The Open
Ocean
The middle of the ocean has less life per cubic metre than the Sahara Desert. Yet just kilometres away, where nutrients reach the surface, you'll find some of the densest food chains on Earth. By the end of this topic you'll know why, where, and what happens around our coast.
Every winter, Southern Right and Humpback whales swim thousands of kilometres from Antarctic waters to South Africa to give birth — and they barely eat for the entire trip. So how do they know it's safe to come?
They front-load the year. From November through March, the Southern Ocean explodes with krill — tiny shrimp-like animals that bloom in their billions when summer light hits nutrient-rich polar water. The whales gorge for months, building a thick layer of fat.
That fat is the entire winter budget. They mate, calve, and rear their newborns in the warm shallow bays around Hermanus and Algoa Bay, living almost entirely off summer reserves. In years when the krill bloom fails — usually strong El Niño years — the whales may not breed at all and may not even make the trip.
Where the Food Web Begins
Light + nutrients = phytoplankton = everything else
Almost every marine food chain starts the same way: phytoplankton, microscopic plant-like drifters, photosynthesise sunlight at the ocean's surface. They need two things — light and nutrients. Where both are abundant, life explodes. Where either is missing, the ocean turns into a watery desert. The map of where the ocean is alive is really a map of where nutrients reach the surface.
The open ocean is an ecosystem — a community of organisms plus the physical conditions they live in — and it is worth reading it with the ecological ideas you already have. Producers at the bottom, consumers stacked above them, energy lost at every step. What makes this ecosystem distinctive is that the producers are microscopic and drifting, so the whole web moves with the water.
What makes a coastal margin so much richer than mid-ocean
Out in the middle, conditions are stable and poor. Closer to coastal margins — and particularly over continental shelves (see Topic 3) — conditions become dynamic, and complex communities can evolve. Three things do it:
- Strong currents — which keep water, and nutrients, moving.
- Wave action, and the surface mixing that follows it — the quiet second route through the thermocline lid, and the one most people forget.
- Offshore winds along some coastlines, generating strong upwelling events.
You met the currents and the upwelling in Topic 2. This topic is what lives in the water those processes produce.
Three Coasts, Three Rules
South Africa is famously a country with three different oceans on its doorstep. Each one feeds its phytoplankton — and therefore its food chain — by a different rulebook. Tap each coast to see what's going on offshore.
Local connection — Algoa Bay's east-shift
Gqeberha sits right where the south coast hands off to the east. Twenty years ago the big pelagic schools — pilchards, anchovies, mackerel — peaked further west off Cape Town. The schools now regularly extend as far as Mossel Bay, and move further still during the sardine run, which follows the continental shelf right past us up the KZN coast every winter. Cold nutrient-rich water is reaching the south-coast shelf more often than it used to, and our local fishing and tour-boat operators have followed.
Mossel Bay is the landmark to remember — it is the one CAPS names for how far west-coast pelagic schools now travel.
Up the Food Chain
Open-ocean food chains are mostly planktonic — every link drifts. Closer to coastlines, the rules change: the larvae of crabs, fish and mussels temporarily join the drift as meroplankton before settling onto the bottom, and the whole web becomes more complex.
Phytoplankton
~ the producersmicroscopic photosynthesisers — diatoms, dinoflagellates, cyanobacteria. Need sunlight and nutrients. Bloom whenever both arrive.
Zooplankton
~ first consumersradiolarians, foraminiferans and copepods drift with the phytoplankton and graze on them. Near coasts, larvae of crabs, fish and mussels join in as meroplankton.
Krill, jellies & small fish
~ second consumersshrimp-like krill, bluebottles, jellies and tiny fish hoover up the zooplankton. Krill alone feed every large animal in the Southern Ocean.
Pelagic fish schools
~ third consumerspilchards, anchovies, mackerel and maasbanker (horse mackerel) form vast schools that follow the food. 📌 "Pilchard" and "sardine" are the same fish — CAPS uses "pilchards" in the food chain and "sardine run" in the pelagic-fish section, and a learner who treats them as two species will misread both.
Top predators
~ apex consumerstuna, hake, yellowtail, elf (shad), dolphins, sharks, and diving seabirds — gannets, cormorants, penguins — all converge wherever the schools run thickest.
Southern Right and Humpback whales — a one-year cycle in two halves
CAPS names two examples for this migration, and they behave the same way: Southern Right and Humpback whales. If a question asks for examples, one is half an answer.
Feed in the Southern Ocean
Long polar days drive a krill explosion in the cold, nutrient-rich water around Antarctica. Whales feed almost continuously, packing on a thick fat layer that has to last the rest of the year. Bigger krill bloom = better breeding.
~ if El Niño is strong, this bloom shrinks. Whales may not breed.
Mate & calve on the SA coast
Southern Right and Humpback whales swim thousands of kilometres north into warmer, sheltered SA bays — Hermanus, Walker Bay, Algoa Bay. They mate, give birth, and rear newborn calves in calm shallow water. Almost no feeding happens. The whole season runs on summer fat.
~ visible from shore at Hermanus from Jul–Oct.
Why the open ocean is mostly a desert
Out in the middle of an ocean basin, far from any coast, surface water is warm and stable. A warm surface layer floats on top of cold nutrient-rich deep water — and the boundary between them, the thermocline, acts like a lid. Without wind-driven upwelling or a coastal current to break the lid, nutrients stay trapped down deep, sunlight stays trapped up top, and phytoplankton populations stay tiny. Whole stretches of mid-Pacific are this way: blue, beautiful, and almost empty.
The Lid on the Ocean
Everything above comes down to one question: can nutrients get from the deep water up into the sunlight? This cross-section is the answer. Tap the numbered points.
Where the Pelagic Fish Are, and Why
CAPS gives this its own page, and the opening line is the whole idea: "Variable nutrient supplies around our coastline dictate the distribution of pelagic fish." Follow the food, and you have found the fish.
West coast — two reasons it should be richest
CAPS names exactly two factors:
- The generally higher nutrient content of water in the Benguela Current — true at all times, not just seasonally.
- Frequent summer upwelling events along the western shoreline, which push nutrient levels higher still.
Result: a rich food supply, and therefore a potential abundance of pelagic fish.
South coast — the shelf carries it east
The cold nutrient-rich water often extends along the continental shelf of the south coast — and it has done so more often over the past decade or two. So the schools travel: they now regularly reach Mossel Bay, and further during sardine runs.
East coast — steady, but thin
Relatively stable conditions and warm water mean a much lower abundance of pelagic schools — but a constant one, present for most of the year rather than booming and crashing.
The word "potential" is doing a lot of work
CAPS says the west coast's rich food supply results in a potential abundance of pelagic fish. It is potential rather than actual because of one thing the food chain cannot control: how many fish people take out. Overfishing in earlier decades had a significant detrimental effect on west-coast pelagic populations.
Now join the two halves, because they are one event. Overfishing damaged the west-coast stocks, and at the same time cold nutrient-rich water began reaching the south-coast shelf more often — possibly linked to climate change. The west-coast decline and the south-coast increase are not two separate stories. They are the same story, and an exam question about "why have pelagic fish moved east" wants both halves.
What the sardine run actually is
Not just "lots of sardines". It is an annual event linked to the reproductive cycle of sardines, during which they form massive schools that follow the continental shelf up towards KZN. As the shelf narrows, the schools become more densely packed and driven closer to shore — which is exactly why it is visible from a beach.
That density attracts everything: sharks, larger pelagic fish, dolphins, whales and diving seabirds, which go into a feeding frenzy for the few weeks the sardines are on the move. 📌 The shelf-narrowing detail is a direct callback to Topic 3 — the shape of the seafloor is what concentrates the fish.
In the Lab — the CAPS Investigations
CAPS's Investigations column for this block is short: "Blue Planet and other documentary excerpts" and "Provide worksheet and questions. The worksheet should include supplementary notes." So the practical sheet supplies exactly that worksheet — plus the data work the Depth column asks for.
- 🟢 desk Build the food web. Turn CAPS's five-tier food chain into a proper web, label the trophic levels, and work out what happens when one link fails.
- 🟢 desk Three coasts, one year. Plot a year of phytoplankton data for the west, south and east coasts and explain the three different shapes.
- 🟢 desk The pelagic shift. Two decades of west-coast and south-coast catch data on one graph — and the argument that they are one event, not two.
- 🟢 desk 📺 video The documentary worksheet that CAPS asks for, with its supplementary notes, usable with any ocean documentary you can get hold of.
→ Open the Topic 4 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. The three marked 📕 are the definitions your booklet flags as examinable; the rest are terms CAPS uses in the key concepts for this block.
Microscopic photosynthesising drifters — diatoms, dinoflagellates, cyanobacteria. They form the base of almost every marine food chain and produce roughly half the oxygen on Earth.
Tiny drifting animals — radiolarians, foraminiferans, copepods, krill larvae — that graze on phytoplankton. They are the first consumers in the planktonic food web.
Simple crustaceans, 1 mm to 1 cm long, that form dense planktonic communities and are an important food source for larger predators. Probably the most numerous animals on Earth.
Shrimp-like crustaceans that form dense planktonic communities, particularly in polar oceans. The single biggest food source for whales, seals, penguins and many fish in the Southern Ocean.
The larvae of coastal animals — crabs, mussels, fish, sea urchins — that drift as part of the zooplankton only for a phase of their life before settling onto the bottom or growing into adults.
The boundary layer where temperature drops sharply between warm surface water and cold deep water. Acts as a lid that traps nutrients below, especially in summer on calm coasts.
Belonging to the open water column, away from the bottom and the coast. Pelagic fish — pilchards, anchovies, tuna — live their lives swimming in the open sea, not on the seafloor.
A weather condition that forms over the Pacific Ocean but reaches around the planet — disrupting upwelling, krill blooms and whale migration patterns, even in the Southern Ocean and around SA.
An annual event linked to the reproductive cycle of sardines, in which massive schools follow the continental shelf up towards KZN, packing denser and closer inshore as the shelf narrows — and drawing in every predator in reach.
A community of organisms together with the physical conditions they live in, and all the interactions between them. CAPS asks you to read each marine ecosystem using the ecological ideas from Grade 10.
All the interconnected food chains in an ecosystem. A chain is one path; a web is the real thing, because most animals eat more than one kind of prey and are eaten by more than one predator.
A feeding step in a food chain — producers, then first consumers, then second, and so on. Energy is lost at every step, which is why there are far fewer whales than there is krill.
Horse mackerel — one of the four pelagic schooling fish CAPS names, alongside pilchards, mackerel and anchovies. Worth knowing by both names, since papers use either.
Quick Check
Question 1
Well done!
Move on to Topic 5 when you're ready.