Distinguish phytoplankton from zooplankton, explain their role as the base of marine food webs, and describe the microbial loop and how plankton support fisheries and the carbon cycle

What this dot point is asking

QCAA wants you to define plankton, distinguish the photosynthetic phytoplankton from the animal zooplankton, and explain why plankton are the foundation of almost every marine food web. You should also understand how plankton link to fisheries and to the ocean's role in the carbon cycle, which connects Unit 3 biology to Unit 4 climate.

What plankton are

The word plankton refers to lifestyle, not a single group. Plankton are organisms that float and drift with the water because they are too small or too weak to swim against currents. They contrast with nekton (strong swimmers such as fish and whales) and benthos (organisms living on or in the seabed). Plankton range from microscopic single cells to drifting jellyfish, but the small forms dominate.

Phytoplankton and zooplankton

• Phytoplankton are microscopic, single-celled photosynthetic organisms such as diatoms and dinoflagellates. They are the primary producers that begin most marine food webs, fixing energy by photosynthesis in the sunlit surface layer.
• Zooplankton are the animal plankton. They include tiny crustaceans (such as copepods and krill) and the larvae of fish, corals, crabs and many other animals that spend part of their life drifting before settling. Zooplankton graze phytoplankton and are themselves eaten by larger animals.

Because larvae drift as zooplankton, plankton are also the main way coral, fish and invertebrate offspring disperse between reefs, which is the basis of the connectivity studied elsewhere in Unit 3.

Plankton at the base of the food web

A typical marine food chain runs: phytoplankton to zooplankton to small fish to large fish to top predators. Because energy is lost at each transfer (the ten per cent rule from the energy-flow dot point), the vast biomass of phytoplankton supports progressively less biomass at each level above. This is why productive plankton zones, such as upwelling regions, support the world's biggest fisheries and feeding grounds for whales.

The microbial loop

Not all energy flows through the simple grazing chain. Much of the organic matter released by phytoplankton and by dead organisms is taken up by bacteria, which are then eaten by tiny protists and returned to the food web. This pathway, the microbial loop, recycles dissolved organic matter and nutrients that would otherwise be lost, and is especially important in nutrient-poor waters such as the open ocean and reef systems.

Plankton, fisheries and carbon

Plankton link Unit 3 biology to Unit 4 issues in two big ways.

• Fisheries. Fish stocks ultimately depend on phytoplankton productivity. Changes in plankton, from warming, nutrient shifts or pollution, propagate up to the fish that people catch.
• The carbon pump. When phytoplankton photosynthesise they remove carbon dioxide, and when they and their grazers die some of that carbon sinks to the deep sea. This biological pump is a major way the ocean stores carbon and slows climate change, which is developed in the Unit 4 carbon and acidification material.

Australian context

Krill and copepods in productive Australian waters feed fish, seabirds and migrating whales such as the humpbacks that pass the Queensland coast each year. Harmful algal blooms, when certain phytoplankton multiply rapidly under high nutrients, can produce toxins and oxygen crashes, an outcome linked to the nutrient-runoff impacts studied in Unit 4.