Pond Boss Forums Thread Like Summary - 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)?

Theo - After stratification the dead productivity and organics that develop in the upper warmer water will settle to accumulate above the thermocline. Decomp of these materials releases dissolved organics. The rain event did mix the upper layer/s of the pond. These dissolved organic chemicals and types of nutrient enrichment are favored by at least one to several types of Euglena that often has various hues of red. This algae group is usually an indication of excessive amounts of dissolved organics in the water due to various reasons. From link below. ""As reliable indicators of organic contamination, the euglenophytes are investigated primarily for their ecological significance as phytoplankton."" The redness can even change during the day to green based on amount of sunlight and the type of food consumed by these algae. This interesting algae has been reported to sometimes produce toxins from my limited bank of knowledge for this 'critter'. When the dissolved organics get depleated the dominant red form of Euglena dies out. FYI this algal group of Euglena is a lot different type of algae than the algae that causes "red tide".

Advanced reading with some pictures of red water conditions. Later in the article it shows the water color change that can happen during the day.
https://pmc.ncbi.nlm.nih.gov/articles/PMC11063437/