Mosquito fern is a small, free-floating fern, less than ½ inch across. Newer leaves are gray green, but can turn rusty-red, especially in winter. It spreads vegetatively and can rapidly form large, floating mats. It occurs throughout Florida in still or sluggish waters. The leaves are tiny and lacy-looking, and often overlapping.
Mosquito fern is the only native Azolla in Florida (Wunderlin, 1998). Azolla caroliniana is present throughout the majority of the eastern U.S., and in Ontario and British Columbia, Canada, as well (Kartesz, 1999).
If water fern is completely covering your pond, you may have fire ants. Read Interference of Natural Regulation of the Aquatic Weed Mosquito Fern (Azolla caroliniana) by the Red Imported Fire Ant, reprinted from Aquatics magazine, Vol. 26(2), Summer 2004.
View the herbarium specimen image from the University of Florida Herbarium Digital Imaging Projects.