Watermoss

Watermosses are persistent and stubborn plants, which makes them a successful invader. Watermosses can grow outside of their optimal conditions including temperatures, access to light, and nutrient availability (Coetzee & Hill, 2020) – and therefore can successfully grow outside of their native range. Watermosses are distributed through vegetative propagation, which occurs when a fragment of the plant is moved from one area to another and spawns at the new site, growing rapidly (Coetzee & Hill, 2020). Watermosses are therefore very easily propagated as they only require a fragment of the plant for a new plant to reproduced, and accordingly, all parts of the plant must be controlled so as not to spread. Distribution of watermosses can also occur due to water currents or by physical contact, such as by swimmers or boats. Once situated at a new site, watermoss populations can double their population size in just three days when under favourable conditions (Coetzee & Hill, 2020).

Watermosses undergo an exponential growth and has three growth stages: initial plant invasion and growth, secondary growth stage, and tertiary growth stage (Coetzee & Hill, 2020). Watermosses are more difficult to visually detect during their earlier growth stages, as they mostly grow in small populations under the water’s surface, but as they mature, will reach the water’s surface and form dense mats of vegetation spanning the surface (Coetzee & Hill, 2020).

Watermosses will fail to grow under -4°C or over 40°C but are hardy and will still grow under non-optimal conditions within this temperature range (Coetzee & Hill, 2020). When watermosses experience winter temperatures, plants will die off. However, plants have also been observed as re-sprouting in spring temperatures when waters warm, from seemingly dead plants (Coetzee & Hill, 2020). It is therefore very important to keep in mind that watermosses can still very much grow and persist in Canada’s colder climate.

Watermosses tend to grow in warm (30°C), high sunlight, and high nutrient waters, similar to their native range (Coetzee & Hill, 2020). Watermosses are an aquatic freshwater plant, and therefore only occur in freshwater ecosystems. The optimal growth of watermosses occurs in minimal water current conditions, but due to the plant’s hardiness, it has been found in still waters, canals, rivers, lakes, reservoirs, and swamps. Optimally, watermosses grow in conditions that are high in sunlight access and nutrient availability but have been observed to persist in areas lacking high levels of sunlight and nutrients (Coetzee & Hill, 2020). Water temperatures also have an optimal temperature of 30°C for watermosses to grow the most rapidly, but this plant has been observed growing in temperatures between -4°C and 40°C (Coetzee & Hill, 2020). This plant does not overwinter but has been observed sprouting from seemingly dead populations after the winter, showing its persistence and hardiness to less than optimal temperature conditions (Coetzee & Hill, 2020). Due to watermosses not having a root system, they can therefore grow in a variety of soil and substrate conditions.

Watermosses are native to throughout mainly South America, with one species native throughout Europe, Asia, and Africa as well. Salvinia has been introduced to the southern United States and along the East Coast, including New York and Massachusetts. Globally, watermosses have been found in parts of Africa, India, Indonesia, Malaysia, Singapore, Papua New Guinea, Australia, New Zealand, Fiji, Cuba, Trinidad, Borneo, Columbia, Guyana, Philippines, Puerto Rico and the Virgin Islands.

Watermosses have been distributed globally through commercial trade as an aquarium and ornamental plant. In fact, Salvinia species are easy to find on the Canadian market. This accessibility speaks volumes for the necessary outreach and education that is needed around this plant and how it threatens our ecosystems.

Watermosses are typically distributed into the wild through either accidental or intentional release from aquariums and ornamental ponds. Once in the wild, Salvinia is distributed by water currents and by physical contact. Fragments of watermosses can get stuck to swimmers, boats, and even wildlife, and deposited elsewhere, creating a new population.

It important to clean, drain, and dry your boats after exiting a waterbody, as to not transfer numerous invasive or pest species from region to region. It’s also important to remember: Don’t Let It Loose!

Ecological Impacts:

Due to the watermosses abilities to grow in dense patches at the water’s surface, this blocks out sunlight to any native species dwelling below. Watermosses also use up valuable and sometimes scarce nutrients in the waterbody, denying nutrients for native organisms. The blocking of sunlight and use of nutrients often decreases plant, invertebrate, and wildlife diversity. Losing biodiversity in an ecosystem often leads to low oxygen levels, which can cause the system to uninhabitable for long periods of time. The alteration of water chemistry can be irreversible and often has deeper impacts on nearby ecosystems.

Social and Economic Impacts:

The dense growth of watermosses can impact the recreational use of waterbodies, such as swimming and boating. Watermosses have the potential to become entangled around motors and paddles. This can in turn decrease waterfront property values. Watermosses can also clog irrigation channels which may cause flooding or drought in some areas. Due to a decrease in biodiversity, watermosses cause disruption in aquacultural cultivation. It has been known to disrupt the cultivation of rice, catfish, and crawfish, but has the potential to affect more species.

The Ontario government has proposed watermosses to be prohibited under the Invasive Species Act (New invasive species proposed for regulation under the Invasive Species Act | ontario.ca). Prohibited species are illegal to bring into the province, which includes the possession, sale, and propagation of the species within or into the province.

The prevention of species introduction is much easier and cost-effective than controlling a species once it is established. Thus, it is of high importance that watermosses are not introduced to Canada, as they pose a significant risk to Canada’s native species and would cause significant economic impacts for their removal and management.

The United States have previously found the introduction of the salvinia weevil, Cyrtobagous salviniae, has been successful in managing Salvinia populations as a biological control agent (Coetzee & Hill, 2020). This weevil feeds on the Salvinia plants but may pose a new threat as it is a non-native insect introduced to areas where Salvinia is introduced. The salvinia weevil is native to Brazil, and therefore should not be considered an ideal management tactic. However, chemical control of Salvinia via herbicide has also caused significant struggle, as the plant is persistent in growth even after treatment (Coetzee & Hill, 2020). Herbicide is also a very costly management option for invasive plants and can have detrimental effects on any non-target native plant species nearby the chemical treatment. Given the persistence and difficulty to manage watermosses, it is imperative that they not be introduced to Canada’s delicate and economically significant fresh waterways.

Watermosses are difficult to detect in their initial growth stage, due to its small size and its main growth occurring underwater, however, during its secondary and tertiary stages of growth, watermosses reach the water’s surface and forms dense mats that can span meters wide and up to one meter thick. The main sign of watermosses present is the death of native plants. Due to the rapid growth and size of watermosses, the plants both block out sunlight to native aquatic plants and take up valuable nutrients that native plants require. This decrease in plant biodiversity in turn will have whole effects on the ecosystem, causing decreases in insect, fishes, and wildlife diversity. Reptiles and amphibians are commonly used as indicator species, as they are most susceptible to environmental changes. Therefore, the most obvious sign of watermosses growing is a decrease in native plant diversity and a decrease in the nearby wildlife populations.

Outside of Ontario’s proposal for watermosses to be a prohibited plant under the Invasive Species Act, it is important to determine how other Canadian provinces are handling the threat of this plant.

Currently, Alberta has declared Giant Salvinia (Salvinia molesta) as prohibited under the Fisheries (Alberta) Act. Quebec’s Ministere de l’Environnement categorizes Salvinia as a “Species not listed in Quebec” but does recognize its threat.

Other Canadian territories and provinces have yet to make legal recognition of watermosses. Ontario is arguably the most threatened province by this plant, as its distribution is found in the connected and close-by states of New York and Massachusetts.