This groundwater temp map suggests the groundwater is warm enough to support TP year round. The average winter air temps are cold enough to kill them (just so no mods intervene ... temperature is not a piscicide smile ). So it will depend I think on flow through. If enough warm ground water is flowing it could provide warm water refuge or even keep the entire pond warm enough to carry TP. This will be interesting.

[Linked Image from www3.epa.gov]

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers