IB ESS SLTopic 4 — Aquatic Food Production SystemsPaper 1 & 2Core idea~9 min read
Food Webs in Aquatic Systems
Almost every fish you have ever eaten owes its existence to organisms too small to see. Aquatic food webs rest on a base of microscopic photosynthesisers, and understanding that base is what makes the rest of this sub-topic — overfishing, aquaculture, acidification — make sense.
📚 What you need to know
Aquatic food webs show how energy and nutrients move through freshwater and marine ecosystems.
Phytoplankton are microscopic photosynthesising organisms — and they are not plants. They include algae such as diatoms, and cyanobacteria.
Macrophytes are aquatic plants big enough to see, and come in emergent, submerged and floating forms.
Both groups are producers: they capture solar energy as biomass, release oxygen and drive nutrient cycling.
Energy then passes to primary consumers (zooplankton, small fish), secondary consumers (larger fish and birds) and tertiary consumers (top predators such as sharks).
Decomposers — aquatic bacteria and fungi — break down dead material and return nutrients to the system.
Phytoplankton: the base of nearly everything
Phytoplankton are microscopic organisms found in both marine and fresh water that can photosynthesise. The word “plankton” describes how they live — drifting — not what they are.
They are not plants. They are a mixed group of autotrophic microorganisms.
They include algae, such as diatoms, and cyanobacteria, which are prokaryotes also known as blue-green algae.
They form the base of most aquatic food webs, capturing solar energy and converting it into biomass.
They are eaten by primary consumers such as zooplankton and small fish.
They contribute a large share of the planet’s oxygen production and drive nutrient cycling.
Calling phytoplankton “tiny plants” will cost you a mark. Cyanobacteria are bacteria, and diatoms are not plants either. Say “photosynthetic microorganisms” and you are safe.
Macrophytes: the visible producers
Macrophytes are aquatic plants visible to the naked eye. They split into three types by where they sit relative to the water surface.
All three photosynthesise and all three cycle nutrients, but they also do something phytoplankton cannot: provide physical habitat. Seagrass beds in particular are nursery grounds for young fish, which is why losing them costs a fishery as well as an ecosystem.
Energy flow through the web
Decomposers sit outside the ladder because they feed on dead material from every level. Bacteria and fungi break it down and return the nutrients to the water, where producers take them up again.
Level
Who is there
What they do
Producers
Phytoplankton and macrophytes
Capture sunlight by photosynthesis, produce oxygen, build biomass
Primary consumers
Zooplankton, small fish, some invertebrates and birds
Feed directly on the producers
Secondary consumers
Larger fish and birds
Eat the primary consumers
Tertiary consumers
Top predators such as sharks and birds of prey
Eat secondary consumers; usually few in number
Decomposers
Aquatic bacteria and fungi
Break down dead organisms and return nutrients to the ecosystem
Why the pyramid narrows. Only a small fraction of the energy at one level is passed on to the next — the rest is lost in respiration, movement and heat. That is why top predators are rare, why they are so vulnerable to overfishing, and why eating lower down a food chain feeds more people from the same energy.
Worked examples
EXAM Q1
Distinguish between phytoplankton and macrophytes. [2]
PhytoplanktonMicroscopic photosynthetic organisms — algae and cyanobacteria — that drift in the water and are not plants.
MacrophytesAquatic plants visible to the naked eye, growing emergent, floating or submerged.
Both are producers, but only one group is actually plantsthe size difference alone is not enough for both marks
EXAM Q2
Explain why there are far fewer sharks than small fish in a marine ecosystem. [3]
Point 1: energy is lost at each transfer
Only a small proportion of energy passes from one trophic level to the next; the rest is lost mainly as heat from respiration.
Point 2: less energy available higher up
Sharks are tertiary consumers, several transfers above the producers, so far less energy reaches them.
Point 3: consequence
That energy supports a much smaller total biomass, so top predator populations stay small.
Fewer sharks because there is less energy left at the topthis also explains why top predators recover so slowly from overfishing
💡 Exam tip
Use the correct level names: producer, primary consumer, secondary consumer, tertiary consumer, decomposer.
Say phytoplankton are not plants if the question gives you any opening. It is a classic discriminator.
Arrows in a food web always point in the direction energy flows, from the eaten to the eater.
Mention oxygen production and nutrient cycling when asked about the role of producers — not just food.
Learn the three macrophyte types with an example each.
Link energy loss to why fishing pressure on top predators is so damaging.
⚠ Common mix-up
Calling phytoplankton plants. They are algae and cyanobacteria.
Confusing phytoplankton with zooplankton. “Phyto” photosynthesises; “zoo” eats.
Drawing food web arrows backwards. They follow the energy, not the eating.
Forgetting decomposers. They are part of the web and carry a mark of their own.
Assuming macrophytes only matter as food. Their bigger role is habitat and shelter.
Treating a food web as a single chain. Most species feed at more than one level.
Up next: Rising Demand for Seafood — what we take out of these webs, and how fast that amount has grown.
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