Black Mat Alga
Photo credit: FWC Fish and Wildlife
(Lyngbya wollei [Plectonema wollei])
Report this Species!
If you believe you have found this species anywhere in Pennsylvania, please report your findings to state and federal authorities.
Species at a Glance
Black mat alga is a hardy, quickly multiplying, toxic cyanobacterium that masquerades as a filamentous blue-green alga. It is considered the prehistoric “beast of water algae” because of its potentially deadly impacts. It forms large benthic and floating mats that have been shown to degrade water quality, aquatic habitats, and produce dangerous toxins.
Colonies of clustered filaments, usually visible to the naked eye, form dense dark green to black mats along the bottom of the water. These mats remain inconspicuous until they are either dislodged by wave or wind action, or gasses produced during photosynthesis become trapped underneath and allow them to float to the surface. The mats start out looking like brown smoke in the water and as they surface, they ball up into the shape of marbles with the texture of coarse hair. The algae filaments are unbranched, untapered, and are encased in sheaths. Cells have been observed at 2-12 µm in length. While lacking specialized cells, this alga is still able to fix nitrogen from the atmosphere.
May be difficult to distinguish from Phormidium, which also forms mats that can be dark brown, green, purple, or blue-green. Phormidium has short cylindrical cells in a fine sheath. Cells are at least as long as they are wide and the ends of the cell often taper with a thickened “cap” at the end.
While it thrives in warm, slightly alkaline waters with abundant nutrients, black mat alga is found in a wide range of habitats, from clear, mineralized, thermally and chemically stable water bodies in Florida, to turbid waters with variable nutrients in the St. Lawrence River. It is highly resistant to freezing and cold weather, and is also able to survive without light once established.
Black mat alga grows in sediments and rapidly multiplies in warm water. The stringy filaments can cling to fishing gear, boats, trailers, and other recreational equipment and gets moved to new locations where mats can persist for several years.
Commonly found in ponds and lakes in the southeastern United States, a genetically similar subgroup of algae has been proliferating in the Great Lakes Region and the St. Lawrence River. Many believe black mat algae arrived in these locations because the chemistry and water temperature changed, or it arrived on recreational boats. This alga is also found in locations in North Carolina, Virginia, and the northern Chesapeake Bay area of Maryland.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
Black mat alga is considered a harmful cyanobacterium because of its ability to smother native vegetation, negatively impact fish communities, and produce toxins that are a threat to livestock, dogs, and other animals. In humans, the toxins have caused skin irritations, allergic reactions, gastrointestinal symptoms and respiratory problems. These toxins are responsible for a musty-earthy taste and odor in water, which can affect aesthetics and recreational water uses. It’s also nearly impossible to control with current physical, chemical, and biological tools.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).