These 3 links have the info you need on winter aeration in northern ponds. There are more threads and posts if you find a point not answered. Its all about depth , temp and O2

http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=20;t=001274;p=1

http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=11;t=000096

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=12;t=000064#000004