Investigations with duckweed (Lemna minor)
Estimating the number and biomass of a population of plants
(from Osmosis 20, Spring 2001)

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. You can grow them quite simply in water in plastic cups, where they grow and reproduce vegetatively on the surface (see OSMOSIS 5 & 6). 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 following technique shows how you can estimate the number of plants and the biomass of a duckweed population. For equipment, you need a plastic tea strainer, a white plastic tray*, wax pencil, tape measure and balance. Then follow the instructions below.

1. Measure surface area of tea strainer and your aquarium or pond. Decide from where and how many samples you will take

2. Lower the tea strainer carefully into pond, allowing duckweed to distribute itself evenly above it

3. Raise the tea strainer and allow water to drain. Remove the duckweed attached to the outside of it

4. Scrape plants from interior of tea strainer into weighing boat. Blot dry before weighing fresh mass

5. Mark the base of a shallow plastic tray with a wax pencil to create a grid of squares 5 cm x 5 cm. Flood tray with at least 1 cm depth of tap water.

6. Empty contents of tea strainer into tray and spread the duckweed out evenly. Count the number of plants over one or more squares and then calculate total number in tray

* White flat rectangular trays (such as those available from garden centres as window-box holders) make excellent trays for the counting process. The procedure is essentially a form of quadrat-sampling, using the grid marked on the base. As an alternative technique, you could trap the plants under a small inverted plastic or glass dish, such as a petri dish.

 

Some hints to help you with your practical work

SAFETY NOTE - When collecting Lemna from the pond, you are advised to wear rubber gloves and wash your hands thoroughly afterwards. Don't fall in the pond!

• If the duckweed does not form a continuous layer across the whole pond, then use a string or rope as a dragnet to collect them into one area for sampling. If you don’t do this, it may be necessary to make a subjective estimate of the percentage area of the pond covered by the duckweed.
  
  
• The layer of duckweed will be disturbed by the entry of the tea strainer to the water. Make sure you allow sufficient time for the plants to become re-distributed, before you raise the tea strainer to collect the sample.
  
  
• Lots of plants will stick to the outside of the tea strainer, but you must discard these, or your results will be misleading. Use a paintbrush if you don't like touching the plants.
  
  
• If more than one sample is to be taken with the same tea strainer, you should rinse off the plants from the first sample into a jam jar (or beaker) of water.
  
  
• The plants are wet, so before measuring the fresh mass, it is essential to blot the contents of the weighing boat. Do this carefully with tissue paper. You also need to be careful to avoid plants sticking to the tissue – and thus being removed from the weighing boat.

Doing the calculations (and some sample class results)

4. Total biomass of duckweed on pond = P x M g (if you give the mass in grams)

Look at this example of some class results – how many millions of Lemna plants and how many kg of biomass do you think there were in the pond? Have a guess before you start the calculation! The tray measured 25 cm x 35 cm, and 129 plants were counted in an area of 60 cm2, covered by an inverted petri dish. The area of the pond was 3 100 000 cm2. The tea strainer used for sampling had a diameter of 7 cm, and the fresh mass of the sample in the tea strainer sample was 5.5 g. Now calculate the biomass and numbers of duckweed per unit area of the pond (e.g. per m2). Answer given at the foot of this page.

Some further suggestions for Lemna minor investigations

Pollutants (such as copper ions) in the pond water are known to affect the growth and biochemistry of duckweed. The inhibitory effects on growth are reflected in a reduced concentration of photosynthetic pigments (such as chlorophyll) and an increase in activity of antioxidant enzymes (such catalase and peroxidase), within the leaves. Responses to substances such as copper sulphate in the water are measurable within as little as 24 hours (peroxidase), and effects on population growth of fronds will be detected within a week. Suggestions for measuring the rate of growth of individual duckweed plants are given by Tomkins (OSMOSIS 5). Methods for quantitative measurements of photosynthetic pigments (such as chlorophylls and carotenoids), and for secondary compounds such as anthocyanins, can be found on OSMOWEB. A simple colorimetric assay for peroxidase is given on OSMOWEB. Using these methods, you could carry out investigations into enzyme levels (such as the peroxidase enzyme), pigments or growth rate. Possible investigations include:-

• The effects of different concentrations of pollutants (such as copper sulphate or detergents)
  
  
• The effects of eutrophication, simulated by adding different concentrations of fertilisers to the water
  
  
• The relationship between enzyme levels, pigments and growth rate during cultivation of duckweed
  
  
• The effects of disturbance (water movement) on growth

Roger Delpech
Haberdasher's Askes' School

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Answers to calculation:

Estimated total number of plants = 1.52 x 10⁸ (approx)

Estimated biomass = 443 kg (approx)

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