Could likely be either iron or tannic acid from decomposing vegetation, based on the color...
BeezFun said:Is this pond charged by rain water or a spring? I don't think rainwater has iron in it. Not sure how water picks up iron, I assume it has to sit long enough for it to dissolve minerals in the soil and rocks.
Chilly807 said:It could be manganese as well, often the stuff will float on top, looks like an oil spill until you notice that it doesn't spread around much.
Sean