they may be feeding on "brown trout"....
The birds like to hang out in the center of the river in the area that you mentioned. It looks like they are feeding, but they aren't. I fish and run this area frequently. It is a hang out spot for the birds, maybe there is some sewage coming up out there, but probably not (most comes along the shore).
Most of those swirls, especially the big ones are carp. They gulp air and surface even in the coldest months. This isn't feeding activity. But it can show you where schools are. Carp are catchable year round, but get tough when it is this cold and the cold makes them fight like boots. Surface activity doesn't usually happen outside of may/june here. During the fall you can often see clouds of juvi herring, but the chases are pretty much sub-surface. There are many species in the chuck that feed on them.
Slappy the baitshop boy