Practical 4
Marine Ecology
CAPS block 54 · Term 3 · 2 hours
The Open Ocean
Four investigations, all desk work. Build the food web CAPS describes, test whether krill really drive whale breeding, follow the pelagic fish as they move east — and fill in the documentary worksheet CAPS actually asks for.
What CAPS asks for
Investigations column, block 54, in full: "Blue Planet and other documentary excerpts" and "Provide worksheet and questions. The worksheet should include supplementary notes."
That is Investigation 4. It is a thin column, so the other three are built to the Depth column instead, which asks that "individual ecosystems should be viewed with reference to the ecological concepts introduced in Gr10" and that learners understand "the complexity of ecosystems". That means food webs, trophic levels and real data — the things an exam can actually mark.
What you need
- 🟢 kitchen Pencil, ruler, eraser and a calculator
- 🟢 kitchen Graph paper, or the printed grids in Investigations 2 and 3
- 🟢 kitchen Coloured pencils — useful for keeping two data series apart
- 📺 video Investigation 4 only: any ocean documentary you can get hold of
No documentary? Investigation 4 still works.
CAPS names Blue Planet, but any ocean documentary will do — a series episode, a museum film, an aquarium presentation, a 20-minute clip. The worksheet is written so that it works with whatever you can actually watch, because it asks you to identify things the ocean always contains rather than things one particular film happens to show. The supplementary notes it needs are printed with it. Investigations 1–3 need nothing but this sheet.
1
Build the Open-Ocean Food Web
Depth column · Gr10 ecological concepts
Aim
To convert the open-ocean food chain given in CAPS into a food web, label its trophic levels, and predict what happens to the whole community when one link is removed.
Method
- Read the organism list in the supplied-data panel. Every organism there is named in CAPS block 54.
- In the drawing box, place the organisms in trophic levels, producers at the bottom.
- Draw arrows to show the flow of energy. Arrows point from the eaten to the eater — that is, in the direction the energy travels. Getting this backwards is the single most common mistake in food-web questions.
- Follow the drawing conventions from Topic 5: pencil, ruled label lines, no arrowheads on label lines. ⚠️ The energy arrows in the web itself do need arrowheads — that is what makes them energy arrows, not labels.
Supplied data — the organisms, and the three coasts
Producers: phytoplankton (diatoms, dinoflagellates, cyanobacteria).
Drifting consumers: radiolarians, foraminiferans, copepods. Near the coast, meroplankton — the larvae of coastal invertebrates and fish — join them temporarily.
Larger carnivores: krill, jellies, bluebottles, small fish.
Pelagic schools: pilchards (= sardines), mackerel, anchovies, maasbanker (horse mackerel).
Top predators: dolphins, tuna, hake, yellowtail, elf (shad); seabirds — gannets, cormorants, penguins; and whales, which feed mainly on krill.
| Coast | Surface nutrients | Phytoplankton | Why |
|---|---|---|---|
| West | High all year, higher still after upwelling | Abundant year-round | Benguela water plus frequent summer upwelling |
| South | Good in spring and autumn, restricted in summer | Blooms in spring, early summer and autumn | A summer thermocline blocks nutrients from below |
| East | Consistently low | Small but stable all year | Constant supply of warm, nutrient-poor Agulhas water |
Results — draw your food web here
Analysis
- Draw the food web. Marks: producers correctly placed at the base (2), all five feeding groups present and correctly ordered (4), at least eight energy arrows drawn in the correct direction (4), trophic levels labelled (2), neat pencil work with no crossing-out (1). 13
- Name the producers in this ecosystem, and state the two things they need in order to thrive. 3
- Explain the difference between a food chain and a food web, and give one reason a web is a better description of the open ocean. 4
- What is meroplankton, and why does it make coastal food webs more complex than open-ocean ones? 4
- Using Data set 1.1, decide which coast could support the largest pelagic fish population, and justify your answer with two pieces of evidence from the table. 5
- A disease wipes out the krill in the Southern Ocean for one summer. Trace the effect through three steps of your web, ending with an effect on whales. 6
- There are far fewer whales in the ocean than there are krill. Explain why, in terms of trophic levels and energy. 3
2
Does Krill Really Drive Whale Breeding?
CAPS concepts 7–11 · correlation vs causation
Aim
To investigate whether Southern Right whale calving success is related to the density of Southern Ocean krill, and to decide what that relationship does and does not prove.
⚠️ Read this before you use the numbers
This data set is constructed for this exercise. It is not a set of measurements. It is built to show the pattern that published research describes — your booklet cites a 2016 study by Seyboth and others on exactly this question — so that you can practise plotting and interpreting. Do not quote these figures as real data anywhere. If you write this up properly, find the actual study.
Method
- Plot both series in the supplied data on one set of axes. They are in different units and different ranges, so you will have to solve the plotting problem — a second y-axis, a scaled series, or two graphs stacked with a shared x-axis are all acceptable.
- Mark the three El Niño years on your graph.
- Answer the analysis questions.
Supplied data — krill density and whale calves (constructed)
| Year | 1 | 2 ★ | 3 | 4 | 5 | 6 ★ | 7 | 8 | 9 ★ | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| Summer krill density (g/m³) | 18 | 7 | 16 | 21 | 19 | 6 | 15 | 20 | 8 | 17 |
| Calves counted the following winter | 142 | 61 | 128 | 165 | 150 | 54 | 121 | 158 | 66 | 134 |
Results — plot your graph here
Analysis
- Plot both series. Marks: axes chosen and labelled with units (3), the scale problem solved and your solution stated (3), points plotted accurately (3), El Niño years marked (1). 10
- Describe the relationship between krill density and the number of calves counted the following winter. 3
- In which years is krill density lowest? What do those years have in common? 3
- Explain the biological mechanism that links a poor krill summer to fewer calves the following winter. Use the words fat reserves and migration. 5
- A learner writes: "This graph proves that El Niño causes fewer whale calves." Using what you know from Topic 5, explain why the graph on its own does not prove that, and state one thing that would strengthen the claim. 5
- Whales are not the only animals that depend on the Southern Ocean krill bloom. Name two other groups from your food web that would also be affected by a poor krill year. 2
3
The Pelagic Shift — One Event, Not Two
CAPS pelagic fish, concepts 1–5
Aim
To use catch data to describe how pelagic fish abundance has changed on the west and south coasts, and to argue whether the two changes are separate events or one.
⚠️ Constructed data again
As in Investigation 2, these numbers are a teaching data set, not real catch statistics. They reproduce the direction and rough scale of the change that CAPS and your booklet describe. Real catch records are published by the national fisheries department — use those for any real report.
Method
- Plot both coasts on the same axes, using a different colour or line style for each.
- Identify the year in which the two lines cross.
- Answer the analysis questions, then write the argument in Q19.
Supplied data — pelagic catch by coast (constructed)
| Year | 2000 | 2004 | 2008 | 2012 | 2016 | 2020 | 2024 |
|---|---|---|---|---|---|---|---|
| West coast | 310 | 268 | 205 | 160 | 128 | 104 | 92 |
| South coast | 48 | 72 | 110 | 149 | 186 | 212 | 228 |
Results — plot your graph here
Analysis
- Plot both series on one set of axes. Marks: axes labelled with units (2), suitable scale (2), both series plotted accurately (3), a key identifying the two lines (1). 8
- Between which two data points do the lines cross? Estimate the year and the catch value at the crossing point from your graph. 3
- Calculate the percentage decrease in the west-coast catch from 2000 to 2024. Show your working. 4
- CAPS says two factors make the west coast's food supply richer than anywhere else. Name both. 2
- If the west coast has the richest food supply, explain why its catch has fallen anyway. 3
- The argument. A classmate says: "Two different things happened — the west coast was overfished, and separately the south coast got luckier." Write a paragraph arguing that these are better understood as one event. Use both the overfishing evidence and the change in where cold nutrient-rich water reaches. 6
- Sardines are the same fish as pilchards. Explain, using the shape of the continental shelf, why the sardine run becomes visible from the shore as the schools move north. 4
4
The Documentary Worksheet
CAPS Investigation, verbatim
Aim
To watch an ocean documentary as a marine scientist rather than as an audience — identifying the ecosystem, its producers, its trophic structure and the conditions that make it work.
Method
- Choose any ocean documentary or excerpt — 20 minutes is enough. CAPS names Blue Planet; anything comparable works.
- Read the supplementary notes below before you watch. CAPS asks specifically for a worksheet that includes them, and they are what turn watching into observing.
- Complete the table as you watch. Pause freely.
- Answer the questions afterwards, from your own notes.
Supplementary notes — what to look for
- Find the producer. Every scene has one, even when the camera never shows it. If you are looking at a shark, the energy on screen arrived through phytoplankton several steps earlier. Ask yourself how many steps.
- Watch for the nutrient source. Rich scenes are almost always near upwelling, a coast, a shelf, or a polar summer. Barren blue scenes are mid-ocean. The film will rarely say so; the biology tells you.
- Count trophic levels, not species. A crowded reef may be three levels deep; an Antarctic scene may be two. Depth of the chain matters more than the number of animals.
- Notice what is drifting versus what is swimming. Plankton drift by definition. An animal that swims against a current is not plankton, however small.
- Beware the narration. Documentaries compress time, cut between oceans, and use music to imply drama. A shot of a predator and a shot of prey do not prove that one ate the other. Note what you actually saw.
Results
| What to record | Your observation |
|---|---|
| Title of the documentary or excerpt | |
| Which ocean or region | |
| Open ocean, coastal, or both? | |
| Producers shown or implied | |
| Two consumers, and their trophic level | |
| Top predator shown | |
| Evidence of upwelling, a shelf, or a bloom | |
| One human impact mentioned or visible |
Analysis
- Complete Table 4.1. 8
- Write out one food chain from the documentary, with at least four links, using arrows in the correct direction. 4
- Was the ecosystem you watched nutrient-rich or nutrient-poor? Give two pieces of evidence from what you saw. 4
- Identify one moment where the narration stated something the footage did not actually show. Explain the difference between the two. 3
- Compare the ecosystem in the documentary with the South African coast nearest to you. Give one similarity and one difference. 4
Memo — worked answers and mark allocation (does not print)
Total: 119 marks — Investigation 1: 38 · Investigation 2: 28 · Investigation 3: 30 · Investigation 4: 23.
Investigation 1 — the food web
- Q1 (13): Phytoplankton at the base; then radiolarians/foraminiferans/copepods (+ meroplankton near coasts); then krill, jellies, bluebottles, small fish; then pilchards, mackerel, anchovies, maasbanker; then dolphins, tuna, hake, yellowtail, elf, gannets, cormorants, penguins — plus whales branching directly off krill. That whale branch is the mark most learners miss: whales are top predators that feed near the bottom of the chain.
- Q2 (3): Phytoplankton — diatoms, dinoflagellates, cyanobacteria (1). They need light (1) and nutrients (1).
- Q3 (4): A chain is a single feeding pathway; a web is all the interconnected chains (2). A web is better because most organisms eat more than one prey and are eaten by more than one predator, so a single chain misrepresents what happens if one species is removed (2).
- Q4 (4): The larvae of coastal invertebrates and fish, which join the zooplankton temporarily before settling or maturing (2). They add extra organisms and extra feeding links near coastlines, and they connect the bottom-dwelling community to the drifting one (2).
- Q5 (5): The west coast (1). Evidence, any two (2 each): nutrients high all year rather than seasonally; frequent summer upwelling raises them further; phytoplankton abundant year-round, so the producer base never collapses.
- Q6 (6): Any valid three-step trace (2 per step), e.g. krill collapse → the animals that eat krill directly (small fish, seabirds, whales) lose their main food → whales cannot build fat reserves over summer → they arrive on the SA coast in poor condition, or do not migrate at all, and calving falls.
- Q7 (3): Whales are several trophic levels above krill... in fact whales feed directly on krill, so the better answer is about energy loss generally: only a small fraction of the energy at one trophic level passes to the next (2), so the biomass that can be supported shrinks at every step, and a single whale needs an enormous mass of krill to sustain it (1). Accept either framing if the energy-loss principle is stated.
Investigation 2 — krill and calves
- Q8 (10): Year on x. The two series differ by roughly a factor of eight, so a single linear y-axis wastes the grid — award the scale marks for any stated solution (second axis, scaled series, stacked graphs) and withhold them if the learner simply squashes both onto one axis without comment.
- Q9 (3): A clear positive relationship (1): years with high krill density are followed by winters with many calves, and low-krill years by few (2).
- Q10 (3): Years 2, 6 and 9 (2) — all three are marked as strong El Niño years (1).
- Q11 (5): Whales feed almost exclusively during the Southern Ocean summer (1) and build fat reserves (1). Those reserves fund the entire migration north and the whole breeding season, during which they barely feed (2). If the krill bloom fails, the reserves are insufficient — the whales may not breed, and may not make the trip at all (1).
- Q12 (5): The graph shows correlation, not causation (2). Both could be driven by some third factor, the sample is only ten years, and calf counts may reflect survey effort rather than calf numbers (1). Strengthen it with, any one (2): more years of data; a proposed and tested mechanism; data from an independent whale population; controlling for survey effort.
- Q13 (2): Any two of: seabirds (penguins, gannets, cormorants), seals, small fish, other whale species. (1 each)
Investigation 3 — the pelagic shift
- Q14 (8): Both series fit comfortably on one linear axis here (0–320), so no scale trick is needed — but a key is essential and is worth a mark on its own.
- Q15 (3): The lines cross between 2012 and 2016 (2), at roughly 2014 and about 165–170 thousand tonnes (1). Accept any reading consistent with the learner's own plotted graph.
- Q16 (4): (310 − 92) ÷ 310 × 100 (2) = 218 ÷ 310 × 100 (1) = 70.3 % (1).
- Q17 (2): The generally higher nutrient content of Benguela Current water at all times (1), and frequent summer upwelling events along the western shoreline (1).
- Q18 (3): Because a rich food supply only gives a potential abundance (1). How many fish are actually present also depends on how many are removed, and overfishing in earlier decades significantly damaged west-coast pelagic populations (2).
- Q19 (6): Full marks need both halves joined (2 each) plus the joining statement (2): overfishing damaged the west-coast stock, and cold nutrient-rich water has been reaching the south-coast shelf more often over the past decade or two, possibly linked to climate change — so the fish are both fewer in the west and better supported in the south. The decline and the increase are the same redistribution, which is why the two lines mirror each other rather than moving independently.
- Q20 (4): The schools follow the continental shelf northward (1). As the shelf narrows, the same number of fish is squeezed into a smaller volume of shallow water (2), so they become densely packed and are driven close to shore, where they are visible — and where the predators concentrate too (1).
Investigation 4 — the documentary worksheet
- Q21 (8): One mark per completed row, marked for reasonableness against whatever was watched, not against a fixed answer.
- Q22 (4): At least four links, arrows pointing from eaten to eater. Deduct for reversed arrows — that is the outcome being tested.
- Q23 (4): Evidence might include: dense plankton or bait balls, large aggregations of predators, kelp or algae cover, murky green water (rich); or clear blue water, sparse life, coral in nutrient-poor settings (poor). (2 per piece of evidence)
- Q24 (3): Any defensible example of narration outrunning the footage — cuts implying a hunt that was filmed separately, a stated cause the images do not demonstrate, compressed time presented as continuous. The mark is for identifying the gap between observation and interpretation, which is the Topic 5 skill.
- Q25 (4): Marked on reasoning, not on which coast is chosen. A strong answer names a specific mechanism — for example, both depend on phytoplankton, but the filmed reef is nutrient-poor and productive through symbiosis while our west coast is nutrient-rich and productive through upwelling.
Marking note. Investigation 4 cannot have a fixed memo, because the learner chooses the film. Mark it on whether the observation is specific and whether the biology is right — a vague answer that could describe any documentary earns little, however long it is.