Just to add a little more mud to the water....
Don't cut them if they are larger (4" or greater like RC said). Too risky. Not only will the rooting rots make way for piping water, they will also decrease the compaction of the soil they're in when the die. I've got a pine in my yard that we cut down and left the stump. If you go out there today and walk within 3 to 5 feet from the stump, the ground is literally mushy. Also, pine trees, unlike most hardwoods, have an exceptionally deep tap root.
I say let the big ones live. In 20 or 30 years when you have to dredge the silt out of the pond, just grub the stumps out then.
