Water Fern: A Useful Alien

Water Fern, Azolla filiculoides

The Water Fern Azolla filiculoides belogs to the only genus of floating ferns. A native of the West Coast of South, Central and some of North America (1) it has become naturalised in Asia and at least 16 countries in Europe (2). It has been recorded in 14 of the 40 vice counties in Ireland (3) where its presence has been attributed to discarding of material from ornamental ponds and aquaria. Ongoing colonisation may be mediated by wildfowl carrying frond on their feet from site to site (2).

Frond and Roots of the Water Fern, Azolla filiculoides

Individual plants are floating fronds 1-2 cm in diameter with hair-like roots dangling into the water (4). Sori are produced at the base of side branches, producing spores that ripen from June to September. As the plants get older, they produce anthocyanins which lend a purple-pink tinge to the fern. The numbers of fern from year to year at a site may fluctate (3).
Ecologically, the Water Fern poses a threat to many native plants and invertebrates due to the tendancy of the fronds to clump together, often in their millions. This habit has been shown have a significant negative effect on submerged macrophytes such as Potamogeton crispus (5). Successful biological control has been achieved using a frond-feeding weevil, Stenopelmus rufinasus (6).

Water Fern, Azolla filiculoides, Covering a Pond

A filiculoides obtains its nitrogen from by a symbiotic relationship with the blue-green alga Anabaena azollae (7). This poses a problem in its aquatic habitat as its presence may lead to eutrophication (2), but this feature of A. filiculoides has been exploited in the cultivation of rice where it has been suggested that a fallow season crop of the fern in paddyfields could supply up to 50% of the nitrogen requirements for rice (8). While it is also used as a feed for pigs in areas such as Colombia, it has proved to have limited digestibility (9).

Water Fern, Azolla filiculoides

A. filiculoides has interestingly also been shown to be an excellent biosorbent due to its ability to bind and concentrate metal ions from aqueous solutions (10). Indeed, in mining operations in South Africa its ability to recover gold from effluents has shown to be quite efficient (11).

References:

1. Houghton Cambell, 1893 Annals of Botany 7 pp. 155-184
2. O'Mahony, 2009 Wild Flowers of Cork City and County p. 330
3. Reynolds, 2002 A Catalogue of Alien Plants in Ireland pp. 48-49
4. Phillips, 1980 Grasses, Ferns, Mosses and Lichens of Great Britain and Ireland p. 109
5. Janes et al., 1996 Hydrobiologia 340 pp. 23-26
6. McConnachie et al., 2003 Biological Control 28 pp. 25-32
7. Page, 1988 Ferns: Their Habitats in the British and Irish Landscape
8. Talley and Rains, 1979 Agronomy Journal 72 pp. 11-18
9. Leterme, 2010 Animal Feed Science and Technology 155 pp. 55-64
10. Fourest and Roux, 1992 Applied Microbiological Biotechnology 3 pp. 399–403
11. Antunes et al., 2001 Biotechnology Letters 23 pp. 249-251