Student Sheet 21 – Measuring the biomass of duckweed (Lemna minor)

As you gaze across a pond covered in duckweed, have you ever wondered just how many plants there are . . . or how many kilograms of biomass this represents?

Duckweeds are robust, tiny flowering plants. They frequently form an unbroken layer on the surface of bodies of still water, such as aquaria, ponds, ditches and lakes.

In the classroom, these plants are very useful for investigations on population growth and the increase of biomass. You can grow them quite simply in water in plastic cups, where they grow and reproduce vegetatively on the surface (see OSMOSIS 5 & 6 in the Library section of the SAPS site). In this practical, you can either collect duckweed from the surface of a pond, or from an aquarium in the lab.

Beyond the classroom, duckweeds are used for the bioremediation of fresh water – they can lower the concentration of toxic ions in the water, incorporating them into their tiny bodies.

So, despite their rather insignificant appearance, duckweeds are really quite versatile and useful little plants.

The technique in the attached resources shows how you can estimate the number of plants and the biomass of a duckweed population in a pond. This can be repeated over the course of several days, allowing the increase in biomass to be measured.