Red algae bloom/pond

My in laws have a pond in New Hampshire which is probably 
}fifty feet across and eight feet deep in places.  It is 
full of crawfish and salamanders and 6 18" koi.  This pond
is fed by a 2" pvc pipe from a near by stream and overflows 
back into this stream.  This spring they are having a 
problem with a red colored haze in the pond, which I assume
is some form of algae.  Now because I have some success with 
aquarium plants they want me to tell them what to do. If I had an
If I had an algae bloom in my tank I'd do a water change,
decrease the light intensity maybe, add more plants.  This
obviously doesn't work in a pond this size. Suggestions??