Four Cups of Duckweed: Which Resource Runs Out First?

Investigation question: When a real duckweed population gets light but no added nutrients, nutrients but no light, or everything it needs but a crowded start, how does it change over two weeks, and which resource limits each cup?

Scientific purpose

Students census a real population. Each group floats common duckweed, a tiny plant that multiplies by budding off new fronds, in four identical cups. Cup B gets nutrient water and bright light; each of the other cups differs from B in exactly one way: no added nutrients (A), no light (C), or four times as many fronds at the start (D). Over two weeks students count fronds, score their color and estimate how much of the surface is covered, then calculate growth factors, pool the class data, and name the resource that limited each cup. The lab gives the chapter its evidence from living organisms, both for population size and for the condition of individual plants.

Why this helps students with this chapter

Students have been told that populations need resources; here they watch a population respond to them. Duckweed is ideal because it doubles in days, floats where it can be counted, and each frond is an individual. Comparing every cup with one full-resource cup makes each comparison a fair test of one resource, and the crowded cup shows something the textbook only asserts: individuals of the same species compete with each other, so more plants means less for each. Because real data are messy, pooling eight groups and using the median teaches students how scientists separate a real effect from one cup's bad luck.

Standards connections

MS-LS2-1 (assesses, the whole PE: analyzing data on how light, nutrient and space availability affect a population and the individual organisms in it): Students collect and analyze their own counts of a living population under abundant and scarce resources, compute growth factors, compare against a full-resource cup and pooled class medians, and use the data as evidence for the effect of each resource on population size and on frond health. That is the PE as written.

MS-LS1-5 (supports, environmental factors (light, nutrients, crowding) influencing the growth of individual plants): Frond color and size in the dark cup and the unfed cup show that resources change how individual plants grow, which is one of this PE's own examples (fertilizer increasing plant growth). In the California integrated sequence MS-LS1-5 is a Grade 6 PE, so it appears here as background students should already have met, not as a target.

Roles (rotate each session)

• Cup keeper: Sets up and labels the cups, measures water to the line, tops up evaporation on count days, and reads the thermometers in the light and inside the dark box.
• Counter 1 (cups A and C): Counts fronds with a toothpick and hand lens using the counting rule, and gives the color score and surface estimate for those two cups. Works fast on cup C so it spends as little time in the light as possible.
• Counter 2 (cups B and D): Counts the two fastest-growing cups, counting cup D one quarter at a time on the '+' paper, and gives their color scores and surface estimates.
• Recorder and checker: Fills the tables, recounts any cup whose number looks wrong, photographs the cups from above if a camera is allowed, and computes the growth factors.

Materials

Estimated cost: 12 to 30 per class without a grow light; add about 15 to 25 for a clip-on LED grow light (estimates) (an estimate; A starter portion of duckweed costs about $5 to $15 and some aquarium shops give it away; a bottle of liquid houseplant fertilizer is about $5 and is often already on hand; a sleeve of 50 clear cups is about $4 and a roll of plastic wrap about $2. Boxes, toothpicks, hand lenses, thermometers and measuring cups are classroom stock or donated.)

Safety

• Fertilizer concentrate irritates eyes and skin and is harmful if swallowed. Only the teacher handles it, wearing goggles and gloves. The diluted nutrient water is still not for drinking; if it splashes in an eye, rinse with running water for several minutes and tell the teacher.
• Water that stands for two weeks grows bacteria and mold. Wash hands with soap after touching the cups, the water or the fronds. No food or drink at the lab table. Students with mold allergies can count from photographs instead of handling the cups at the end.
• Keep cups and water well away from the lamp, its cord, outlets and power strips. Dry your hands before touching the lamp switch.
• Spilled water is a slip hazard; wipe it up at once.
• Never pour duckweed down a drain or release it into a pond, creek or storm drain. It spreads quickly, and aquarium stock can carry snails, other plants or diseases into local water. Bag it and put it in the trash, or compost it at school.
• Cover each cup with the punched plastic wrap to keep out dust and insects; mosquitoes can breed in uncovered standing water.

Teacher preparation and setup

1. One to two weeks ahead, float the starter duckweed in a shallow tub of nutrient water in the bright spot so it multiplies to about 600 healthy fronds. Remove any brown or white fronds before class.
2. Two days ahead, fill an open jug with tap water and let it stand. If your district treats its water with chloramine, which does not leave standing water, use bottled spring water instead for everything.
3. The day before, mix the nutrient water. Wear goggles and gloves while you handle the concentrate. One quarter of the label's houseplant strength is enough; stronger mixes encourage algae and can burn the fronds.
4. Check the growing spot: small cups in direct afternoon sun can heat well above room temperature. Put a thermometer in a test cup there for an afternoon; if it rises above about 30 °C, move the cups back from the glass or use the lamp.
5. Plan the top-ups. Water evaporates over two weeks. On each count day, and once midweek if you can, top every cup back to its 200 mL line with plain aged water (never with nutrient water, which would add extra nutrients).
6. Practice the counting rule on the starter culture so you can show it under a document camera: a frond counts once its outline sticks out past the edge of its parent.

Investigation design

Student procedure

1. Session 1, day 0. Label four cups A, B, C and D. Measure 200 mL into each and draw the waterline with a marker: cup A gets aged tap water only; cups B, C and D get nutrient water.
2. Using a toothpick, lift healthy green fronds from the starter tub and float 10 fronds in cup A, 10 in cup B, 10 in cup C and 40 in cup D. Fronds often come joined in little clusters; count every frond in a cluster using the counting rule.
3. Record the day-0 count, color score and surface covered for each cup and the starting temperature. Then answer q1 and q2: predict how the cups will rank by growth and which resource might run short in each.
4. Cover each cup with punched plastic wrap. Set cups A, B and D side by side in the bright spot. Put cup C inside its box with the lid closed, right next to the others so it stays at the same temperature. Put one thermometer beside the cups and one inside the box.
5. Session 2, day 7. Read and record both thermometers. Count every cup (cup D one quarter at a time on the '+' paper), score color, estimate surface covered, and note roots, algae or anything unusual. Count cup C last and quickly, then return it to its box. Top every cup up to its line with plain aged water. If a camera is allowed, photograph each cup from directly above.
6. Session 3, day 14. Repeat every count and observation. Compute each cup's growth factor (day-14 count ÷ day-0 count), and for cups B and D the two weekly factors (day 7 ÷ day 0 and day 14 ÷ day 7).
7. Graph the number of fronds against day for all four cups on one set of axes, one color per cup, and draw a top view of each cup in the drawing box.
8. Add your four growth factors to the class table. When every group has reported, find the class median for each cup (list the values in order and take the middle one, or the average of the two middle ones).
9. Answer the analysis questions on your own.
10. Clean up: lift the fronds out with a spoon or pour each cup through a paper towel so no fronds reach the drain, bag the fronds for the trash or compost, pour the water into a sink, rinse the cups and wash your hands.

Expected observations

• These are expected ranges, not tested results, and depend on light and temperature. Cup B usually grows the most: about 4 to 10 times in two weeks (10 fronds to roughly 40 to 100), with deep green fronds.
• Cup A grows less, often 1.5 to 3 times (10 fronds to roughly 15 to 30), because fronds use nutrients they stored before the lab; by day 14 its fronds are often smaller and paler, and their roots are often longer than in cup B.
• Cup C, in the dark, barely changes or shrinks (roughly 8 to 13 fronds) and turns yellow or white over the two weeks.
• Cup D gains the most fronds but usually has a smaller growth factor than cup B (40 fronds to roughly 100 to 250, a factor of about 2.5 to 6), and its week-2 factor is often lower than its week-1 factor as the surface fills. In a cool or dim room, B and D may grow at nearly the same rate.
• Some green algae may cloud cups B and D, which share the nutrients and light with the duckweed; note it as another competitor rather than ignoring it.

Likely difficulties

• Counting is the biggest source of error. Fronds hide under each other and tiny daughter fronds are easy to miss. Insist on the counting rule, have the checker recount any number that seems wrong, and count crowded cups by quarters or from a photo.
• Groups top up with nutrient water by mistake, which adds extra nutrients. Keep the plain water jug and the nutrient jug in different places with large labels.
• Cup C gets left on a windowsill 'for a minute' and is forgotten. Count it last, quickly, and return it to the box before writing the numbers down.
• Students compare raw counts instead of growth factors and conclude that cup D 'grew best' because it has the most fronds. Ask what cup D started with and why the growth factor is the fair comparison.
• If the room is cold over a weekend, all cups grow slowly and differences are small. Compare with the class medians and discuss temperature as a condition that affects growth without being a resource.

Structured debrief

1. Put the class medians on the board in a single row: A, B, C, D. Ask which comparison each pair of cups is (A against B is nutrients, C against B is light, D against B is crowding) and have students say the limiting resource for each before anyone looks at their answers.
2. Look at the spread in the class table. Ask why eight cups that were set up the same way did not give eight identical numbers (different starting fronds, counting differences, position in the window) and why the median is a fairer summary than the highest or lowest cup.
3. Hold up cup C beside cup B. The dark cup had more nutrients than cup A and still did worst: a plant cannot use nutrients without the energy from light to build new tissue. Link it to the lake-layers slide: the bottom of a lake has nutrients but no light.
4. Discuss cup D. If its weekly factor fell in week 2 while cup B's did not, the fronds were starting to compete with each other for surface, light and nutrients, the same shape as the leveling-off part of the deer graph. If D and B grew at the same rate, the cup's carrying capacity had not been reached yet, which is also a real result.
5. Close with the frogs: pale, weak fronds in the dark cup and pale fronds in the unfed cup show that a resource changes what an individual organism is like, which is exactly what the missing alkaloid prey does to a captive dart frog.

Recording sheet: what students fill in

Lab 2 recording sheet: Four cups of duckweed

Analysis questions: answer key

Individual evidence of learning

Each student hands in their own counts and conditions tables, the four-line graph, the day-14 drawing, the class table with medians, and written answers to q1 to q11. The growth factors with q5 and q11 are the evidence for analyzing data on resource availability and population size; q6 and q7 are the evidence for competition and carrying capacity; q8 is the evidence for effects on individual organisms.

Assessment criteria

Accessibility adaptations

• Pair a student with low vision or limited fine-motor control with a partner who moves fronds with the toothpick while the student counts from an enlarged phone or tablet photo and records.
• Provide a growth-factor helper: a worked example (10 fronds to 60 fronds is 60 ÷ 10 = 6 times) printed at the top of the counts table.
• Offer a sentence frame for q5: 'Cup __ grew __ times while cup B grew __ times. The only difference was __, so the limiting resource was __.'
• Let students who need movement act as the class 'cup keeper team' for the midweek top-up, and accept a labeled photo series in place of the day-14 drawing.

Low-cost alternative

Cut the cost by asking an aquarium shop or a pond-keeping family for spare duckweed (many give it away), using a bright window instead of a grow light, and running cups A, B and C in every group but only one cup D per table. If no duckweed is available at all, run the same four conditions with mustard or radish seeds on damp paper towels in clear zip bags (A: plain water, B: nutrient water, C: nutrient water in a dark box, D: four times as many seeds) and, after 10 to 14 days, count the seedlings whose seed leaves have opened and turned green and score their color. Do not use height as the measure: seedlings in the dark grow tall and pale while they use up the food stored in the seed, which makes the dark bag look like the winner. Seedlings grow as individuals rather than multiplying, so the crowded bag tests individual growth more than population growth.

Model limitations

This investigation has not been physically tested by the authors; the expected numbers are ranges based on published descriptions of how fast duckweed grows, and a classroom's light and temperature can change them a great deal. A cup is a tiny, simplified pond: it has no grazers, no water flow bringing in new nutrients, and only one duckweed species, so it shows competition within a species far more clearly than a real pond would. Cup D changes several things at once (surface per frond, light per frond and nutrients per frond), so it shows that crowding limits growth but cannot say which of those three ran short first. Two weeks is long enough to see differences in growth but usually not long enough for any cup to level off completely.

Teacher notes

All numeric keys are group-specific; the worked examples show the calculation and the expected ranges. The most common reasoning error is comparing raw counts instead of growth factors, which makes cup D look like the winner; push every group back to growth factors. If cup C's thermometer reads several degrees different from the light spot, use it as the q10 discussion rather than discarding the data. Duckweed is a flowering plant, but in this lab it reproduces by budding, so each new frond is a genetic copy of its parent; students do not need that detail but sometimes ask why there are no seeds.