Almost everything that lives in water depends on organisms too small to see. Before you can explain overfishing, aquaculture or coral bleaching, you need to know who eats whom in the sea — and how little energy survives each step up the chain.
📘 What you need to know
Phytoplankton are microscopic photosynthetic organisms, and they form the base of most aquatic food webs.
Phytoplankton are not plants — they include algae such as diatoms, and cyanobacteria.
Macrophytes are aquatic plants big enough to see: emergent, submerged or floating.
Both are producers: they convert sunlight into biomass and release oxygen.
Energy passes from producers to primary, secondary and tertiary consumers, with heavy losses at each step.
Decomposers break down dead material and return nutrients to the system.
Phytoplankton and macrophytes
Both groups do the same job — capture sunlight — but they are completely different kinds of organism. That distinction is worth a mark on its own.
Calling phytoplankton “tiny plants” is the classic slip. They are autotrophic microorganisms, and cyanobacteria are not even in the same kingdom as plants.
Energy flow through the web
Aquatic food webs work like any other: energy enters through producers and moves upwards, losing most of itself at every transfer. Roughly 10% of the energy in one trophic level ends up in the next. The rest is lost as heat, in respiration, or in the parts that are never eaten.
The numbers are illustrative but the ratio is real. It takes an enormous amount of phytoplankton to support one shark, which is why heavily fished top predators recover so slowly.
Who sits where
Producers — phytoplankton and macrophytes, capturing sunlight by photosynthesis.
Primary consumers — zooplankton, small fish and some invertebrates and birds that feed on producers.
Secondary consumers — larger fish and birds that eat the primary consumers.
Tertiary consumers — top predators such as sharks, tuna and birds of prey.
Decomposers — aquatic bacteria and fungi that break down dead organisms and recycle nutrients back into the water.
Nutrients cycle, energy does not. Decomposers send nitrogen and phosphorus back round for reuse. Energy makes a one-way trip: in as sunlight, out as heat. Getting that distinction right is worth a mark in almost any food web question.
Notice how much of the rest of 4.3 sits on this page. Overfishing removes upper levels, acidification hits shell builders in the middle, and productivity (the HL sections) is about how much energy enters at the bottom.
Worked examples
WORKED EXAMPLE 1
Energy up the trophic levels
Phytoplankton in an area fix 12 000 kJ m−2 yr−1. Assuming 10% efficiency at each transfer, calculate the energy available to top predators at the fourth trophic level.
Step 1: producers to primary consumers12 000 × 0.1 = 1200 kJStep 2: to secondary consumers1200 × 0.1 = 120 kJStep 3: to top predators120 × 0.1 = 12 kJ12 kJ m−2 yr−1, or 0.1% of the originalthis is exactly why aquatic food chains rarely have more than four or five links
WORKED EXAMPLE 2
Placing organisms in the web
Give the trophic level of each: (a) a diatom, (b) copepod zooplankton, (c) a mackerel that eats zooplankton, (d) a seal that eats mackerel, (e) bacteria on the seabed.
(a) and (b)
Diatom = producer; copepod = primary consumer(c) and (d)
Mackerel = secondary consumer; seal = tertiary consumer(e)
Bacteria = decomposer, outside the numbered levels
Follow what each organism eats, not its sizea huge basking shark eats plankton, so it is a primary consumer despite its size
WORKED EXAMPLE 3
Explain why a fall in phytoplankton affects fish stocks [3]
Write a linked answer.
Link 1
Phytoplankton are the producers at the base of the web, so all other levels depend on them
Link 2
Fewer phytoplankton means less energy passing to zooplankton and small fish
Link 3
With only about 10% transferred at each step, the shortage is amplified upwardsCommercial fish populations fall, and top predators fall hardestuse the 10% figure — it turns a vague answer into a quantified one
💡 Exam tip
State clearly that phytoplankton are not plants. It is a favourite examiner check.
Give named examples: diatoms and cyanobacteria for phytoplankton; seagrass, bulrushes and water lilies for macrophytes.
Use the 10% rule whenever energy is mentioned. It explains pyramid shape, chain length and predator rarity.
Say energy flows, nutrients cycle.
Do not forget decomposers. They complete the system and appear in mark schemes.
Work out trophic level from diet, not size.
⚠ Common mix-up
Calling phytoplankton plants. They are autotrophic microorganisms, including bacteria.
Confusing phytoplankton with zooplankton. Phyto photosynthesises; zoo eats.
Thinking energy is recycled. Only nutrients are.
Assuming big animals are top predators. Whale sharks and baleen whales feed near the bottom of the web.
Leaving out macrophytes. They matter in shallow and freshwater systems, where they also provide habitat.
Drawing a food chain when asked for a web. A web shows multiple feeding links.
Up next: Rising Demand for Seafood — why the amount of aquatic food we eat has multiplied several times over in one lifetime, and where all of it now comes from.
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