Explain energy flow through marine food webs (producers, consumers, trophic levels, productivity) and the cycling of carbon and nitrogen in marine ecosystems

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.

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.

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.