A review of some ecological factors affecting the growth of Azolla spp.

The genus Azolla forms a group of small-leafed, floating aquatic ferns native to the tropics, subtropics, and warm temperate regions of Africa, Asia, and America. For several decades, these ferns have been utilized for various purposes: e.g. as green manure, feed for animals, but also for the removal of different metals (e.g. Hg, Pb, Cr and Cd) through wastewater treatment or for elimination of nitrogenous compounds from surface water. Notwithstanding, these many advantages of Azolla, it has invaded many natural habitats, thus becoming an obnoxious weed. Azolla can grow quickly with a doubling time of only 2-5 days and form very dense mats in favourable habitats, causing many difficulties for boat transport, water animals and native plant species and becoming a source of eutrophication. The present paper gives an overview of some important ecological factors affecting Azolla’s growth over the past few decades. Moreover, for the most ecological variables discussed in this study, the authors refer to their recent publications for the habitat requirements of Azolla in Anzali wetland. Water availability is the key factor for its growth. Growth is further promoted by optimal light intensity (15-18 Klux), temperature (18°- 28°C) and relative humidity (55-83%). Wind and turbulent water can fragment and kill Azolla. The importance of both macro (e.g. phosphorus, nitrogen, potassium, calcium and magnesium) and micronutrients (e.g. molybdenum, cobalt and etc.) has also been confirmed from literature. Various types of insects (e.g. caterpillars), bacteria, fungi and viruses can affect Azolla growth. As a conclusion, understanding the habitat requirements of Azolla is very helpful for managing this aquatic fern, also for decision making in the context of wetland restoration and conservation management