Zooplankton are heterotrophic organisms (mainly copepods, krill, and invertebrate larvae) that feed on phytoplankton and transfer primary production up the food web to fish and marine mammals. Zooplankton abundance, body size, and community composition are sensitive to water temperature, phytoplankton food quality, and predation, making them important indicators of ecosystem change.
From your study of marine phytoplankton and primary production, you know that photosynthetic microorganisms in the surface ocean fix carbon and form the base of marine food webs. But phytoplankton cannot directly feed a whale or a tuna — that energy must be packaged and transferred upward through the food web. Zooplankton are the critical link that makes this transfer happen, and their community structure determines how efficiently solar energy captured by phytoplankton reaches fish, seabirds, and marine mammals.
The term zooplankton covers an enormous diversity of organisms united by two traits: they are heterotrophic (they eat rather than photosynthesize) and they drift with currents rather than swimming strongly against them. The most abundant group is copepods — tiny crustaceans typically 1–5 mm long that dominate the zooplankton in nearly every ocean. A single copepod can filter thousands of phytoplankton cells per day using feathery appendages that create feeding currents. Krill (small shrimp-like crustaceans, 1–6 cm) are the dominant zooplankton in polar waters and the primary food of baleen whales, penguins, and many fish species. Other zooplankton include jellyfish, arrow worms (chaetognaths), salps, and the larval stages of organisms that become bottom-dwelling adults — crab larvae, sea urchin larvae, and fish larvae all spend weeks as zooplankton before settling.
The structure of the zooplankton community profoundly affects how energy flows through the food web. A key concept is food chain length. In productive upwelling regions with large diatoms, large copepods and krill eat the phytoplankton directly, and fish eat the zooplankton — a short, efficient chain with only two or three steps between sunlight and a commercially harvested fish. In nutrient-poor subtropical gyres where tiny picoplankton dominate, the food chain is longer: picoplankton are eaten by microzooplankton (protists), which are eaten by small copepods, which are eaten by larger zooplankton, which are finally eaten by fish. Each transfer loses roughly 80–90% of the energy as metabolic heat, so a five-step chain delivers far less energy to top predators than a three-step chain. This is why highly productive upwelling zones support major fisheries while the vast subtropical gyres, despite their enormous area, support comparatively little harvestable fish biomass.
Zooplankton also perform one of the ocean's most remarkable daily migrations. Many species undertake diel vertical migration — rising to the surface at night to feed on phytoplankton under cover of darkness, then descending hundreds of meters at dawn to avoid visual predators like fish. This nightly migration is the largest synchronized animal movement on Earth, and it has major consequences for ocean carbon cycling: zooplankton feed in the surface, then metabolize and excrete at depth, actively transporting carbon from the sunlit zone to the deep ocean. Changes in zooplankton community composition — driven by warming, acidification, or shifts in phytoplankton availability — therefore ripple through the entire marine ecosystem, affecting everything from fisheries yields to the ocean's capacity to sequester atmospheric carbon.