Spirogyra is a kind of filamentous algae of the Charophyta division of the Zygnematophyceae class. The weed represents a nonbranching filament consisting of a series of cylindrical cells with the size of up to 0.01 mm each. Each cell of spirogyra is enclosed in a cellulose wall covered with mucus. The algae are easily recognizable by their spirally twisted chloroplasts.
This type of filamentous algae was discovered nearly two centuries ago. Spirogyra is one of the most common algae and is found in freshwater (rarely in salty) waters worldwide. It forms large velvet-like clusters that float on the surface of the water, drift along the bottom or mix with the algal mat of lakes, swamps, ponds, creeks, etc.
Some believe that the cause for the proliferation of spirogyra were favorable warm weather conditions that characterized one of the seasons at Lake Baikal. However, with the onset of cold weather the number of the filamentous algae has not decreased. Ice water did not become an obstacle for the formation of a thick green film on the Lake.