Mosquito Fern
Azolla filiculoides ?
Photographed on ponds used for aquaculture, Eldoret, Kenya. July 2014.
These small ferns float on the water surface of slow ponds and marsh edges. This appears to be the exotic species from the Neotropics (Central or South America). Each individual plant is just a centimeter or so in diameter and as new leaves are added the plants break apart increasing the spread of the plant. The plants are capable of fixing nitrogen, a factor that sometimes offsets its roll as an aquatic weed.