Mapping Species Distributions

WHAT IS THAT STUFF COVERING THE WATER?

Azolla filiculoides

by Sharon Hartwell

 

Photo by Bruce Stewart

That thick, reddish ‘porridge’ covering the Chatterton fields in Rither's Bog in Victoria is not an algal bloom. It is actually hundreds of thousands of mosquito ferns or Azolla filiculoides. The Azolla appeared in late July, 2004, and covered the entire Chatterton field, completely obscuring the water. It also appeared at Swan Lake, and there is concern that it may harm the Wolffia borealis population there. The origin is interesting - the species is known from the lower mainland, and Oregon and Washington, but had not been officially recorded on Vancouver Island before (although Anne Scarfe says she saw it at Beckwith Pond a couple of years ago). It may have arrived via waterfowl, but there is also the possibility that it is the result of someone dumping or flushing their pond or water garden, since Azolla is sold as an aquatic in some garden centres. 

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!