Filamentous Green Algae (Pond Scum)

Cladophora, Pithophora, and Spirogyra spp., and others


Picture of a patch of filamentous green algae floating in a stream.
As the base for aquatic food chains, green algae feeds and shelters tiny animals, insects, and young fish.
Other Common Name

Includes families in divisions Chlorophyta and Charophyta


Filamentous green algae forms green, cottony masses that are free-floating or attached to rocks, debris, or other plants. It consists of fine, green filaments that have no leaves, roots, stems, or flowers. They often form dense mats. On warm, sunny days, they commonly float when bubbles, generated by the plant or created by its decay, get trapped in the mats and make them buoyant. The three genera listed below are examples.

Cladophora feels cottony and can form balls that float when the core decays; magnification reveals long, slender cells and a branching habit.

Pithophora is sometimes called “horsehair algae” because of its coarse texture, which can feel like steel wool.

Spirogyra is bright green and slimy; magnification reveals the chlorophyll-bearing pigments are shaped as spirals; the filaments do not branch.

Note: Not all types of green algae are filamentous; they occur in an great variety of forms, including single cells and colonies.

Habitat and conservation

Green algae grow in practically any water that is capable of supporting life and receives good light. In early spring and again in late fall, hairlike strands of filamentous green algae have a heyday in forest streams, when leafless trees permit light to reach the water. When a body of water receives too many nutrients — such as decaying animals or fertilizer or manure runoff — green algae can grow (“bloom”) in quantities that unbalance the ecology of the lake, pond, or stream.

image of Filamentous Green Algae Moss Pond Scum distribution map
Distribution in Missouri


Human connections

As a base of aquatic food chains, algae are tremendously important for healthy rivers, streams, ponds, and lakes, and all the human interests that involve water, including boating and fishing.

When filamentous algae gets out of hand, it can impede fishing and swimming and clog water intake screens.

Ecosystem connections

Green algae is vital for healthy aquatic ecosystems. As the base for aquatic food chains, it feeds and shelters tiny animals, insects, and young fish.

Greenside darters spawn in late March to the end of April, earlier than most Missouri darters. Timing is everything: this species of fish typically attaches its eggs to strands of filamentous algae (Cladophora spp.), which has its big "bloom" in early spring before trees gain their leaves and shade forest streams, causing the algae to decline.

Algae used to be called “primitive,” but a more accurate term is “ancestral” for organisms that have not changed much for billions of years.