Practical 17 — Plankton
CAPS lists video clips as sufficient for this block, so what follows goes further on purpose. You live twenty minutes from one of the most productive plankton coastlines on Earth — so investigation 1 is a plankton net you build from a stocking and a jar, and tow off a jetty in Algoa Bay.
But the Depth column is where the real requirements sit: "The depth of this topic is at a level where pupils appreciate and understand: Evolutionary trends · The definitive differences between phyla · Specific adaptations, relating structures to functions · The richness of marine biodiversity and why it must be conserved." Investigations 2 and 3 target those directly — and your booklet's own sample question on the adaptations is worth 8 of its 14 marks.
A working plankton net costs about R50: one leg of an old stocking, a wire coat hanger, a small clear jar and some string. Full build below.
Can't get to the water? Investigation 1 supplies a described tow sample from Algoa Bay — enough to complete the identification, the drawings and every analysis question. Investigations 2 and 3 need only a pencil.
Aim
To collect a plankton sample from Algoa Bay using a home-made net, and to identify and draw the organisms present.
Apparatus
- 🟢 kitchen One leg of an old nylon stocking (the finer the denier, the better) — this is the mesh
- 🟢 kitchen A wire coat hanger, bent into a circle ~20 cm across — the mouth
- 🟢 kitchen A small clear jar or plastic bottle top — the cod-end, where the sample collects
- 🟢 kitchen String, elastic bands, a tape measure
- 🟠 microscope Microscope, slides, cover slips, dropper (a strong hand lens shows the larger zooplankton)
- 🌊 site A jetty, harbour wall or boat
Build it
- Bend the coat hanger into a circle about 20 cm across. Leave the hook to twist into a bridle point.
- Cut the toe off the stocking. Stretch the open end over the hoop and fold it back, securing with elastic bands or stitching.
- Tie the cut toe end tightly around the mouth of the jar so everything the net strains ends up inside it.
- Tie three equal lengths of string to the hoop at 120° apart, gather them into a bridle, and attach the tow line.
- Measure your hoop diameter and write it down. You need it to calculate how much water you filtered.
Tow it
- Lower the net into the water and tow it slowly — walking pace along the jetty, or drifting behind a slow boat — for a measured distance (say 10 m) or a measured time.
- Lift it out, let the water drain through the mesh, and unscrew the jar. Hold it to the light. Anything moving?
- Take a dropper sample from the jar onto a slide, cover slip on, and examine — low power first.
- Record the date, time of day, tide state, water temperature if you can, and the weather. Plankton varies enormously with all of these, and a sample without its conditions is close to meaningless.
| Date, time and site | |
|---|---|
| Tide state and weather | |
| Hoop diameter (m) and tow distance (m) | |
| Colour / cloudiness of the sample |
| Sketch ref | Approx. size | Moving? How? | Phyto- or zooplankton? | Holo- or meroplankton? |
|---|---|---|---|---|
🔬 Supplied observations — a real 20 cm net tow, Algoa Bay, 10 m, mid-morning in spring
The jar held about 40 mℓ of faintly green-brown cloudy water with visible specks drifting in it.
Under × 100 — the phytoplankton
Enormous numbers of chain-forming diatoms: golden-brown rectangular cells, roughly 40 µm long, joined end to end in ribbons of six to twelve. Also single disc-shaped diatoms, ~60 µm, some with fine radiating spines. Scattered dinoflagellates, ~30 µm, spinning slowly on their own axis, each with two flagella and a visible armoured plate pattern.
Field of view 1.8 mm across. Diatoms far outnumber everything else.
Under × 40 — the zooplankton
Copepods, about 1.5 mm, tear-drop shaped with long trailing antennae, darting in short jerks. Several crab zoea larvae (~2 mm) with an unmistakable long curved rostral spine. Two barnacle nauplii, triangular, ~0.5 mm. One transparent fish larva about 5 mm with a visible eye and yolk sac. A few threads of polychaete worm larvae.
Orange oil droplets visible inside several copepods.
Your drawings
Two labelled scientific drawings — one phytoplankton, one zooplankton — following the nine drawing conventions from Topic 5. Underlined heading, stated magnification.
Analysis
- 6Submit both labelled drawings with headings and magnifications. (3 each.)
- 4Complete the organism log for at least four distinct types, classifying each as phyto/zoo and holo/mero.
- 4Calculate the volume of water your net filtered: V = π r² × distance towed. Give your answer in m³ and then in litres. (1 m³ = 1 000 ℓ.)
- 3Explain why a stocking works as plankton mesh, and what would happen if you used (a) a much coarser mesh, (b) a solid plastic bag.
- 3Your sample is dominated by one group. Identify which, and explain why that is what you would expect at the base of a food web.
- 4Explain why recording the time of day matters, referring to diel vertical migration — and predict how a tow at midnight would differ.
- 3Several copepods contained visible orange oil droplets. Name the adaptation and explain its function.
Extension
Tow at the same site at midday and again after dark, and compare the zooplankton counts. You will be measuring diel vertical migration yourself.
Aim
To produce an annotated drawing showing the adaptations that keep planktonic organisms in the sunlit surface layer, relating each structure to its function.
Your booklet's sample question on this is worth 8 of its 14 marks — more than any other item in the topic. CAPS concept 8 has five sub-points, more than any other concept in the block.
The problem every drifter has
A phytoplankton cell is denser than seawater. Left alone it sinks — and below the sunlit layer it can no longer photosynthesise, so it starves. Every adaptation below is a way of losing that race more slowly. That framing is what turns a list into an explanation, and it is what earns the marks.
Your task
Draw a single composite "super-drifter" — an imaginary organism carrying all five adaptations — and annotate each one. An annotation is not just a label: it names the structure and states its function.
| Adaptation (CAPS 8a–e) | How it works | A real organism that has it |
|---|---|---|
| a) Weak self-propulsion | ||
| b) Neutrally buoyant jelly | ||
| c) Food stored as fats and oils | ||
| d) Gas-filled bladders | ||
| e) Long spines or frills |
Analysis
- 8Submit the annotated drawing. (5 for the adaptations shown and correctly annotated, 3 for drawing conventions: single lines, ruled label lines, no arrows, underlined heading.)
- 10Complete both remaining columns of the table for all five adaptations. (1 for the mechanism, 1 for the example.)
- 3Explain why storing food as oil rather than starch is an adaptation to a planktonic life, referring to density.
- 3Cold water is more viscous than warm water. Explain how that helps plankton, and what it implies about productivity in cold seas.
- 3Adaptation (a) is described by CAPS as "an ability to propel themselves, although weakly". Explain why weak propulsion still counts as planktonic, and what would make an organism nektonic instead.
Aim
To interpret productivity data from the two South African coasts, and to explain the difference using the limiting factors.
The data
Chlorophyll-a concentration is the standard proxy for phytoplankton abundance — more chlorophyll, more phytoplankton. Below, one year of monthly satellite readings from two sites at the same latitude on opposite coasts.
| Month | West Coast (Benguela, off St Helena Bay) | East Coast (Agulhas, off Durban) |
|---|---|---|
| Jan | 8.4 | 0.22 |
| Feb | 11.2 | 0.19 |
| Mar | 9.6 | 0.24 |
| Apr | 6.1 | 0.31 |
| May | 3.8 | 0.44 |
| Jun | 2.4 | 0.52 |
| Jul | 2.1 | 0.58 |
| Aug | 2.9 | 0.49 |
| Sep | 5.7 | 0.36 |
| Oct | 9.1 | 0.27 |
| Nov | 12.6 | 0.21 |
| Dec | 10.3 | 0.20 |
Analysis
- 7Plot both data sets on one line graph, month on the x-axis. You will need to think about the scale — one series is roughly 20× the other. Follow all seven graph rules from Topic 5.
- 3Calculate how many times more productive the West Coast is than the East Coast in November.
- 4Describe the seasonal pattern on each coast, and state in which months each peaks.
- 6Explain the difference between the two coasts using all three limiting factors CAPS names.
- 3The West Coast peaks in summer, which is the opposite of most northern-hemisphere seas. Explain why, referring to the wind that drives upwelling.
- 3Tourists describe East Coast water as "beautiful and clean" and West Coast water as "murky and dirty". Explain why that description is biologically backwards.
- 4The Depth column requires you to understand why marine biodiversity must be conserved. Using this data, explain what would happen to South Africa's West Coast fisheries if upwelling weakened — and name two things that could weaken it.
Extension
Find a satellite chlorophyll map of southern Africa online (NASA Earth Observatory publishes them free). Locate the Benguela plume, the Agulhas Current, and Algoa Bay sitting between them. Then find the same map for a different season and describe what moved.
✅ Memo & marking guide — try it first
Total: 84 marks — Investigation 1: 27 · Investigation 2: 27 · Investigation 3: 30.
This memo does not print with the worksheet.
Investigation 1 — the net tow
- Q1 (6): 3 each — recognisable drawing, correct labels, underlined heading with magnification.
- Q2 (4): Using the supplied data: chain diatoms (phyto, holo) · dinoflagellates (phyto, holo) · copepods (zoo, holo) · crab zoea / barnacle nauplii / fish larva / polychaete larva (zoo, mero).
- Q3 (4): r = 0.10 m, so V = π × 0.10² × 10 = 0.314 m³ = 314 ℓ. Full marks require the working and both units.
- Q4 (3): Stocking mesh is fine enough to retain plankton while letting water pass through, so the sample is concentrated (1). (a) Coarser mesh would let the smaller plankton — including most phytoplankton — wash straight through, biasing the sample toward large zooplankton (1). (b) A solid bag would filter nothing: water cannot pass, so the bag simply pushes water aside and collects almost nothing, and it would drag badly (1).
- Q5 (3): Diatoms / phytoplankton dominate (1). They are the producers — the base of the food web — and a pyramid of numbers must be widest at the base because energy is lost at every trophic transfer, so producers must vastly outnumber consumers (2).
- Q6 (4): Zooplankton perform diel vertical migration: down during the day to avoid visual predators, up at night to feed (2). A midnight tow would therefore capture far more zooplankton — copepods, larvae — because they have risen into the surface layer (2).
- Q7 (3): Storing food reserves as buoyant oils (CAPS 8c) (1). Oil is less dense than seawater, so it adds lift and slows sinking, keeping the copepod in the productive surface layer (2).
Investigation 2 — the adaptations drawing
- Q8 (8): 5 for all five adaptations drawn and annotated (structure named AND function stated — a bare label scores half); 3 for conventions.
- Q9 (10): a) Weak propulsion — cilia or flagella beat to swim slowly upward, countering sinking; dinoflagellate, copepod. b) Neutrally buoyant jelly — watery low-density tissue makes overall density almost equal to seawater, so it neither rises nor sinks; jellyfish, salps, comb jellies. c) Fats and oils — less dense than water, so stored energy doubles as a float; copepods, many diatoms. d) Gas-filled bladders — a pocket of gas is far less dense than water and provides strong lift; bluebottle (Physalia sp.), some cyanobacteria. e) Spines and frills — increase surface area and therefore drag, slowing the downward drift; diatoms, radiolarians, crab zoea.
- Q10 (3): Starch is denser than seawater and would make the organism sink faster; oil is less dense, so the same stored energy provides buoyancy instead of ballast. The organism gets its energy store and its flotation from one substance.
- Q11 (3): Higher viscosity means greater resistance to a body moving through it, so plankton sink more slowly and stay longer in the sunlit layer (2). Combined with upwelling nutrients, this is part of why cold seas are so productive (1).
- Q12 (3): Planktonic means unable to swim against the current — weak propulsion only allows small vertical adjustments, not horizontal travel against the flow (2). An organism strong enough to determine its own horizontal course against the current is nektonic — fish, squid, whales (1).
Investigation 3 — the two coasts
- Q13 (7): Line graph, month on x. The scale problem is the point: either use a second y-axis for the East Coast series, or a logarithmic y-axis, or plot the East Coast on an inset — any of these earns the mark provided it is labelled. Both axes labelled with units, sensible scale, accurate points, both lines distinguished with a key, figure number and title.
- Q14 (3): 12.6 ÷ 0.21 = 60 times more productive.
- Q15 (4): West Coast peaks in summer (Nov–Feb, max 12.6 in November) and troughs in winter (min 2.1 in July). East Coast is the reverse and far flatter — a weak winter peak (0.58 in July) and a summer minimum (0.19 in February).
- Q16 (6): Nutrients (2) — the Benguela upwells cold nutrient-rich deep water into the surface; the Agulhas is a warm, nutrient-poor current with no upwelling. Temperature (2) — the warm Agulhas is strongly stratified, forming a thermocline that seals nutrients below; the cold Benguela mixes readily. Sunlight (2) — comparable at the same latitude, so light is not the limiting factor here; nutrients are. Recognising that light is NOT the limiter on either coast is worth full credit.
- Q17 (3): Upwelling on the West Coast is driven by south-easterly winds, which are strongest in summer. They push surface water offshore and cold nutrient-rich water rises to replace it — so the bloom peaks in summer, not spring.
- Q18 (3): The "murk" on the West Coast is the phytoplankton — the water is turbid because it is full of life, and that life supports the fisheries. The "clean" clear blue East Coast water is clear because it is nutrient-poor and comparatively empty. Clarity is a sign of low productivity, not of health.
- Q19 (4): Weakened upwelling → less nutrient supply → phytoplankton collapse → the anchovy, sardine and hake stocks that feed on them collapse, along with seals, gannets and penguins, and the fishing industry and jobs that depend on them (2). Two causes, any of: climate change altering wind patterns or strengthening stratification; ocean warming reducing the density difference that drives upwelling; large-scale shifts such as Benguela Niño events; overfishing removing the consumers that recycle nutrients (2).