Pond Boss Forums INTRO Questions & Observations Red Pond Dye, Red Tide, or What?

Yesterday I drove by a nearby small pond (perhaps 1/4 acre) for the first time in a few days. This was one day after we received 3" of rain (with air temp in the low 70s) in an hour or so. The entirety of the pond was a vivid, dark red. Around the edges were patches of a slightly lighter color red, with some texture (algae like).

This is a fairly shallow pond, built maybe 20 years ago. It has never had any aeration; the fairly cold rain after several weeks of temps in the upper 80s and 90s might well have caused a turnover. It has a very small watershed which, combined with a leaky dam that was not cored during construction (and which has settled in the middle by about a foot over the years) keep it almost always below full pool. However, the 3" of rain combined with 4" or so over the previous week had it completely full. (And completely red!)

I drove by this property everyday on the way to work for 35 years, before, during, and after its construction. The only color it has ever been is the blue from the pond dye the owner has used on it every year. I don't recall ever seeing any floating algae on the pond at all. The only plants apparent around the shore are a few willow trees which were allowed to grow unchecked for many years.

Where the heck did the red color come from? Is there some red pond dye that I have never heard of? Or some species of fresh water algae/bacteria which can color an entire pond red? If a turnover event (or the winged phantasmagoria) stirred up the bottom, our local clay color is a light brown that I don't think could have turned red.

Any ideas, especially on algae/bacteria (calling Dr. Perca and other resident experts)?