Welcome to the posting portions of the forum Redspruce!!!

Is there any way to add aeration to your pond? Air tubing is pretty cheap and can be run LONG distances from the power supply. Aeration would end your concerns of winter survival also.

I would be concerned most with the oxygen since springs, like wells are often devoid of oxygen and can be high in nitrogen.

Since I can get nothing to grow in my clay soils, I have no suggestions to offer there.