Floating gardens have long been a staple of agriculture for many waterside communities that lack access to enough land. Yet these waterborne gardens can have benefits beyond providing food security. They can also improve water quality, scientists say.

Floating gardens usually involve rafts that allow certain plants to be grown hydroponically with their roots extending into the water. These gardens are used either for growing vegetables or for decorative purposes, or both.

Yet the environmental effects of these gardens are not always well understood. Research has already shown that floating gardens can improve water quality over time in bodies of water such as wastewater treatment ponds. But what happens in moving water such as rivers?

A team of researchers at Illinois State University’s Department of Geology, Geography and the Environment in Chicago set out to find that out by examining the impacts of one small floating garden on the quality of river water.

They did so by taking samples of water on a weekly basis at the surface of the water and 30cm deep both upstream and downstream of a 3m-by-50m floating garden installed along a shoreline of the Chicago River at the edge of the city’s urban center.

Local water quality is affected by agriculture upstream and the scientists wanted to find out how the plants in the floating garden affected the nutrient content of water, including nitrogen, chloride, sulfate, and phosphate.

Over time they documented “modest but definitive improvement” in water quality despite the small size of the water garden. “For example, nitrate as nitrogen dropped from 4.69 milligrams per liter in surface water just upstream of the garden to 4.43 milligrams per liter just downstream, a drop of about 1 percent. Phosphate was also lower downstream of the garden,” they explain in a statement.

“Despite how small this garden was there was measurable improvement in water quality from upstream to downstream, especially for nitrates,” stresses Abigail Heath, a scientist at the university who led the research.

“Even this tiny garden makes a difference,” Heath says, adding that similar gardens can be installed to improve water quality in bodies of water.

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