Floating Treatment Wetlands - Why Use Them?

And what exactly are we trying to prevent?

As more and more bodies of water fall victim to declining water quality, the call to reverse this ecosystem degradation increases. Floating treatment wetlands are islands of wetland plants that are created to float in affected bodies of water and remove excess nutrients and pollutants through passive uptake. The goal of this project is to create a floating island system of native plants to help reduce pollution levels and improve water quality in Ottawa’s iconic Dow’s Lake. By implementing 63 floating treatment wetlands connected together as three larger islands perpendicular to a large storm drain that dumps wastewater directly into the lake, we hope to improve water quality and remove some of these pollutants as they enter. Native plants have been carefully selected for their ability to absorb nutrients, withstand ice encasement and regrow in the following spring. The dynamic design of the islands themselves provide resilience to the weather and natural elements while remaining adaptive and flexible to the changing water levels from draining or refilling throughout the year.

Factors influencing water quality

Agricultural runoff from nearby farms, such as the Central Experimental farm, decreases the water quality. This sort of runoff carries pesticides and fertilizers into nearby waterways. This introduces excess nutrients, particularly phosphorus, into the water, leading to imbalances in the aquatic ecosystem (Pericherla et al., 2020). The accumulation of these nutrients plays a significant role in the deterioration of water quality and contributes to issues such as eutrophication. Additionally, urban areas have limited natural spaces to absorb water, causing stormwater to flow directly into waterways while collecting pollutants along the way (Urban Stormwater, n.d.). This runoff carries contaminants such as oil, heavy metals, microplastics, and other harmful substances, further degrading water quality. Without proper stormwater management, urban runoff contributes to the ongoing pollution and disruption of aquatic habitats (Rain Ready Ottawa, n.d.)

Moreover, microplastics are a major contributor to water contamination and originate from sources such as synthetic clothing fibers, plastic litter, and microbeads found in personal care products (Murphy & Vermaire, 2016). Urban runoff carries these plastic particles into rivers and lakes. Microplastics are not biodegradable, meaning that they remain in the water and are consumed by organisms. These may cause significant issues for animals such as reproductive problems, tissue damage, and can even be fatal (Issac & Kandasubramanian, 2021). In addition, microplastics can absorb and transport harmful pollutants such as heavy metals, pesticides, and industrial chemicals, which further threatens aquatic life (Rafa et al., 2024).

Erosion and sedimentation introduce soil particles, minerals, organic matter, and harmful bacteria into waterways. These particles can bind to toxic chemicals and transport them across different areas of a lake, spreading contaminants (Droppo et al, 2011). Increased sedimentation raises water turbidity, reducing the amount of sunlight that reaches the lakebed. This limits photosynthesis, increases water temperature, and disrupts the aquatic ecosystem (Bilotta & Brazier, 2018).

Ottawa’s heavy use of road salt during winter, averaging 156,000 metric tonnes annually, significantly impacts freshwater systems (5 Years of Road Salt Monitoring, 2024). Chloride from road salt accumulates in waterways, creating harmful conditions for aquatic species such as amphibians, freshwater invertebrates, and fish. Exposure to high chloride concentrations can lead to stress, reproductive issues, and even mortality in these organisms (Cañedo-Argüelles et al., 2013; Elphick et al., 2011). Chloride levels in some Ottawa creeks have exceeded chronic toxicity thresholds (120 mg/L), with certain areas surpassing acute toxicity levels (640 mg/L), posing a serious threat to freshwater ecosystems (Sorichetti et al., 2022; 5 Year Road Salt Monitoring, 2024).

Eutrophication

Eutrophication is a process when aquatic environments become overly enriched with inorganic nutrients, causing excessive plant and algae growth. This can be a naturally occurring process but is being exacerbated by anthropogenic activities such as erosion and runoff from fertilized agriculture, sewage runoff, and industrial waste water. Plant and algae growth is dependent on the availability of limiting nutrients such a s phosphorus and nitrogen. When aquatic ecosystems become enriched with these nutrients, growth is favoured. phosphorus-rich sources include fertilizers, untreated sewage, detergents containing phosphorus, and industrial waste. Eutrophication causes the degradation of water quality. As plants decay, they sink to the bottom of the water body and begin to decompose. Decomposition depletes dissolved oxygen and consequently results in hypoxic zones. These ‘dead zones’ no longer support biodiversity. There are many ecological effects of eutrophication such as altering the vertical conditions of the water body, increase in water toxicity, invasion of new species, and overall disruption of the food chain. 

Floating treatment wetland metrics

Monitoring the effectiveness of the floating treatment wetlands requires a clear understanding of how these function throughout the year. To track their success, we will collect data at three key periods every year: spring, summer, and fall. This monitoring plan will assess water quality, plant health, and overall performance of how well the floating treatment wetlands are working.

In the spring, the primary focus is to measure the effect of runoff and snowmelt, which accumulates during the winter. Water samples will be collected to measure total phosphorus, nitrogen, chloride, turbidity, dissolved oxygen, pH, temperature, and conductivity levels. The spring measurements provide important data for the management of nutrient cycling and potential impacts of runoff. 

During the summer, plants in the floating treatment wetland will be at their peak in terms of taking up nutrients and particles from the water (Lee et al., 2013). The same samples collected during the spring will be collected in addition to water samples of chlorophyll-a that will ideally be collected every two weeks in the months of August to September. This is due to the fact that algal blooms occur most commonly in late summer to early fall (Blue-green algae, n.d.; Monitoring Lake Water Quality, n.d.). The measurements taken in the summer will assess how well the floating wetlands are able to function under environmental stress. 

As the season transitions into fall, the final round of measurements are taken before the fall turnover of the lake to evaluate how the floating treatment wetlands performed overall during the season. Measurements of total phosphorus, nitrogen, chloride, turbidity, dissolved oxygen, pH, temperature, conductivity levels, as well as root length are done. Root length determines how much the plants have grown and whether their root system has been effective in filtering pollutants (Read, et al., 2009). This helps to determine if the floating treatment wetlands have been able to sustain themselves and if they improved water quality.

FTW materials

Firstly, aluminum 5052 hollow pipes were selected as the base frame because of its high corrosion resistance, making it suitable for marine environments. With a lifespan ranging from 20 to 40 years and a relatively low density of 2.68g/cm3, it provides a lightweight but sturdy structure (Zhang et al, 2011). Other floating island projects often use polyvinyl chloride (PVC) pipes or high density polyethylene (HDPE) pipes as a frame because of their low density, meaning they have a high buoyancy, and cost effectiveness compared to aluminum. However, these materials pose a risk of leaching microplastics into the water (Lourmpas et al., 2024). Furthermore, perforated wood made up of western red cedar is used to the base frame as well. Western red cedar wood was chosen because it has high moisture, decay, and insect infestation resistance due to the natural oil and acids it possesses called polyphenols (Edwards, 2025; Hofmann et al., 2020). Because of these resistances, this type of wood is able to last 10 to 20 years outdoors. 

In addition, to ensure durability and structural integrity, stainless steel 316 was chosen for the mesh. This type of stainless steel has high corrosion resistance, UV radiation resistance (Guo et al., 2022), weathering resistance, and a lifespan of 20  to 50 years (Kim et al., 2014). Stainless steel 316 made to withstand the conditions of water compared to other types of steel. The stainless steel mesh is also used for the bottom of the floating treatment wetland to protect the roots from aquatic organisms from damaging them.

For the matrix, granulated cork was chosen due to its very low density of around 0.13 to 0.25 g/cm3, which means it won’t interfere with the buoyancy of the aluminum pipes all while providing a sturdy base. Additionally, cork is biodegradable and can last for decades (Anjos et al., 2014). 

Synthetic geotextiles, commonly made from polyester (PET), polypropylene (PP), polyethylene (PE), and polyamide all carry the risk of releasing microplastics into the water as they break down over time  (Rollin, 1999). This is why non-woven coir (coconut fiber) geotextile is a better alternative to use. Coir geotextile is a biodegradable fabric that can last 4 to 6 years (Prambauer et al., 2019). Lastly, biodegradable plant inserts, typically made of peat, however, peat harvesting is harmful to the environment. This is because it destroys carbon-rich peatland ecosystems and releases stored carbon into the atmosphere (Daigle et al., 2001). Coir or jute biodegradable pots are a better alternative for this reason.

Ideal Size

Size is an important factor to consider when designing floating wetland treatments, specifically the total surface area of the island. The first impact to consider is how size can improve the resistance of the island. If the island is too small, it is much more susceptible to damage from wind, storms, and other natural elements that could break up smaller islands with ease (White & Cousins, 2013). A smaller size will also make the islands more susceptible to wave damage, since it will be much easier to topple and potentially even flip over. The height of the plants is also important to consider, as they are susceptible to the weather as well. Taller plants such as cattails were more likely to be destroyed when they were planted on smaller wetlands, showing that a larger surface area is necessary to protect and support these tall plants from the elements. The total surface area that these floating wetland treatments cover has a direct correlation to its ability to remove nutrients from the water and improve water quality. Studies have shown that the islands must cover at least 5-8% of the total surface area of the affected area to remove enough nutrients to create an anoxic environment needed for denitrification (Byanju, Pradhanang, & Pradhananga, 2024). If the percent of coverage were to exceed 50%, this could then lead to the depletion of dissolved oxygen levels, suffocating aquatic organisms (Landon & Hunt, 2020). To allow for a large surface area for structure and resilience, but not too large to deplete dissolved oxygen, areas of open water can be created within the floating treatment wetlands. Creating large islands of small hexagon pieces allows for connection and stability between the pieces, but also the ability to have empty gaps within the larger island. Having open areas allows for oxygen production via photosynthesis in the water column as well as increased oxygen dissolution in the water from aeration via wind and waves (White & Cousins, 2013). Many floating wetland treatments are implemented to combat pollution and improve water quality in affected areas. To maximize the island’s ability to improve water quality and nitrate removal, the island needs to have a minimum surface area of 50m² (White & Cousins, 2013). While designing a floating treatment wetland may seem relatively simple, something as straightforward as the total surface area of the islands can be crucial to the functioning of the final product. It is a fine line between balancing all the different factors that rely on the size, and the success of the floating treatment wetland relies heavily on the decisions made to incorporate all these important implications. 

Plants for FTW

The effectiveness of floating islands lies in the ability of the plants carefully chosen that have the ability to uptake excess nutrients and pollutants in the water, thereby improving the water quality of Dow’s Lake and supporting ecosystem restoration.  

Grasses, such as Cattails (Typha spp.), are widely used for bioremediation in aquatic ecosystems because of their ability to remove and absorb excess nutrients. This is due partly to their extensive and fibrous root systems that cover large surface areas for root and soil contact. Studies show that grasses tend to absorb heavy metals and other pollutants the most efficiently by having the most retention rates with the shortest amount of time. 

Thlaspi sp. are rapid growing plants that tolerate high levels of toxicity. They accumulate heavy metals such as zinc, cadmium, and sometimes nickel by concentrating it in the shoot. These species can be specially grown in hydroponic conditions and do not rely on soil for their root system, making them ideal plants for phytoremediation of aquatic ecosystems such as Dow’s Lake and makes it a great option for the floating island. 

Pickerelweed is a monocot aquatic plant that has proved to be resilient by surviving winter, including ice encasement, and regrowing the next season. In a study (provided by Marina in the other document), researchers found that it produced the most biomass compared to other species and removed the most phosphorus from the body of water. This makes it a great candidate for phytoremediation in the floating island in Dow’s Lake. Similarly, soft-stemmed bulrush was able to survive winter conditions and regrew in the following season. 

Blue flag iris, Marsh hibiscus, Marsh marigolds, and Swamp milkweed are exemplary plants used for phytoremediation and also aesthetic purposes. Each of the plants listed have flowering petals and bright colours to attract pollinators. They are endemic to southern Ontario and are adapted to thrive in moist to wet environments such as wetlands and floodplains. Additionally, Swamp milkweed is essential for monarch butterflies.  Monarchs are listed as an endangered species under the federal Species at Risk Act and so the planting of Swamp Milkweed should act to improve their populations. 

Evidence from Past Studies

The creation of floating islands, often referred to as floating treatment wetlands, is a water quality improvement technique that is quickly gaining traction around the world. They are highly customizable to the environment and the desired outcome, can be made from inexpensive materials, and have been proven effective in reducing nutrient loads in a variety of different environments. Field studies and trials have been conducted across North America and have shown hopeful results and provided invaluable evidence of what plants are ideal for this purpose.

A study was conducted in a nutrient enriched urban wet pond in a residential area of City of Fairfax, Virginia to increase the community’s knowledge on the effectiveness of floating treatment wetlands in reducing pollutants from runoff. Specifically, this study evaluated phosphorus accumulating and distribution within the plants, sustainability under the stress of ice encasement, and evaluated which plant species might be most useful to fulfill this role. Based on fluctuations of biomass and removed phosphorus, pickerelweed performed the best in growth and nutrient removal (Wang, Sample, Day, & Grizzard, 2015). It was also observed that harvesting the above-ground portion of the plants either during June to remove the absorbed phosphorus, or in September to reduce phosphorus rerelease due to natural degradation of cells, was an important maintenance practice to maximize efficacy (Wang, Sample, Day, & Grizzard, 2015). Finally, the study showed that the underground portions of pickerelweed and softstem bulrush survived ice encasement and were able to regenerate in the spring (Wang, Sample, Day, & Grizzard, 2015). This study was local and provided evidence that was directly applicable to the environment of Dow’s Lake, addressing concerns such as ice encasement in this temperate environment. Understanding the biological characteristics of native plants that can be used and the benefits of different species is crucial to the effectiveness of this project. Furthermore, real-life evidence showed that some basic maintenance efforts will also improve results, practices that weren’t previously considered before this study began. 

A similar study was conducted to evaluate how a floating treatment wetland could be used to reduce acid mine drainage and removal of metals from polluted waters. This study was conducted in Sudbury, Ontario in water that had been impacted by mining. The purpose was to evaluate how anaerobic processes could be employed for iron and sulphate reduction that would remain functional even during freezing. Canadian rush, common cattails, and lake sedge were the three plant species employed in this study. Not only did the results show sulfate concentrations to be significantly lower in the areas containing the floating treatment wetlands, but also a 30% increase in sulfate-reducing bacteria richness as well as a 100% increase in sulfate-reducing bacteria abundance (Gupta, Courtemanche, Gunn, & Mykytczuk, 2020). These FTWs created an environment for SRBs to thrive, using plants that were able to survive and regenerate following winter. While most floating treatment wetlands use aerobic processes to improve water quality, often struggling in the frigid winters of temperate areas, this study showed the versatility of floating treatment wetlands. It also explored a novel use beyond their already invaluable function in restoring water quality and combating eutrophication. 


© 2025 Dow's Lake Floating Wetlands: A Scalable Model for Urban Ecological Restoration by Reema Adan, Sharon Chen, Ella Gem DeFrancisco, Eva Domond, Calvin Elisen, Madison Lucente, Donya Nadoushan, and Marina Spitz is licensed under CC BY-NC-SA 4.0
© 2025 Dow's Lake Floating Wetlands: A Scalable Model for Urban Ecological Restoration by Reema Adan, Sharon Chen, Ella Gem DeFrancisco, Eva Domond, Calvin Elisen, Madison Lucente, Donya Nadoushan, and Marina Spitz is licensed under CC BY-NC-SA 4.0
© 2025 Dow's Lake Floating Wetlands: A Scalable Model for Urban Ecological Restoration by Reema Adan, Sharon Chen, Ella Gem DeFrancisco, Eva Domond, Calvin Elisen, Madison Lucente, Donya Nadoushan, and Marina Spitz is licensed under CC BY-NC-SA 4.0