Short description of Azolla filiculoides, Water Fern

Water fern forms floating rosettes up to a few centimetres diameter, composed of branched fronds with rows of imbricate leaves covered with dense papillae. It is initially green, becoming reddish late in the season. Globular spore-bearing structures sometimes develop below the fronds.

Impact summary: Azolla filiculoides, Water Fern

The long-term impact of water fern on any water body is difficult to assess. In the short-term it can form a layer up to 30 cm deep covering the whole water-surface of standing or slow-flowing water, excluding light from the water column.

Habitat summary: Azolla filiculoides, Water Fern

Water fernis most frequent in ponds, lakes, canals, ditches and slow flowing rivers. It will also occur in seepages and in the backwaters of rivers and streams with faster flow but is rarely abundant in these habitats.

Overview table

Environment	Freshwater
Species status	Non-Native
Native range	Northern America, Southern America, Central America, Alaska
Functional type	Land plant
Status in England	Non-Native
Status in Scotland	Non-Native
Status in Wales	Non-Native
Location of first record	v.c.21 (Pinner)
Date of first record	1883

Origin

Water fern is native to western North America south to Central America and throughout much of South America. It is also generally considered native to Australia and New Zealand, although there is some disagreement. It was apparently native to GB in previous interglacials.

First Record

First recorded at Pinner in Middlesex in 1886 but did not become well established until 20 years later.

Pathway and Method

It was apparently cultivated in botanical gardens from which it was dispersed into the wild. The dispersal vector is not known but it has been shown to be transported on machinery and may well survive dispersal on the clothes of people working in gardens.

Species Status

Water fern occurs as an introduction Europe from Ireland to Denmark in the north, south to Portugal and east into Romania. It is also naturalised in southern Africa and reported from tropical Asia, although there is some uncertainty about which species is involved in the latter.

Dispersal Mechanisms

There is little confirmed information in dispersal vectors and most available information is speculative. It is certainly carried by machinery and boats, it may also be carried by wildfowl; however the maximum distance that the plant can survive such transport is unknown. There are also records of deliberate introduction to the wild in Denmark.

Reproduction

Most reproduction in water fern in GB appears to be by vegetative fragmentation and most sources suggest that it overwinters as plants or plant fragments. There is evidence that production of sporocarps may be frequent, but there is no information on its capacity to reproduce sexually in GB.

Known Predators/Herbivores

There are no native organisms in GB which are known to feed on control water fern. A weevil (Stenopelmus rufinasus) native to North America which feeds on Azolla species, has been established in GB since 1921.

Resistant Stages

None known.

Habitat Occupied in GB

Water fern typically occurs in ponds, lakes, ditches, canals and slow flowing parts of rivers, usually in eutrophic conditions. It is also capable of exploiting nutrient-poor waters, apparently due to its symbiotic association with the alga Anabaena azollae.

Water fern is widespread and abundant throughout the lowlands of GB, south of a line from Cumbria to the Humber and to the east the Cambrian Mountains. Outside this area, it occurs at scattered localities on the coast of Wales, the Isle of Man, Northumberland, the Scottish Borders and Ireland.

Environmental Impact

Water fern can achieve 100% cover over the water surface as a carpet occasionally up to 30cm thick. At such times, it blocks out light, preventing photosynthesis in aquatic plants and preventing or compromising oxygen diffusion. It also prevents amphibians and invertebrates from reaching the surface and may disrupt movement of animals on the water surface

Health and Social Impact

The main reported health and social impacts of water fern involve disruption of water-based recreation and adverse effects on aesthetics and recreation associated with water bodies. There are also occasional reports of death of pets which mistake carpets of water fern for land, jump onto the surface and are then unable to get out of the water and drown.

Economic Impact

Members of the genus Azolla are used as green manure in rice fields in Asia and in some areas it is used as livestock fodder. There is increasing research into the use of Azolla species for wastewater treatments both through bioaccumulation of heavy metals and nutrient treatment. It may have adverse economic impacts where it forms dense mats on the water surface, obstructing water-borne transport, navigation and flood defences.

Identification

Lumpkin, T.A. (1993) Azollaceae. In: Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. Vol. 2. Oxford University Press, New York and Oxford.

Biology, ecology, spread, vectors

Hussner, A. (2010) NOBANIS – Invasive Alien Species Fact Sheet – Azolla filiculoides. – From: Online Database of the North European and Baltic Network on Invasive Alien Species – NOBANIS www.nobanis.org, Date of access 15 November 2010.

Preston, C.D. & Croft, J. (1994) Aquatic plants in Brtain and Ireland. Harley Books, Colchester.

Management and impact

Hussner, A. (2010) NOBANIS – Invasive Alien Species Fact Sheet – Azolla filiculoides. – From: Online Database of the North European and Baltic Network on Invasive Alien Species – NOBANIS www.nobanis.org, Date of access 15 November 2010.

General

Hussner, A. (2010) NOBANIS – Invasive Alien Species Fact Sheet – Azolla filiculoides. – From: Online Database of the North European and Baltic Network on Invasive Alien Species – NOBANIS www.nobanis.org, Date of access 15 November 2010.

Preston, C.D. & Croft, J. (1994) Aquatic plants in Brtain and Ireland. Harley Books, Colchester.