Shut them down! Not only are you wasting electricity you will stress your fish. They need a relatively warmer layer near bottom as a thermal sanctuary. Maybe if you had heavy ice cover and deep persistant snow it may be advisable to set up a diffuser in a shallow end of the pond but I no longer even do that.
The first winter I had my pond I mixed the water column all winter. I had disease problems the following spring due to winter stress.
Now all I do is remove snow in strips to allow light to penetrate and allow the rooted plants and phytoplankton to produce oxygen, and my fish come out of winter in great shape.
Redears are even less tolerant of cold water than other species.
As far as your leaves there won't be much decompostion going on in cold water anyway so running the diffusers won't help break them down much this time of year.
If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.