Even if you can’t see didymo, you could be spreading this invasive freshwater microscopic alga.
This diatom (a type of singlecell algae) can form massive blooms. They begin as small circular brown blotches with a gooey appearance, giving them the nickname “rock snot.” Unlike other algae, didymo is not slimy. It develops into thick, nuisance mats that can be over 20 centimeters (10inches) thick, with the appearance of fiberglass or strands of wet toilet paper. These mats have texture much like wet wool. Because didymo lacks chlorophyll, it is generally brown, tan, yellow or white. Because it can smother the bottom of streams, rivers and lakes, it adversely affects freshwater fish, plants and invertebrate species by depriving them of food and destroying their habitat.