This is not an Algae at all, but rather a bacteria named Cyanobacteria. It's known by several more popular names - Blue/Green Algae and Pond Scum, to name just two. (Spirogyra is also known as "Pond Scum", but Spirogyra is a true Algae). There are about ten different species of Cyanobacteria, some is stringy, some spongy, some slimy, but it all appears to the naked eye as scummy, and not at all how it appears in this microscopic image. It can also be bright green or very dark green - almost black. Usually caused by high nitrates and phosphates. In backyard ponds, it is best removed simply by pulling the stuff out. It also thrives in backyard waterfalls and stream returns because it is being fed a constant supply of food from the running water. The best defense is a good filtration system and plenty of plants.

Also appearing in the image, just off the tip of the pointer, is a colony of Eudorina. Each cell is spherical, and they form into ball-shaped colonies encased within a jelly-like substance. I didn't find either of these specimens in my pond. They came from a drainage ditch in the field behind my house. I saw what looked like scum, and I just picked up some on my finger.

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