Energy flow and nutrient cycling in marine ecosystems (QCE Marine Science Unit 3)
“Explain energy flow through marine food webs (producers, consumers, trophic levels, productivity) and the cycling of carbon and nitrogen in marine ecosystems”
Energy enters marine ecosystems through producers such as phytoplankton and zooxanthellae, which fix it into biomass (gross primary productivity), and passes up food webs at roughly 10 per cent efficiency per trophic level, with the rest lost as respiratory heat and waste. Carbon and nitrogen, by contrast, cycle: they are taken up by producers, passed through consumers, and returned to the water by decomposers and physical processes such as the biological carbon pump.
Jump to a section
What this dot point is asking
QCAA wants you to track energy from the sun through marine producers and consumers, read and build marine food webs, calculate productivity using the 10 per cent rule, and explain how carbon and nitrogen cycle through the ocean. Numerical productivity questions and food-web stimulus appear most years.
Energy flow in marine food webs
Marine producers
Marine producers fix energy into organic molecules.
- Phytoplankton (diatoms, dinoflagellates, cyanobacteria) drift in the sunlit surface and are the dominant producers of the open ocean. They account for roughly half of all photosynthesis on Earth.
- Macroalgae and seagrasses (such as Zostera and Halophila in Moreton Bay) are major producers in shallow coastal systems.
- Zooxanthellae, the symbiotic dinoflagellates inside coral polyps, produce most of the energy that powers a coral reef.
- Chemosynthetic bacteria at hydrothermal vents fix energy from chemicals, not light, supporting deep-sea communities with no sunlight.
Marine consumers and trophic levels
- Primary consumers (zooplankton, krill, herbivorous fish, dugongs) eat producers. Trophic level 2.
- Secondary consumers (planktivorous and small predatory fish) eat primary consumers. Trophic level 3.
- Tertiary and apex consumers (tuna, sharks, dolphins) sit at the top. Trophic levels 4 and 5.
- Decomposers and detritivores (bacteria, sea cucumbers, crabs) break down dead matter and recycle nutrients.
A simple Great Barrier Reef chain runs zooxanthellae to coral polyp to parrotfish to grey reef shark. A Southern Ocean chain runs phytoplankton to Antarctic krill to baleen whale, one of the shortest and most efficient food chains on Earth.
Productivity and the 10 per cent rule
Gross primary productivity (GPP) is the total energy fixed by producers. Net primary productivity (NPP) is what remains after the producers' own respiration: NPP equals GPP minus respiration. Only NPP is available to consumers.
On average only about 10 per cent of the energy at one trophic level is built into biomass at the next. The other 90 per cent is lost as respiratory heat, movement and indigestible waste. This is why food chains are short and why apex predators such as tuna are rare relative to the plankton that ultimately feed them.
Suppose reef phytoplankton fix an NPP of 5000 kJ per square metre per year.
- Primary consumers (zooplankton): 10 per cent of 5000 = 500 kJ per square metre per year.
- Secondary consumers (small fish): 10 per cent of 500 = 50 kJ per square metre per year.
- Tertiary consumers (reef shark): 10 per cent of 50 = 5 kJ per square metre per year.
After three transfers only 5 kJ per square metre per year (0.1 per cent of the original) reaches the shark, which sets the upper limit on how many top predators the reef can support.
Nutrient cycling
Energy flows one way and is lost, but matter cycles. Two cycles are central in Marine Science.
The marine carbon cycle
Carbon dioxide dissolves into surface water and is fixed by producers during photosynthesis. It passes up the food web as organic carbon and is returned to the water through respiration and decomposition. A key marine process is the biological carbon pump: dead plankton, faecal pellets and other matter sink from the surface into the deep ocean, carrying carbon down and storing it for centuries. Carbon is also locked into the calcium carbonate skeletons of corals and shells, some of which becomes limestone over geological time. The ocean is the largest active carbon sink on Earth, which links directly to climate change in Unit 4.
The marine nitrogen cycle
Nitrogen is often the limiting nutrient for marine producers. Nitrogen-fixing cyanobacteria (such as Trichodesmium, which forms blooms in the Coral Sea) convert dissolved nitrogen gas into ammonium. Bacteria then convert ammonium to nitrite and nitrate (nitrification), which producers take up. Decomposers release nitrogen from dead matter (ammonification), and denitrifying bacteria return nitrogen gas to the water and atmosphere. Upwelling returns deep nitrate to the surface, fuelling productive fisheries.
- Confusing GPP and NPP
- GPP is total photosynthesis; NPP is what is left after producer respiration. Only NPP feeds consumers.
- Saying nutrients flow like energy
- Energy flows one way and is lost; carbon and nitrogen cycle and are reused.
- Drawing food-web arrows backwards
- Arrows point from prey to predator, the direction of energy flow.
- Forgetting chemosynthesis
- Not all marine producers use light; vent bacteria fix energy from chemicals.
Why this matters
Energy flow and nutrient cycling explain why coral reefs thrive in nutrient-poor water (tight internal recycling), why removing an apex predator can reshape a whole food web, and why the ocean is central to the global carbon budget. These ideas underpin the connectivity of marine systems in the rest of Unit 3 and the climate and fisheries issues of Unit 4.
Exam-style questions
Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice5 marksPhytoplankton in a coastal system fix a net primary productivity of . Using the 10 per cent rule, determine the energy available to a tertiary consumer (trophic level 4), and explain why marine food chains are typically short.Show worked answer →
Five marks: the calculation across trophic levels, then the explanation.
Apply the 10 per cent rule (3 marks).
Primary consumers (level 2): .
Secondary consumers (level 3): .
Tertiary consumers (level 4): .
Explain short chains (2 marks). About 90 per cent of energy is lost at each transfer as respiratory heat, movement and indigestible waste, so after a few transfers too little energy remains to support another level. This limits chains to roughly four or five links and makes apex predators rare.
Markers reward correct working to and the energy-loss explanation.
Practice4 marksExplain the role of the biological carbon pump in the marine carbon cycle and why it makes the ocean an important carbon sink.Show worked answer →
Four marks: the mechanism, then the significance.
The pump (2 marks). Surface phytoplankton fix dissolved carbon dioxide into organic carbon during photosynthesis. When they die, sinking dead cells, faecal pellets and aggregates carry this organic carbon from the surface into the deep ocean.
Carbon sink (2 marks). Because the carbon is transported below the surface mixed layer, it is stored in the deep ocean for centuries before returning, removing carbon dioxide from contact with the atmosphere and making the ocean the largest active carbon sink on Earth.
Markers reward the downward transport of organic carbon and the link to long-term storage away from the atmosphere.