You need Dave Willis, he knows his stuff about northern ponds. I'm sure he will chime in or you can find one of his posts and send him a personal email.

sounds to me like you are having dissolved oxygen problems due to rotting vegetation. I remember Dave and I had this discussion on the board, with some research you can find it. Based on what I have learned from Dave you probably dont want to eradicate all the weeds from your pond. But with watermeal and duckweed you may have no choice. It will be difficult to control these weeds and not kill all the other plants in the pond. There are two products called Sonar and Avast that can eradicate water meal and duckweed, but they take a long time to work (90 days) and if the pond has flowing water all the time the herbicide will be washed out it has a chance to work. you may consider this option if your pond does not flow much water, then consider introducing native plants for fish habitat. It will take between 12-18 months depending on the flow rate of your pond to wash all the floridone out so your introduced plants wont be killed by the treatment. The idea here is to eradicate the watermeal and duckweed then introduce a more desirable plant for fish habitat.

another option = There is a product called Reward, we have discussed this herbicide here as well. it will kill duckweed very well and some species of water meal. it also works very well on submersed vegetation. Its a contact herbicide so you may be able to treat certain areas of the pond and leave some weeds in other areas for fish habitat. think of treating the pond like mowing the grass. Start early in the spring and try and maintain about 20 % weed coverage all summer. You will probably not be able to eradicate water meal with Reward, I have eradicated duckweed with Reward but it takes some very consistant treatments. I think as long as you stay on top of things during the warm part of the year you can avoid oxygen depletion during the cold months. A good aerator will help the cause.keeping the snow off the ice will help too. The plankton can still produce oxygen under the ice as long as the snow does not pile up.

I hope this makes some sense, you have a complex problem and I'm just a dumb redneck from Georgia.