An Introduction to Floating Wetlands for Stormwater Ponds

Floating wetlands are artificial floating platforms planted with vegetation that can be deployed in stormwater ponds and other surface waters to help improve water quality. Floating wetlands absorb excess nutrients, provide habitat for aquatic and terrestrial wildlife, and may reduce the frequency of algal blooms. This publication contains information that may be useful to municipalities, stormwater professionals, pond owners, pond managers, homeowner associations, and stormwater educators, among others, on what floating wetlands are, including benefits, and where to find more information.

Introduction

Stormwater Runoff

Contaminants, such as excess nutrients from fertilizers, chemicals, sediment, and fecal bacteria from the improper disposal of pet waste or incomplete treatment of human waste, can be moved over land and transported with stormwater runoff into waterbodies (such as ponds) during storm events. While stormwater ponds are primarily designed to manage flooding, they can also effectively prevent pollution, such as nutrients, from reaching downstream water bodies. While nutrients like nitrogen (N) and phosphorus (P) are essential for aquatic life, excess nutrients can stimulate algal blooms, which can release toxins and decrease dissolved oxygen concentrations in the water upon decomposition.^1–3 Management of stormwater ponds often focuses on dealing with the impacts of nutrients, both on the landscape and once they reach the pond.

Green Stormwater Infrastructure

Green stormwater infrastructure helps to manage stormwater by capturing and storing runoff to reduce the volume of water entering nearby surface waters. Green stormwater infrastructure can also be an effective management tool to improve water quality before it reaches surrounding waterbodies. Detention basins and constructed wetlands are types of green stormwater infrastructure that work to reduce runoff volumes and improve water quality.⁴ Constructed wetlands are designed to mimic natural wetlands in terms of the vegetation, soils, and microbial communities that interact to improve water quality and temporarily store stormwater runoff. The plants can absorb nutrients, while the roots help trap larger particles and support microbiological processes like denitrification. These microbial processes can transform nutrients into forms that are not mobile in the soil and water and cannot be taken up by plants.^5,6

Finding the space to install green stormwater infrastructure in urban and suburban areas can be challenging. Constructed wetlands can require a large land area for installation, depending upon the volume of water to be treated. Rooted vegetation in traditional wetlands can only tolerate short periods of total submergence and relatively shallow water depths (typically < 20 inches) over time.⁷ Floating wetlands, which provide similar water quality benefits to traditionally constructed wetlands but don’t take up any space on land, may be a good option for treating water within stormwater ponds when space is limited.^1,7

What is a Floating Wetland?

A floating wetland consists of a buoyant platform planted with emergent wetland plants that can float on the surface of a slow-moving waterbody like a pond or lake (figure 1). Plants installed in these systems grow hydroponically, meaning without soil; plant shoots grow above the water while plant roots extend downward into the water column. The floating platforms are anchored but have enough slack to adjust to changing water levels.

Floating platforms can be created using various synthetic (e.g., polyethylene foam, polyvinyl chloride tubing, and plastic mesh) and natural (e.g., bamboo, coconut coir) materials.^8,9 The platform needs a material that can support vegetation growth and provide stability through various weather conditions.¹³ The choice of material can impact the overall cost, durability, and lifespan of the floating wetland.¹⁰ Figure 2 shows two floating wetlands made with different materials, including jute and bamboo (a) and synthetic foam (b). The variety of construction materials allows for flexibility in the size and shape of floating wetlands. This flexibility provides opportunities to design floating wetlands based on aesthetic preferences and management goals for the area where it will be deployed.¹¹

Properly designed floating wetlands can enhance habitat and add recreational and aesthetic value to the landscape.^12,13 The platform and vegetation growing above and below the mat can potentially provide additional habitat for pollinators, birds, turtles, invertebrates, aquatic fish, and other species.^12,13 Occasionally, high water temperatures in ponds and lakes can negatively impact aquatic species (reduced health and reproduction).^14,15 The presence of the floating wetland vegetation on a pond’s surface creates additional shade that can result in more stable year-round water temperatures that support overall fish and macroinvertebrate health.¹⁶

Figure 1. Diagram of a floating wetland matrix. Image credit: Clemson Cooperative Extension.

Figure 2. Two different floating wetlands designs: (a) a floating wetland made of natural materials and (b) a floating wetland made of synthetic materials. Image credit: Clare Escamilla, Clemson University.

Floating Wetlands in Stormwater Ponds

While stormwater ponds provide mechanisms for pollutant removal, the primary pollutant removal mechanism in detention basins is sedimentation through the settling of solid, larger particles and their attached pollutants at the bottom of the basin.¹⁷ However, they are less effective at treating pollutants in a dissolved form.¹⁸ The addition of floating wetlands provides an opportunity to enhance water quality treatment performance in wet ponds by helping to remove dissolved and fine particulates that are typically harder for stormwater ponds to remove. See the Land-Grant Press publication “An Introduction to Stormwater Ponds in South Carolina” for more information about types of stormwater ponds and sedimentation.

Based on how floating wetlands are designed, several features can make them an attractive option to include in stormwater ponds (figure 3). Since the floating wetland is installed on the waterbody instead of on the land, it can be used in highly developed areas where space for green stormwater infrastructure is limited.¹⁹ A floating wetland does not appreciably reduce the storage volume of a waterbody, which is important for stormwater ponds that are designed to hold a certain volume of water during rain events.¹² The water depth in stormwater ponds can also fluctuate following rain events, which could damage sediment-rooted plants, but floating wetlands are designed to adjust to variable water levels, protecting the plants installed on the mat.^12,20 These benefits, as well as the low operational and maintenance costs, have contributed to the recent popularity of floating wetlands as a tool for pond management.²¹

Figure 3. Three floating wetlands recently installed in stormwater ponds. Image credit: Clare Escamilla, Clemson University.

Water Quality Benefits of Floating Wetlands

Floating wetlands remove nutrients and other contaminants from water through three primary mechanisms (figure 4)²²:

1. Direct plant uptake of nutrients (N and P) from the water
2. Biofilm (i.e., beneficial microbes) growing on the plant roots break down and reduce nitrogen in the water^22,23
3. Plant roots trap and enhance the settling of sediment, larger particulates, and their associated pollutants.²⁴ The settling of suspended particles is one of the main mechanisms for reducing total phosphorus and orthophosphate levels in the water.⁹

Figure 4. Diagram of a floating treatment wetland and the three primary mechanisms of reducing pollutants from urban stormwater runoff. Image credit: Clare Escamilla, Clemson University.

Various studies have been performed to determine the nutrient removal efficiency of floating wetlands. Table 1 reviews floating wetland performance from several case studies in the eastern United States. Floating wetlands can provide nutrient removal but vary in the extent of removal, depending upon pond location and size, size of the floating wetland, and water quality, among other factors. Percent removal of TN ranged from 0% to 91.7%, while percent removal of TP ranged from 17.4% to 98.4%.

Table 1. TN and TP removal efficiencies from several mesocosm and field scale studies of the floating wetlands along the East Coast.

Factors Influencing Nutrient Removal by Floating Wetlands

The nutrient removal efficiency of floating wetlands depends on a variety of factors, including current water quality, hydraulic retention time, climate, pond depth, and plant selection.

Current Water Quality

The nutrient concentrations in waterbodies can vary dramatically based on land cover and land uses within the surrounding watershed. Floating wetlands have been shown to effectively treat nutrient-rich agricultural wastewater³⁴ and polluted surface waters.³⁵

Hydraulic Retention Time

The hydraulic retention time of the waterbody (the time that water is in the system before it is discharged) where the floating wetland is installed also plays an important role in the efficiency of the floating wetland. Hydraulic retention time refers to how long water stays within a pond before it flows out. Generally, a longer hydraulic retention time increases the overall removal efficiency of floating wetlands for some pollutants.⁹ However, this pattern may not be observed in deep waterbodies where there is a large volume of water below the plant’s roots, which means proportionally less water will be in contact with plant roots and associated biofilms.³⁶

Climate

Studies have also shown that season and temperature can impact floating wetland performance, with nutrient removal efficiencies lower in the fall and winter and higher nutrient removal efficiencies observed in the spring and summer.^37,38

Pond Depth

The depth of the waterbody can affect floating wetland performance because, in shallow ponds, there is a risk of the plant roots attaching to the bottom of the pond. This attachment can prevent the floating wetland from fluctuating with changing water levels.³⁹ The size of the floating wetland in comparison to the surface area of the waterbody can also impact water quality improvement.³¹ A model by Marimon et al.⁴⁰ suggests that a floating wetland must cover 10% to 25% of the pond surface area to significantly improve water quality.

Plant Selection

Plant selection is another important factor in floating wetland design and performance. Wetland plants that can tolerate a continuously wet root system are the best candidates for floating wetlands.⁴¹ Native plants may offer the best support for local wildlife. If the waterbody is tidally influenced or brackish, plants must also tolerate varying salinity levels. Depending on the project design goals for the floating wetland, plants can be selected to

Figure 6. Ducks enjoying a floating wetland. Image credit: Sarah White, Clemson University.

1. Support habitat creation, such as for birds and pollinators. Turtles, frogs, and snails are commonly found enjoying floating wetlands, and the flowering plants can attract bumblebees, moths, and butterflies. Underwater plant roots provide shelter for juvenile fish and other aquatic creatures. Waterfowl, such as ducks, have been observed nesting on floating wetlands (figure 5).
2. Enhance the overall appearance of the waterbody. Consider choosing plants with unique leaves, different bloom seasons, and colors. The Clemson Home & Garden Information Center factsheet “Floating Wetlands: Container Gardens for your Pond” provides a list of South Carolina native plants that are suitable for floating wetlands in freshwater ponds.⁴²
3. Improve water quality. Plants can also be selected based on their nutrient uptake capabilities.^9,32 For example, nutrient uptake and assimilation by soft rush was higher than pickerelweed through both leaves and roots in a field-scale stormwater wet retention pond study.⁴³

Maintenance Considerations for Floating Wetlands

Floating wetlands typically do not require extensive maintenance and can be left on a waterbody to naturalize. However, if the pond or waterbody is in a more visible public space, periodic maintenance to remove weeds and dead plants will maintain the visual appeal of the wetland. Recommended maintenance includes cutting the crowns of plants at least once a year (preferably in late fall) to prevent dead and decaying plant material from falling into the water.⁴² Herbicides and fertilizers should not be applied to floating wetlands. Depending on where the wetland is installed, volunteer species may colonize the wetland. While volunteer species may not be the target species for overall wetland aesthetics, they can still contribute to increased nutrient removal efficiency since they are likely already well adapted to the nutrient conditions of the pond.⁴⁴ If the species becomes a nuisance or a non-native invasive species appears, then weeds should be pulled by hand. After several years of growth, the plants can be divided to create space on the mat and encourage new plant growth.⁴²

Harvesting the plants on a floating wetland can help increase the uptake and removal of nutrients from the waterbody.⁴⁴ Harvesting a floating wetland involves either removing only the above-mat plant material or removing the whole plant. Pruning and removing the plant material above the mat allows plants to reallocate nutrients to above-ground shoot growth, which results in additional nutrient uptake.⁹ In addition, removing old or dead plant material reduces the amount of material that can end up in the water with the potential to become sediment or release nutrients.⁹ Harvested plant material from the wetland can be used in many ways. If the whole plant is harvested, it can be replanted along the shoreline to increase the buffer. Plant material can also be composted and used as fertilizer, used as feed for cattle, or can be used in some craftwork.⁴⁵ You can learn more about harvesting a floating wetland by watching this short Clemson Extension video “An Introduction to Harvesting a Floating Wetland.”

The lifespan of a floating wetland depends on the materials used in construction and general maintenance and upkeep of the wetland. After a whole plant harvest from a floating wetland, synthetic mats can typically be reused for a new floating wetland installation. Some floating wetland mats are estimated to have a lifespan of over ten years.⁴⁶

References Cited

1. Stewart FM, Mulholland T, Cunningham AB, Kania BG, Osterlund MT. Floating islands as an alternative to constructed wetlands for treatment of excess nutrient from agricultural and municipal wastes – results of laboratory-scale tests. Land Contamination and Reclamation. 2008 Jan;16(1):25–33. doi:10.2462/09670513.874.
2. Beutel, MW. Inhibition of ammonia release from anoxic profundal sediments in lakes using hypolimnetic oxygenation. Ecological Engineering. 2006 Dec;28(3):271–279. doi:10.1016/j.ecoleng.2006.05.009.
3. Sawyer CN, McCarty PL, Parkin GF. Chemistry for environmental engineering science. 5th ed. New York (NY): McGraw-Hill Education; 2002.
4. Eckart K, McPhee Z, Bolisetti T. Performance and implementation of low impact development – a review. Science of the Total Environment. 2017 Jul;607-608:413–432. doi:10.1016/j.scitotenv.2017.06.254.
5. Dorman T, Frey M, Wright J, Wardynski B, Smith J, Tucker B, Riverson J, Teague A, Bishop K. San Antonio River Basin low impact development technical design guidance manual. 2^nd ed. San Antonio (TX): San Antonio River Authority; 2019. https://www.sariverauthority.org/sites/default/files/2019-08/SARB%20LID%20Technical%20Design%20Manual%202nd%20Edition.pdf.
6. Vymazal J, Brezinova T. The use of constructed wetlands from removal of pesticides from agricultural runoff and drainage: a review. Environment International. 2015 Feb;75:11–20. doi:10.1016/j.envint.2014.10.026.
7. Tanner CC, Headley TR. Components of floating emergent macrophyte treatment wetlands influencing removal of stormwater pollutants. Ecological Engineering. 2011 Jan;37(3):474–486. doi:10.1016/j.ecoleng.2010.12.012.
8. Afzal M, Arslan M, Müller JA, Shabir G, Islam E, Tasheen R, Anwar-ul-Haq M, Hasmat AJ, Iqbal S, Khan QM. Floating treatment wetlands as a suitable option for large-scale wastewater treatment. Nature Sustainability. 2019 Aug;2(9):863–871. doi:10.1038/s41893-019-0350-y.
9. Colares GS, Dell’Osbel N, Wiesel PG, Oliveira GA, Lemos PHZ, da Silva FP, Lutterbeck CA, Kist LT, Machado EL. Floating treatment wetlands: a review and bibliometric analysis. Science of the Total Environment. 2020 Apr;714:136776. doi:10.1016/j.scitotenv.2020.136776.
10. Dotro G, Langergraber G, Molle P, Nivala J, Puigagut J, Stein O, von Sperling M. Treatment wetlands. London (UK): IWA Publishing; 2017. Vol 7.
11. McAndrew B, Ahn C, Spooner J. Nitrogen and sediment capture of a floating treatment wetland on an urban stormwater retention pond – the case of the rain project. Sustainability. 2016 Sep;8(10):972. doi10.3390/su8100972.
12. Headley TR, Tanner CC. Constructed wetlands with floating emergent macrophytes: an innovative stormwater treatment technology. Critical Reviews in Environmental Science and Technology. 2011 Nov;42(21):2261-–2310. doi:10.1080/10643389.2011.574108.
13. Knight R, Clarke Jr. RA, Bastian R. Surface flow (SF) treatment wetlands as a habitat for wildlife and humans. Water Science & Technology. 2001 Feb;44(11–12):27–37.
14. Bisson PA, Davis GE. Production of juvenile chinook salmon, Oncorhynchus tshawytscha, in a heated model stream. NOAA Fishery Bulletin. 1976 Oct;74(4):763–774.
15. Hokanson KEF, Kleiner CF, Thorslund TW. Effects of constant temperatures and diel temperature fluctuations on specific growth and mortality rates and yield of juvenile rainbow trout, Salmo gairdneri. Journal of the Fisheries Board of Canada. 1977 May;34:639–648. doi:10.1139/f77-100.
16. Hill DT, Payton JD. Effect of plant fill ratio on water temperature in constructed wetlands. Bioresources Technology. 2000 Feb;71(3):283–289. doi:10.1016/S0960-8524(99)90071-8.
17. Guerrero J, Mahmoud A, Alam T, Chowdhury MA, Adetayo A, Ernest A, Jones KD. Water quality improvement and pollutant removal by two regional detention facilities with constructed wetlands in South Texas. Sustainability. 2020 Apr;12(7):2844–2865. doi:10.3390/su12072844.
18. Shilton AN. Pond treatment technology. London (UK): IWA Publishing; 2006. doi:10.2166/9781780402499.
19. Winston RJ, Hunt W, Kennedy SG, Merriman LS, Chandler J, Brown D. Evaluation of floating treatment wetlands as retrofits to existing stormwater retention ponds. Ecological Engineering. 2013 Mar;54:254–265. doi:10.1016/j.ecoleng.2013.01.023.
20. Sharma R, Vymazal J, Malaviya P. Application of floating treatment wetlands for stormwater runoff: a critical review of the recent developments with emphasis on heavy metals and nutrient removal. Science of the Total Environment. 2021 Jul;777:146044. doi:10.1016/j.scitotenv.2021.146044.
21. Shahid MJ, Arslan M, Ali S, Siddique M, Afzal M. Floating wetlands: a sustainable tool for wastewater treatment. Clean – Soil Air Water. 2018 Aug;46(10):1800120. doi:10.1002/clen.201800120.
22. Tanner CC, Sukias J, Park J, Yates C, Headley TR. Floating treatment wetlands: a new tool for nutrient management in lakes and waterways. Methods. 2011 Feb;2008(2011):1–3.
23. Headley TR, Tanner CC. Floating vegetated islands for stormwater treatment removal of copper, zinc, and fine particulates. ARC Technical Report 2008-030. 2007.
24. Sample DJ, Wang C, Fox LJ. Innovative best management fact sheet no. 1: floating treatment wetlands. Blacksburg (VA): Virginia Cooperative Extension; 2013. Publication BSE-76P. http://hdl.handle.net/10919/70627.
25. Wang C, Sample DJ. Assessment of the nutrient removal effectiveness of floating wetlands applied to urban retention ponds. Journal of Environmental Management. 2014 May;137:23–35. doi:10.1016/j.jenvman.2014.02.008.
26. Chang N, Xuan Z, Marimon, Z, Islam K, Wanielista. Exploring hydrobiogeochemical processes of floating treatment wetlands in a subtropical stormwater wet detention pond. Ecological Engineering. 2013 Feb;(54):66–76. doi:10.1016/j.ecoleng.2013.01.019.
27. Tharp R, Westhelle K, Hurley S. Macrophyte performance in floating treatment wetlands on a suburban stormwater pond: implications for cold climate conditions. Ecological Engineering. 2019 Oct;136:152–159. doi:10.1016/j.ecoleng.2019.06.011.
28. White SA, Cousins MM. Floating treatment wetland aided remediation of nitrogen and phosphorus from simulated stormwater runoff. Ecological Engineering. 2013 Oct;61:207–215. doi:10.1016/j.ecoleng.2013.09.020.
29. Lynch J, Fox LJ, Owen JSJ, Sample DJ. Evaluation of commercial floating treatment wetland technologies for nutrient remediation of stormwater. Ecological Engineering. 2015 Feb;75:61–69. doi:10.1016/j.ecoleng.2014.11.001.
30. Hubbard RK, Gascho GJ, Newton GL. Use of floating vegetation to remove nutrients from swine lagoon wastewater. Transactions of the American Society of Agricultural and Biological Engineers. 2004;47(6):1963–1972. doi:10.13031/2013.17809.
31. Sooknah RD, Wilkie AC. Nutrient removal by floating aquatic macrophytes cultured in anaerobically digested flushed dairy manure wastewater. Ecological Engineering. 2004 Feb;22(1):27–42. doi:10.1016/j.ecoleng.2004.01.004.
32. Spangler JT, Sample DJ, Fox LJ, Albano JP, White SA. Assessing nitrogen and phosphorus removal potential of five plant species in floating treatment wetlands receiving simulated nursery runoff. Environmental Science and Pollution Research. 2019 Jan;26:5751–5768. doi:10.1007/s11356-018-3964-0.
33. Lopardo CR, Zhang L, Mitsch WJ, Urakawa H. Comparison of nutrient retention efficiency between vertical-flow and floating treatment wetland mesocosms with and without biodegradable plastic. Ecological Engineering. 2019 Mar;131:120–130. doi:10.1016/j.ecoleng.2019.01.024.
34. Hubbard RK, Gascho GJ, Newton GL, Rute JM, Wilson JP. Plant growth and elemental uptake by floating vegetation on a single-stage swine wastewater lagoon. Transaction of the American Society of Agricultural and Biological Engineers. 2001 May;54(3):837–845. doi:10.13031/2013.37108.
35. Billore SK, Prashant, Sharma JK. Treatment performance of artificial floating reed beds in an experimental mesocosm to improve the water quality of river Kshipra. Water Science & Technology. 2009 Dec;60(11):2851–2859. doi:10.2166/wst.2009.731.
36. Chen Z, Cuervo DP, Muller JA, Wiessner A, Koser H, Vyzmazal J, et al. Hydroponic root mats for wastewater treatment – a review. Environmental Science and Pollution Research. 2016;23(16):15911–15928.
37. Wang C, Sample D, Bell C. Vegetation effects on floating treatment wetland nutrient removal and harvesting strategies in urban stormwater ponds. Science of the Total Environment. 2014 Sep;499:384–393. doi:10.1016/j.scitotenv.2014.08.063.
38. Garbett P. An investigation into the application of floating reedbed and barley straw techniques for the remediation of eutrophic waters. Journal of the Chartered Institution Water and Environmental Management. 2005 Nov;19(3):174–180. doi:10.1111/j.1747-6593.2005.tb01584.x.
39. Nakamura K, Shimatani Y. Water purification and environmental enhancement by the floating wetland. Public Works Research Institute. 1997 Jan;888–895.
40. Marimon ZA, Xuan Z, Chang N. System dynamics modeling with sensitivity analysis for floating treatment wetlands in a stormwater wet pond. Ecological Modelling. 2013 Oct;267:66–79. doi:10.1016/j.ecolmodel.2013.07.017.
41. Borne KE, Fassman-Beck EA, Winston RJ, Hunt WF, Tanner CC. Implementation and maintenance of floating treatment wetlands for urban stormwater management. Journal of Environmental Engineering. 2015 Nov;141(11): 04015030.
42. Powell B. Floating wetlands: container gardens for your pond factsheet. Clemson (SC): Clemson Cooperative Extension; 2012 Sep. HGIC 1857. https://hgic.clemson.edu/factsheet/floating-wetlands-container-gardens-for-your-pond/.
43. Wang C, Sample DJ, Day SD, Grizzard TJ. Floating treatment wetland nutrient removal through vegetation harvest and observations from a field study. Ecological Engineering. 2015 May;78:15–26. doi:10.1016/j.ecoleng.2014.05.018.
44. White SA, Garcia Chance LM, Bell NL, Chase ME. Potential and problems of floating treatment wetlands for mitigating agricultural contaminants. Wetland Science & Practice. 2019 Apr;36(2):51–56.
45. Olguin EJ, Sanchez-Galvan G, Melo FJ, Hernandez VJ, Gonzalez-Portela RE. Long-term assessment at field scale of floating treatment wetlands for improvement of water quality and provision of ecosystem services in a eutrophic urban pond. Science of the Total Environment. 2017;584(585):561–571.
46. Durability through biomimicry. Shepherd (MT): Floating Island International; 2013 [accessed 2023 Aug 25]. https://www.floatingislandinternational.com/durability-through-biomimicry.html.

Additional Resources

Scaroni AE, Sahoo D, Wallover CG. An Introduction to Stormwater Ponds in South Carolina. Clemson (SC): Clemson Cooperative Extension, Land-Grant Press by Clemson Extension; 2021 Aug. LGP 1119. https://lgpress.clemson.edu/publication/an-introduction-to-stormwater-ponds-in-south-carolina/.

Nix HB, Lunt S, Davis RH. Pond Weeds: Causes, Prevention, and Treatment Options. Clemson (SC): Clemson Cooperative Extension, Land-Grant Press by Clemson Extension; 2021 Dec. LGP 1126. https://lgpress.clemson.edu/publication/pond-weeds-causes-prevention-and-treatment-options/.