Question
Hello, i am dealing with a pond/lake that is 75 meters by 50 meters. it has developed a red scum on the surface, this disappears with rain and comes back when left undisturbed. do you think this could be an algae, if so how can i treat it? it is a huge body of water,

many thanks

Answer
All the surface water has to flow. In your case the surface water is carrying the water into the pond. Once the rain comes the excess of water in skimmed of the surface.
You have to track from were the water is coming to your pond, while coming it is carrying the red scum with it.