Plankton
Most people hear "plankton" and think "whale food". That is wildly wrong. Plankton is the foundation of every marine food web on Earth, the planet's biggest single source of oxygen, and an extraordinary community of organisms ranging from microscopic bacteria to 30-metre jellyfish. They have one thing in common: they drift, they can't fight a current, and the ocean would die without them.
Take a deep breath. Now take another. One of those two breaths came from the ocean — produced not by a tree, not by a forest, but by something so small you can't see it. What is it, and why does it run the world's oxygen supply instead of the Amazon rainforest?
It's phytoplankton — single-celled, drifting, photosynthetic organisms suspended in the upper layers of the ocean. Marine phytoplankton produce between 60% and 70% of all the oxygen on Earth. Land plants — every forest, grassland, crop and pot plant combined — manage the other 30%. The reason is simple maths: oceans cover 71% of Earth's surface, and the entire sunlit top of every one of those oceans is packed with photosynthesising cells. They are so numerous, and so productive, that they make up roughly 80% of all marine biomass.
But "plankton" isn't a single thing. The word covers an enormous drifting community: bacteria, single-celled algae, baby fish, jellyfish, larval crabs, sea-snail eggs — anything that floats with the current and can't swim against it. The biggest organism that fits the definition is the Lion's Mane Jellyfish, which can grow tentacles 30 metres long — longer than a blue whale. The smallest are bacteria 1 micrometre across. They drift up at night, down by day, and underneath everything in the sea, they are doing the most important job in the ocean: producing food, producing oxygen, and recycling nutrients.
Drifters, Not Swimmers
"Plankton" is a lifestyle, not a species. It's the community of marine organisms that drift with the currents and cannot swim against them.
Ecologists sort marine organisms by their ability to move. Sessile organisms (corals, sponges, mussels) are fixed in one place. Motile organisms (fish, whales, octopuses) can propel themselves wherever they choose. Planktonic organisms can float or drift, but cannot fight the current — they go wherever the ocean takes them.
"Plankton" therefore covers every taxonomic group you have ever met: bacteria, archaea, protists, algae, invertebrate larvae, even tiny baby fish. Some are tiny producers (phytoplankton, photosynthesising); most are tiny consumers (zooplankton, grazing or hunting). They include both permanent residents (organisms that drift their whole lives) and temporary residents (organisms that drift only as larvae). When they die and sink, deep-sea decomposer plankton recycle them. The planktonic community is, in every sense, the invisible engine of the ocean.
Built to Float — Plankton Adaptations
Plankton can't swim against the current — but they're not entirely passive either. They have to migrate up and down in the water column, stay at a chosen depth, and resist sinking into the lightless deep. Evolution has given them five physical tools to do it. Tap each adaptation to see how it works.
Two Ways to Sort the Drifters
Marine scientists divide plankton in two different ways, depending on the question they're asking. Sort by how they get energy, or sort by how long they stay planktonic.
By energy source: phytoplankton vs zooplankton
Phytoplankton
~ producers · autotrophic
The producers. Single-celled photosynthetic organisms — they use sunlight + dissolved nutrients (carbon, nitrogen, phosphorus) to make their own food. Like plants, but drifting. They power every marine food chain.
~ diatoms · dinoflagellates · cyanobacteria · coccolithophores
Zooplankton
~ consumers · heterotrophic
The consumers. Animal or animal-like drifters that eat other organisms — usually phytoplankton or smaller zooplankton. The vital second step in every marine food web.
~ copepods · krill · jellyfish · larval fish & crabs
By residency: holoplankton vs meroplankton
Holoplankton
~ permanent residents
Organisms that spend their entire lifecycle drifting. From birth to death they never settle on the seabed and never become motile. Most phytoplankton are holoplankton; so are jellyfish and copepods.
~ diatoms · copepods · jellies · krill
Meroplankton
~ temporary residents
Organisms that drift only as eggs or larvae, then either settle to the seabed (corals, mussels) or grow into active swimmers (fish, octopus). Many of the adult animals you'll meet in later topics started life as meroplankton.
~ fish larvae · crab zoea · sea-urchin pluteus · barnacle nauplii
Two sort-orders, one organism
Any one planktonic organism can be classified under both systems at once. A diatom is phytoplankton + holoplankton. A baby anchovy drifting in the surface waters is zooplankton + meroplankton. A jellyfish is zooplankton + holoplankton. The two systems answer different questions — energy source vs how-long-do-you-drift — and you need both to describe a species fully.
The Vertical Commute
Plankton can't move horizontally — but they can move up and down. And they do it every day, on a daily light cycle, in the largest mass migration on planet Earth. Scientists call this diel vertical migration (from Latin dies = day).
~ surface at night, deep by day · driven by light + predators
Why it happens. At the surface during the day, plankton are easy meals — visual predators (fish, sea birds) can see them clearly in the sunlit water. So they sink to deeper, darker water where it's safer. At night, the surface waters are dark and the predators can't see them, but that's where the food (and warm water) is. So they rise to feed. Every day, billions of tonnes of zooplankton biomass complete this commute — driven entirely by light + predation pressure. It's the largest daily movement of biomass on the planet.
The Foundation of the Marine Food Web
Every marine food chain starts here. Without plankton, every fish, every whale, every shark, every penguin would starve within weeks.
~ phytoplankton at the bottom · only ~10% of energy passes to each higher level
It is common (and wrong) to say "whales eat plankton." Baleen whales actually eat krill and small fish, which eat zooplankton, which eat phytoplankton. Whales depend on the whole pyramid below them, but they aren't directly grazing on phytoplankton. The start of every marine food chain, though, is the single-celled plant-like drifters at the bottom of the pyramid. Take them away and the whole structure collapses.
~ a planetary lung
60–70%
The estimated percentage of Earth's total oxygen produced by marine phytoplankton. The Amazon rainforest gets the press; the ocean does most of the work. Every second breath you take was generated by microscopic ocean drifters.
Recyclers in the Dark
Plankton aren't only at the surface. Far below the sunlit zone, deep in the lightless cold, drift specialised heterotrophic plankton — protists and bacteria — that feed on dead surface plankton sinking down as "marine snow". They decompose the falling nutrients and release them back into the water, where deep-ocean currents eventually carry them upward again. This is why plankton aren't just producers — they're also the ocean's decomposers, closing the nutrient loop you met in Topic 16.
Local connection — the Benguela & the Sardine Run
Two of the most spectacular plankton stories on Earth are in South Africa:
- The Benguela Current (West Coast). Cold, nutrient-rich water rises (upwells) from the deep along the Western Cape coastline, dumping nitrogen and phosphorus into sunlit surface waters. The result is one of the most productive plankton zones on the planet — the reason West Coast fisheries (snoek, anchovy, sardine, hake) are so rich. Aquaculture (mussels, oysters) thrives in Saldanha Bay for the same reason.
- The Sardine Run (East Coast). Every year between May and July, billions of sardines (Sardinops sagax) follow a cold-water tongue up the KwaZulu-Natal coastline — the largest fish migration on Earth. They are following the plankton blooms that ride that water. Behind the sardines come dolphins, sharks, gannets, Bryde's whales and game-fish in one of the most-filmed wildlife events in the world. Phytoplankton triggers the whole thing.
Algoa Bay (Gqeberha) sits in the middle of this productivity belt, which is why this stretch of coastline has unusually high marine biodiversity and high populations of marine mammals.
Everywhere — but Not Evenly
CAPS states this concept twice, as key concept 4 and again as key concept 11 — which is a strong hint about how often it is examined. Plankton are found in every ocean on Earth. But they are not spread evenly, and the reason why explains almost everything about where fish are caught.
Planktonic communities are widespread, but generally limited by nutrient availability, temperature and sunlight.
That is CAPS's own wording, near enough. Wherever those three line up, phytoplankton multiply — and because they feed everything above them, the whole planktonic food chain follows. Wherever any one of them runs short, the water may look identical from a boat and be virtually empty.
The three limiting factors
1 · Nutrient availability
Mainly nitrates and phosphates. Usually the factor that runs out first, because sinking dead plankton constantly strip nutrients out of the surface and carry them down. Unless something brings them back up, the surface starves.
~ this is why upwelling matters so much — it is a nutrient delivery system.2 · Sunlight
Phytoplankton photosynthesise, so they can only live in the sunlit surface layer — roughly the top 200 m, and often much less in murky coastal water. No light, no primary production, no food chain.
~ depth and water clarity set the ceiling. You meet the physics in Topic 20.3 · Temperature
Controls metabolic rate, and — just as importantly — controls whether the water column mixes. A warm surface layer sitting on cold water forms a thermocline that acts as a lid, blocking nutrients from below.
~ CAPS names temperature in concept 4. Cold water is also more viscous, which slows sinking.CAPS adds "although there are other contributing factors" — grazing pressure from zooplankton, trace elements such as iron, turbulence and day length all play a part. Name the three, then mention that others exist.
Deserts and blooms — the same ocean
🏜️ Mid-ocean "deserts"
The centres of the great ocean gyres are warm, calm, permanently stratified and startlingly clear. That famous deep-blue tropical water is blue because there is almost nothing living in it. Plenty of light; almost no nutrients, because a permanent thermocline seals off the deep supply.
Result: very low productivity across enormous areas — the biological deserts of the sea.
🌊 Upwelling zones
Where wind and current drag surface water offshore, cold nutrient-rich water rises to replace it. Now nutrients and sunlight are in the same place at the same time, and phytoplankton bloom in enormous numbers. The water often looks green or murky — that murk is the life.
Result: upwelling covers roughly 1% of the ocean surface but supports around half the world's fish catch.
Local connection — South Africa has one of each coast
You can see both ends of this in one country, and the contrast is the clearest example in the world.
The West Coast (Benguela). Persistent south-easterly winds push surface water offshore and cold, nutrient-loaded water upwells to replace it. The result is one of the four great upwelling systems on Earth, and the reason the West Coast supports the sardine, anchovy and hake fisheries — plus the Cape fur seals, gannets and penguins that live off them.
The East Coast (Agulhas). The Agulhas Current is warm, fast and nutrient-poor, and it is strongly stratified. Beautiful water, superb diving visibility — and biologically far thinner. That is exactly why KwaZulu-Natal has coral and clear water while the West Coast has kelp and fish.
Algoa Bay sits between the two, which is why our productivity swings with the season and why the sardine run passes us on its way up the coast.
Key Vocabulary
Tap each card to flip it. The four "-plankton" distinctions (phytoplankton, zooplankton, holoplankton, meroplankton) come up regularly in CAPS questions — and so do the limiting factors and the production terms added below.
The community of marine organisms — the majority very small, but some up to 30 m — that drift suspended in ocean currents and cannot swim against them. From Greek planktos, "wanderer". A way of life, not a single species.
The autotrophic (producer) plankton. Single-celled photosynthesisers — diatoms, dinoflagellates, cyanobacteria, coccolithophores. The base of nearly every marine food chain and the source of ~60-70% of Earth's oxygen.
The heterotrophic (consumer) plankton. Animal or animal-like drifters that eat phytoplankton or smaller zooplankton. Includes copepods, krill, jellyfish, and the larvae of larger marine animals.
Permanent residents of the planktonic community — organisms that spend their entire lifecycle drifting. Most phytoplankton, plus jellies, copepods and krill. From Greek holos, "whole".
Temporary residents — organisms that drift only as eggs or larvae, then either settle to the seabed or develop into active swimmers. CAPS names the groups: the larval forms of echinoderms (sea-urchin pluteus), crustaceans (crab zoea, barnacle nauplii), worms (polychaete trochophores) and many fish species. From Greek meros, "part".
The daily up-and-down movement of planktonic communities — surfacing at night to feed in moonlit waters, then sinking to deeper, darker waters during the day to avoid visual predators. From Latin dies, "day". The largest daily mass migration on Earth.
The condition of floating suspended at a specific depth without using any energy to stay there — neither sinking nor rising. Achieved through jelly tissues, gas bladders, fat reserves and/or large surface area. Key plankton survival trait.
Able to propel itself independently from place to place. The opposite of sessile (fixed in place) and distinct from planktonic (drifts but can't fight the current). Fish, whales, octopus, dolphins, sharks — all motile.
Whatever is in shortest supply, and therefore sets the ceiling on how much a population can grow — no matter how plentiful everything else is. For plankton CAPS names three: nutrient availability, temperature and sunlight.
Wind and current drag surface water offshore, and cold nutrient-rich water rises from the deep to replace it. It puts nutrients and sunlight in the same place at once, so phytoplankton bloom. Upwelling covers about 1% of the ocean but supports roughly half the world's fish catch — the Benguela is one of four great systems.
Primary production is new organic matter built by producers — phytoplankton photosynthesising. Secondary production is new organic matter built by consumers eating those producers — zooplankton growing on phytoplankton. CAPS names both as roles plankton play. Photosynthetic plankton make up about 80% of ocean biomass.
An animal that strains food particles out of the water rather than hunting individual prey. CAPS's point: the small predators that drift within the plankton — fish, krill and others — themselves become food for large filter feeders such as whales. A baleen whale eats a whole trophic level at a time.
Quick Check
Question 1
Well done!
Next: Protists — the diverse single-celled eukaryotes, many of which spend their lives drifting as plankton.