Science 7–10 · Year 7

Duckweed population growth: exponential start, limited finish

Science understanding: Biological sciences

Practical, model not builtLow risk

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The idea

A population with plenty of resources grows by a constant proportion each day, and growth slows as space or nutrients run out.

What you need

Duckweed (Lemna) from a pond or an aquarium supplier, enough for 10 fronds in each of six containers (60 fronds)
Clear containers holding 200 mL of pond water or tap water left to stand 24 h, three per treatment and so six for the two treatments; dilute liquid plant food for the nutrient treatment as recommended on the SAPS duckweed growing guidance
A bright windowsill or a lamp on a timer (12 h light)
Labels, a hand lens and a counting sheet

How to do it

Place exactly 10 fronds in each container. Set up at least two treatments (for example plain water and water with nutrient; or full light and shade) with three containers each.
Count fronds every second day for three weeks, counting a new frond once it is clearly separate. Top up evaporated water.
Plot frond number against time for each container, then plot the natural logarithm of frond number against time.
From the straight early part of the log plot read the slope r (per day); doubling time = 0.693 divided by r.
Note the day on which growth in each container slows and estimate the frond number at which it levels off.
Compare r and the levelling-off number between treatments.

What you should see

Under good light with nutrient, frond counts rise by a roughly constant proportion at first, then level off as the surface fills. If a container doubles every 3.0 days its r is 0.231 per day; from 10 fronds an unlimited population would reach 101 fronds on day 10, whereas a logistic population that levels off at 200 fronds reaches 69 on day 10 and 168 on day 20. The learner's own counts give their own r and ceiling; the practical worked when the log plot is straight early and bends later, and the treatments can then be compared by their r values and levelling-off numbers.

What changes

What you change: Nutrient level, or light level
What you measure: Frond count over time; growth rate r; final frond number
What you keep the same: Starting fronds (10)
Water volume (200 mL)
Container size
Temperature
Light period

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Populations grow by the same number each day.
A population keeps growing until it uses everything up and crashes; growth slows before that in a resource-limited container.
Slower growth late in the run means the plants are dying.

Safety card

Low riskLearners carry it out

Hazards

Pond water pathogens

Controls

Wash hands after counting; no mouth contact
Dispose of duckweed in the bin, not in a waterway

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-LIV-01 SC4-WS-05 SC4-WS-06
Australian Curriculum v9AC9S7U02AC9S7I04AC9S7I05

Sources

The pages the author read to write this activity.

All Concept Studio activities