Dono,
This is certainly not my field of expertise since I live next to the Gulf of Mexico but...

I'd say your first thought was correct. These plants make the oxygen your fish live on while the pond is covered in ice and need sunlight to photosynthesize. Even if you don't have rooted plantlife or FA growing, you do have the type of algae (phytoplankton?) in the water that feeds newly hatched fish and make the oxygen your pond needs in the winter.

Last edited by djstauder; 01/24/14 08:46 AM.
1 ac pond LMB, BG, RES, CC