I can't answer your question but can tell you my experience. I find that as the weather changes the pond will change also. It is not uncommon for my pond in late summer to get a blackish or brownish look to it.
I've just always assumed it was another form of algae that responds to warmer water temperatures or a die off of the greenish type algae. I don't really know but know the creeks and other BOW's change with the season and temperature also so never was too concerned about it.
