Hey Noel, I had that same film on my pond from spring into early summer. Mine progressively got heavier until it became an algae scum that covered almost the entire pond. I "think" it is a PA bloom starting to die off. I'm not absolutely sure, but as soon as I installed the surface fountain, it got a lot lighter. I still see a little by late evening on bright days, but nothing like it used to be.

If you get an absolute definition of what it is, I would also be very glad to hear it.

.10 surface acre pond, 10.5 foot deep. SW LA. The epitome of a mutt pond. BG, LMB, GSF, RES, BH, Warmouth, Longear Sunfish, Gambusia,Mud Minnows, Crappie, and now shiners!!...I subscribe!!