Just five years ago, Fish Fry Lake was dying. The groundwater flowing into the lake situated 30 miles northeast of Billings, Montana, contained high levels of nitrogen and phosphorous, common ingredients in agricultural fertilisers and animal waste. The nitrogen and phosphorus had fostered an overgrowth of algae, which covered the lake and blocked sunlight from penetrating the surface. The deep water was a dead zone, devoid of oxygen and home to very little aquatic life.

The solution was as simple as it was improbable: cover rafts with plants, and set them afloat in the lake. Within a year-and-a-half, the algal blooms were gone. Water clarity improved. Oxygen levels rose. Today, the lake is home to a thriving community of fish, including black crappie, yellow perch and Yellowstone cutthroat trout.

The story of Fish Fry Lake demonstrates the power of mimicking wetlands to clean up dirty waterways. Wetlands are sometimes called nature’s own water purifiers: as dirty water moves through a sprawling marsh, the bacteria that cling to wetland plants, timber, rocks, and other debris consume and process some common water pollutants. Other contaminants get trapped in the mud and muck. As result of these and other processes, the water that eventually flows out of a wetland is much cleaner than the stream that came trickling in.

By creating floating treatment wetlands out of small, human-engineered rafts of vegetation, researchers and entrepreneurs hope to provide these same ecological services to small, polluted bodies of water that may be far from a natural marsh. “BioHaven floating islands are concentrated wetland systems that are essentially biomimicking nature’s wetland effect,” says Bruce Kania, the founder and research director of Floating Island International, the company behind the Fish Fry Lake rafts.

Cleansing power

To construct a BioHaven island, the company starts with layers of mesh made from recycled plastic. They assemble this mesh into a floating raft – which can be as small as a home aquarium or nearly as large as a football field – and top it with soil and plants. They launch the island into a lake, pond, stream, or lagoon, anchoring it in place. Over time, the plants’ roots grow into and through the raft’s porous matrix, descending into the water below. At the same time, bacteria colonise the island, assembling into sticky, slimy sheets called biofilm that coat the floating matrix and the suspended plant roots.

This bacterial biofilm is the secret to a floating island’s cleansing power. Overgrowth of algae from nitrogen and phosphorus pollution can cause several problems, preventing sunlight from reaching subaquatic plants and starving a body of water of the oxygen needed to sustain fish populations and other animal life. A dead zone, like the one is Fish Fry Lake, is often the ultimate result. The biofilm bacteria consume nitrogen and phosphorous, however, and as polluted water flows through and around a floating island, the bacteria converts these contaminants into less harmful substances. Though the bacteria do the brunt of the work, the plant roots suspended from the floating island also play their part, absorbing some of the nitrogen and phosphorous through their roots.

In Fish Fry Lake, for instance, Floating Island International deployed several islands, which together covered almost 2% of the lake’s 6.5-acre (2.6-hectare) surface area. Over the course of four years, the islands helped reduce nitrogen concentrations by 95% and phosphorus concentrations by nearly 40%. Today, levels of dissolved oxygen are sixty times what they once were.

Clearer, cleaner, healthier

The system also mechanically filters out other pollutants, like metals and particulates. “The sticky biofilm essentially keeps the water clear because all the suspended solids tend to bond to it,” says Kania. Floating Island International, which has deployed more than 4,400 of their artificial wetland systems worldwide, has documented this effect in multiple case studies. For example, the concentrations of suspended solids, copper, lead, zinc, and oil and grease fell dramatically after a floating island was installed in a stormwater pond in Montana. Controlled laboratory studies and research by scientists not affiliated with the company have also found that floating treatment wetlands can reduce the levels of many common water pollutants.

Some scientists are now exploring how to optimise the design of floating islands – probing, for instance, which plants do the best job of removing pollutants. Gary Burtle, an aquaculture specialist at the University of Georgia, thinks we can get even more out of these artificial wetlands by seeding the rafts with plants that are of commercial value, such as lettuces and herbs. Burtle is screening a number of potential plant candidates – if he finds one that grows well on a floating island, we may soon see constructed wetland systems that “give us a little bit more return”, he says, producing saleable crops while purifying the water.

Meanwhile, the removal of contaminants not only improves the water itself, but also helps to foster a healthier ecosystem. Clearer water allows light to penetrate deeper, encouraging the growth of various aquatic plants, which produce oxygen and become part of the food chain, supporting larger populations of fish and other animals. “You end up with a waterway that can be abundant,” Kania says, “that can be verdant even at depth.” The organic debris that attaches itself to the underside of a floating island also becomes a source of food for fish and other aquatic organisms, and the island itself provides new habitat for birds.

“The concept of how to get back to a healthy waterway,” Kania says, “is very simple: nature’s wetland effect.” All we have to do is simulate it.

If you would like to comment on this article or anything else you have seen on Future, head over to our Facebook page or message us on Twitter.