Kay,
The hole most likely will not close up and will get larger again once temps moderate. That has been my experience here in northern Indiana.
I used to run a diffuser 24/7 in my 1/2 acre pond, and have since learned that I although I may have prevented winterkill I did stress my fish by supercooling the water. I have not run mine at all this winter and am monitoring oxygen levels and keep the snow for the most part off the pond. So far so good.
I have noticed since I have not run my diffuser at all this winter my pond temp profile is milder than last year even though the diffuser was offset to the side of the pond for winter to prevent to much mixing.
Bill Cody also has good advice on aeration in ponds in winter. He's from Ohio too.
