Mallards in My Pond

We've recently had a pair of mallards take up residence in our 3,000 gallon
pond. It's been a nice novelty so far but I'm wondering what the long term
effects might be to the health of the pond, plants and fish. We've done
nothing to discourage them from staying and they're becoming accustomed to
our comings and goings.

Has anyone else had experience with ducks on their pond? Any recommendations
on whether or not we should allow them to stay or encourage them to go
elsewhere?

TIA

John