The 17,000 species was just for the number of species of filamentous algae(FA) and those examples of red and green are not types of FA. We won't scare you as to how many species of algae there are of nonfilamentous algae. In outdoorliving's case it was probably bluegreen(Cyanobacteria) and in spkplg's case it was likely a species of Euglena which can change color as day length changes. I would be more concerned with the bluegreen(toxin potential) compared to the Euglena. Although both when dense can cause severe DO sags during long periods of cloudy days or in early morning hours. It depends.
