Pond newbie - Buying home with 3000 gallon pond
I am completely new to the world of ponds. We are purchasing a home on several acres in the town of Duvall, WA (about an hour from Seattle) that has a pond in the backyard. We estimate the size of the pond to be about 3000 gallons. We currently can't tell if this pond is naturally occurring or if the previous owners built it. The house sits on a hill and the pond is at a low spot in the property. We cannot see any liner of sorts, but there are rocks around the edge of it that seem to have been placed there by someone. There is also a pipe that empties into it, but we do not know where this comes from (or if it is even functional). There is quite a bit of life in and around this pond. There are lots of plants growing in it and many frogs coming and going from it, some of which are quite large. One end of the pond appears to once have had a bridge over it, but it is now collapsed and overgrown. We have wondered if the pond does or used to connect to a stream on that side, but currently it is too overgrown to tell. The water is pretty clear, too, although I cannot see all the way to the bottom. I have not had a chance to look closely enough to see if there are any fish in the pond. There are lots of water bugs of different sorts around it, though.
So as a complete newbie to ponds, where do I begin? I would like to maintain and encourage the lovely ecosystem that already seems to be around the pond, as well as cut back the overgrowth and enhance it's aesthetic aspects. I don't know if I need to do anything to encourage the water to be "cleaner" or add nutrients. How do I go about trimming/removing plants in the pond (and should I) as well as those around the edges? Can I safely add creatures like fish or turtles to the pond in the future? It is in quite a rural area, so I would assume deer and other animals probably visit the pond sometimes.
Does anyone know of a good book or resource for working with ponds, particularly in the PNW climate?
I will add a couple more photos shortly.