The photo on the left is of Anabaena separated from the Azolla. The fertilizing relationship was understood long ago in agricultural history, although the actual mechanisms have been studied only recently. In southeast Asia, Azolla was grown in rice paddies as a means of fertilizing the crops. Sometimes called “green manure,” it has been estimated that the use of Azolla more than doubles rice production. Further, there is no need for crop rotation or the addition of other fertilizers to the soil, otherwise done at considerable cost. The plant also has been used as a feed for stock and as a control for mosquitoes because it closes the surface of the water and does not allow the larvae to breathe (leading to another common name - mosquito fern.) On the other hand, the mat decreases production within the aquatic system and lowers oxygen levels in the water. This can have negative effects on other aquatic life, so Azolla may be considered a water weed.