ocean will freeze, and that's all "above-ground/light," too, so I'm unsure.
Have any of you tried this? What were your results?