I had exactly what your describing last spring/summer. I took a sample into a pond store but it was hard to see once you take it out of the pond (even though I thought I tried to skim a bunch of it). They visually (no microscope) looked at it and said it's most likely pollen from my spruce trees. (huh?) I asked them about it being a red algae and they didn't think so. It saw it for a few weeks on and off last spring/summer and didn't experience it this year at all. I would see it looking through my window but when I went out to the pond it seemed barely visible from certain angles like the lighting had some effect on how well I could see it.

Try to scoop some up to look at it, feel it. I have a microscope now but no red scum, lol. I have a clear rain gauge that sometimes shows red in the rainwater, something different and I would like to know what that's about too. Probably some kind of acid rain, I keep meaning to call the county to ask.

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