Floating Ferns Appearing In Freshwater Ponds

Aquatic ferns are been spotted in some of the island’s freshwater ponds, with Dr Mark Outerbridge saying that “a few of Bermuda’s freshwater ponds have been replaced with what appears to be grassy lawn when seen from a distance.”

In the latest issue of Envirotalk, Dr Outerbridge said, “Some readers may have noticed that a few of Bermuda’s freshwater ponds have been replaced with what appears to be grassy lawn when seen from a distance. Closer inspection, however, reveals a dense mat of floating water plants. The plants forming these mats are aquatic ferns belonging to the genus Salvinia.

“There are a few species within this genus, and all appear to be popular in water gardens and the global aquarium trade. Giant salvinia Salvinia molesta and common salvinia [aka water spangles] Salvinia minima are the two found growing here. Nathaniel Britton reported salvinia growing in the ditches of Pembroke Marsh over 100 years ago, however these days it can also be found growing in Cloverdale Pond, the Edmund Gibbons Nature Reserve, and in Seymour’s Pond.

“Introductions into the wild occur as intentional releases or via hitchhiking on waterfowl and waterbirds. Flooding also distributes fragments that can create new colonies. Salvinia can only grow in freshwater wetlands where the salinity is less than 7 parts per thousand [ppt], anything greater and the plants weaken and die.

Photo courtesy of Envirotalk

“Salvinia has small, oval shaped leaves that grow in pairs along horizontal stems. Isolated plants have petite leaves which lie flat on the water surface [primary form] – a typical sight during the early stage of an invasion. As the plants develop, the young leaves grow larger and start to fold [secondary form].

“Eventually the salvinia leaves get bigger, curl upwards and become deeply folded [tertiary form] and large infestations form dense mats which can completely cover the surface of the water, giving the illusion of being solid ground. The upper surfaces of floating leaves are uniformly covered with rows of white, bristly hairs.

“The stalks of each hair divide into four thin branches that are either spreading and free at the tips [common salvinia] or joined at the tips [giant salvinia]. These specialized hairs create a water repellent, protective covering and give the leaves a velvety appearance. The plants reproduce prolifically through vegetative growth and sporadic fragmentation, in other words by forming new branches which break off. Under ideal conditions salvinia grows extremely fast. In nutrient rich waters it may reach a density of 30,000 small plants per m2 and can double its biomass in two days.

“These are just a few of the qualities which have earned it a spot on the list of the 100 most invasive species on the planet. Dense mats of salvinia cause numerous ecological problems. They prevent atmospheric oxygen and sunlight from entering the water, seriously affecting the survival of aquatic flora and fauna. Also, the ferns create a large amount of organic matter which settles and rots on the bottom.

“This rotting matter depletes oxygen in the water, causing fish kills and harming other aquatic wildlife. Furthermore, waterbirds have a very difficult time accessing food resources below the mats and migratory birds are much more likely to overlook our wetlands as they pass by. In other countries salvinia clogs waterways, causing problems to agricultural irrigation and hydroelectric power generation as well as restricting commercial and recreational boating activities.”

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Category: All, Environment