Today I noticed that the grasses that usually grow on the house side of the pond were gone.  Maybe the grass seeds and then dies.  And I noticed that on the woods side of the pond that there was an oily sheen that collect debris and touched the shore.  I also noticed that there seems to be fewer tadpoles and a lack of frogs.  I heard them with their plucked banjo call, but I didn't see any mature frogs at all.  I wonder where the oil came from.  What can I do about it?