The short version
Floating wetlands (also called floating treatment wetlands or ecological floating beds) are rafts planted with wetland plants whose roots hang down into the water, where the roots and the microbes living on them take up nitrogen and phosphorus. Published results range from strong to weak, so three researchers in Beijing pooled 169 studies to find out which designs and conditions work best. The rafts generally lowered nitrogen and phosphorus in nutrient-rich water whatever the design. They did best with two or three kinds of plants, with a fifth to two-fifths of the water covered, in water at 16 to 25 °C (about 61 to 77 °F), in tests lasting one to two months, and where water stayed in place a long time. We read the published abstract and the authors’ earlier preprint, which may differ in detail from the final article.
What the researchers did
The team gathered earlier floating-bed experiments and combined their results statistically (a meta-analysis).
- They searched Web of Science and CNKI, China’s main research database, for papers published before June 2022, and found 320.
- They kept the 169 papers (116 in Chinese, 53 in English) that compared water with a floating bed against water without one, and pulled out 4,745 measurements.
- They compared how much total nitrogen, ammonium, nitrate, total phosphorus and phosphate each bed removed, sorted by plant choice, raft size, how much of the water it covered, planting density, water temperature, how long water stayed in the system (retention time), test length and added aeration.
What they found
- Floating beds generally work. Water with a bed ended up with less nitrogen and phosphorus than water without one, whatever the design or test conditions (Jia et al., 2024).
- Two or three kinds of plants. Beds with two or three plant species removed more than beds with one or with many. Grass-like plants with fibrous roots (monocots, such as irises and cannas) removed more nitrogen.
- Mid-sized rafts, partial cover. Beds of 1.1 to 3.0 square meters (about 12 to 32 square feet) covering 21% to 40% of the water did best. In the preprint, the authors note that beds this size were mostly small outdoor tests, while larger beds in the field did less well, possibly because faster-moving water gave the roots less time to catch nutrients.
- 16 to 25 °C works best. Removal was strongest in water at 16 to 25 °C (about 61 to 77 °F) and weaker in cooler or warmer water. The authors link this to plants and microbes being most active in that range.
- One to two months, and time to soak. Tests of 31 to 60 days showed the biggest effect: in shorter tests the plants had not grown much, and in longer ones they began to age. The longer water stayed in the system, the more nitrogen came out. In the preprint, total nitrogen was about two-thirds lower than in water without a bed when water stayed more than eight days.
- Nitrate is harder. Results for nitrate were mixed, because microbes around the roots both make it and remove it. In the preprint, bubbling air into the water raised nitrate levels.
What it means for Waquoit Bay today
At CPWB’s 2021 Annual Meeting, John Todd proposed floating wetlands for Waquoit Bay and put a system at about $20 million (see our recap of his talk). This analysis is a useful check on that idea. Floating beds clearly take nutrients out of the water, and they do it best in water between 16 and 25 °C, in small setups, over a month or two, where water stays put for days.
Waquoit Bay is colder than that range for roughly half the year, from fall into spring, and its tides move water in and out every day. So the strong results here can’t simply be scaled up to a bay-wide system. A small local test, like the floating wetland pilot on the Moonakis River that CPWB’s Wetlanders committee proposed (see the May 2022 newsletter), would show how much a raft removes in our own water. Another floating approach has already been measured close to home: oyster gear in a Cape Cod salt pond (see floating oyster farms and nitrogen).
Read the paper
Jia, D., Wei, S., & Wang, S. (2024). Meta-analysis revealed the factors affecting the functions of ecological floating bed in removing nitrogen and phosphorus from eutrophic water. Environmental Science and Pollution Research, 31(50), 59712–59726. https://doi.org/10.1007/s11356-024-35241-2
License: © 2024 The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature. The article is not openly licensed, so we link to the publisher’s page rather than host a copy. This summary is based on the published abstract and the authors’ earlier preprint on Research Square, which may differ in detail from the final article. The card image is a title card made from the citation.
Drafted with AI assistance from the linked source.
