brownish red scum on pond surface

Question
I have a medium size water garden with fish. ( approx. 1000 gal.)
It has developed, almost overnight, a reddish-brown scum on the surface of the water.  It began with bubbles on the surface of the pond, almost like someone threw soap in the pond.  Bubbles have disappeared but I now have brown scum.
If it rains or your sprinkle it with a hose it disappears, but reappears within 12-24 hours.
It is a mature pond ( 18 years old) that gets full sun and I have never had this problem before.
I have 2 small bubblers operating.  The water used to circulate into another pond but last summer that changed.  I did not have the problem last year.
It is fed by rainwater from the downspouts.

Answer
It sounds like an algae bloom.  The bubbles contain gasses from the algae's gas exchange.  You probably have a lot of extra dissolved inorganic materials from the downspout.  This fuels algae growth.  Partial water changes also help to control algae growth.