Duckweeds look like simple plants: small, green, and with no flowers. But this simplicity makes them ideal experimental organisms for labs, and they are widely used to assess the toxicity of substances that can get into the ponds and rivers where they live.

Aquatic toxicity tests are used around the world to determine and monitor the toxic effects of substances that might be be harmful to aquatic life in the environment. The results of toxicity tests can be used to determine the need for control of discharges, to set effluent standards, for research, and for other purposes.

You might choose to do a research project focusing on the ways in which watercourses can be polluted as a result of human activity, how this can be measured, and what steps can be taken to prevent this happening.

Various organisations, including Greenpeace, Friends of the Earth and Oxfam, have made high profile statements about the level of contaminants in water sources around the world. To what extent do you agree with their claims? What steps would governments and businesses need to take to address the problems they raise?

Using duckweeds, you can carry out a range of practical investigations of environmental topics, including the potential effects of pollution, the toxicity of specific chemicals in the water, competition, and many more.

Get inside the science

The duckweeds (Family Lemnacea) are a small and cosmopolitan group, found from the sub-polar regions to the tropics. They all prefer ditches, ponds, lakes and slow-flowing rivers. There are, surprisingly, only 22 species in the world and most are very widespread. One can imagine them jet-setting from one continent to another on the feet of birds. All have a few flattened, often rounded, disc-like leaves (they are really leaf-like stems or thalli) floating on the surface. New leaves bud off from these as the plant grows. Once there are half a dozen leaves the plants break apart. Often there is an unbranched rootlet hanging below the leaves. Some of these rootlets are photosynthetic.

The table below describes some duckweeds you may find in your locality, identified by their individual leaf size, leaf shape and rootlet characteristics.

There are at least 5 different species in Britain. Lemna minor is undoubtedly the most often encountered, but the smaller Lemna minuta has recently been spreading rapidly in Britain. Spirodela polyrhiza is less common but much the largest and most elegant.

Species	Leaf size	Leaf character	Rootlets
The great duckweed, Spirodela polyrhiza	large, 5 – 8 mm	shiny and circular, floating	many up to 30 mm long
The lesser duckweed Lemna minor	small, 1.5 – 4 mm	opaque and more or less elliptical, floating	single to 15 mm long
The least duckweed Lemna minuta	very small, 1 – 2.5 mm	elliptical, floating	single to 8 mm
The ivy-leaf duckweed Lemna trisulca	elongated, 5 to 15 mm	spear-shaped with opposite branches, submerged and translucent	single and hooked
The gibbous duckweed Lemna gibba	very small, 3 – 5 mm	ovate, convex below, fat and bouyant	single to 6 mm

Find out more

• Polluted legacy: Repairing Britain’s damaged landscapes, (BBC News, 26 June 2012). This article from the BBC introduces the long-standing legacy of industrial pollution across the UK, and how the Industrial Revolution of the 19th century is still effecting our rivers and lakes.
• OECD Guidelines for the testing of chemicals, section 2: effects on biotic systems, This is a Test Guideline from the Organisation for Economic Cooperation and Development (OECD), designed to assess the toxicity of substances to Lemna. It outlines how professional ecologists would go about using duckweeds to assess the potential effects of chemicals that might get into watercourses.
• Biological Test Method: Test for Measuring the Inhibition of Growth Using the Freshwater Macrophyte, Lemna minor. This protocol is from the Canadian Government’s Environment Canada department.
• Toxicity of single and combined herbicides on PSII maximum efficiency of an aquatic higher plant, Lemna sp., (Toxicology and Environmental Health Sciences
  June 2011, Volume 3, Issue 2, pp 97-105). This article is an example of how environmental researchers can use duckweed to investigate the effects of herbicides on the environment.
• The most polluted rivers and streams in Europe, (New Scientist, 16 June 2014). This article reports on a research article reporting water quality data from over 4000 monitoring sites across Europe.

Practical Investigations – step-by-step protocols

We’ve put together step-by-step protocols for you to use for a practical investigation on this topic.