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Rithet's Bog LogoWhat is that Stuff Covering the Water?

BARA Bugle (Broadmead Area Residents Association Newsletter),
Fall 2004 - Sharon Hartwell

That thick, reddish ‘porridge’ covering the Chatterton fields is not an algal bloom. It is actually hundreds of thousands of ferns – mosquito ferns, to be precise.

Mosquito ferns, the smallest ferns in North America, are tiny, free-floating plants completely adapted to life in the water. Each fern is composed of branching fronds with dense, overlapping, scale-like leaves, and resembles a chubby miniature cedar branch with dangling roots. The species growing on the Chatterton wetlands is ‘Large Mosquito Fern’ (Azolla filiculoides). In spite of its name, it is only 1 to 2 centimetres across. The plants are initially bright green in colour, but develop a reddish tinge as they age and become stressed.

Mosquito ferns establish in areas of still water such as ponds or sloughs, and form rapidly expanding mats. Under ideal conditions they are capable of doubling their mass every few days, and can soon dominate entire wetlands, as they now do the Chatterton fields. One of the reasons the ferns are able to reproduce so quickly is that they have a symbiotic relationship (mutually beneficial partnership) with a microscopic blue-green alga or cyanobacterium, called Anabaena azollae. This alga, which is a nitrogen fixer, lives in cavities in the leaves of the mosquito fern, acting like a built-in fertilizer factory. The well-fed ferns are able to grow at a dramatic rate, reproducing vegetatively when fragments break off, or via massive numbers of spores.

Is a bloom of mosquito ferns any better than an algal bloom? In the short run, yes: algal blooms soon decay, causing foul odour and low oxygen levels in the water – a bad combination for aquatic wildlife. Mosquito ferns persist: photosynthesizing, producing oxygen, providing food for waterfowl and creating habitat for a variety of aquatic invertebrates. If you watch the ducks as they plough head down through the mosquito ferns, you’ll see they are actively feeding on the invertebrates on the underside of the matt of ferns – coots will even eat the ferns themselves. It really is ‘duck soup’.

In the long run, Azolla blooms usually collapse and disappear, typically over the hot summer months. Before this happens at Rithet’s, though, the massive bloom may well cause temporary problems. The ferns will likely persist over the winter (Azolla filiculoides is cold tolerant, surviving even under thin ice), and next spring their thick growth might out-compete and eliminate other desirable plants. If the mat is thick enough, it may even block so much light that underlying plants are killed, subsequently creating an oxygen deficit as they decompose. The ultimate effect would be the same as that caused by a short-term algal bloom. Control measures are sometimes attempted for mosquito fern blooms, and have included physical skimming and removal, parasitic weevils, and tarragon oil.

Azolla’s prolific tendencies have also been encouraged and put to use agriculturally. It is employed as a living fertilizer in Asian rice paddies, has been used as a green mulch and fertilizer on land, and a protein source for pigs in South America. The name ‘mosquito fern’ is said to derive from yet another use: in optimum conditions, the foliage becomes so dense it is said to prevent mosquito larva from developing and hatching!

How did the mosquito ferns get to the Chatterton wetlands in the first place? Or to Swan Lake, where they also appeared this summer? Since Azolla filiculoides is native to the Pacific Northwest, it most likely arrived naturally, transported on the feet and feathers of migrating waterfowl. A more disturbing possibility is that it may have been artificially introduced when someone cleaned his or her fishpond or water garden: Azolla is sold in some nurseries as an easy to grow aquatic, and has a reputation overseas as a troublesome North American horticultural invasive.

For more information on mosquito ferns, check the following websites:





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