Zooplankton are heterotrophic organisms that drift in the world’s oceans, seas, and freshwater bodies. These tiny organisms are the animal component of the plankton community, meaning they must obtain energy by consuming other life forms. They are widely distributed across aquatic environments, forming a collective of microscopic to near-visible life that is unable to swim against currents. This distinction is fundamental to understanding their place in every aquatic ecosystem.
Zooplankton are Consumers, Not Producers
The primary difference between producers and consumers lies in how they acquire the energy needed for survival and growth. Producers, or autotrophs, like phytoplankton and terrestrial plants, generate their own food, typically through photosynthesis. This process places them at the first trophic level in the food web.
Zooplankton are classified as heterotrophs, meaning they must ingest organic matter to gain energy and nutrients. As a result, they occupy the next trophic level, making them primary or secondary consumers in the aquatic food chain. They predominantly graze on phytoplankton, the microscopic producers of the sea, establishing them as the first link in the consumer chain.
The group includes a diverse range of feeding behaviors. Some zooplankton are carnivorous, feeding on smaller zooplankton, which elevates them to secondary consumers. This variety means the zooplankton community occupies multiple trophic positions within the aquatic web.
Defining the Zooplankton Group
The term zooplankton describes a vast and diverse collection of drifting organisms, ranging significantly in size and type. This group includes single-celled protozoans (radiolarians and ciliates) and multicellular organisms, such as small crustaceans like copepods and krill. Their size spans from microscopic organisms up to larger, gelatinous species like certain jellyfish.
The zooplankton assemblage is divided into two categories based on their life cycles: holoplankton and meroplankton. Holoplankton spend their entire lives as plankton, such as copepods and water fleas. They are permanent residents of the water column, maintaining their drifting lifestyle throughout development.
Meroplankton are temporary plankton, spending only a phase of their life cycle as larvae or juveniles in the water column. This group includes the larval stages of many larger marine and freshwater animals, such as fish, crabs, sea urchins, and mollusks. Once mature, meroplankton transition out of the plankton community to become free-swimming or bottom-dwelling adults.
The Critical Role in Aquatic Food Webs
Zooplankton serve as a foundational energy bridge, transferring the energy captured by primary producers up to higher trophic levels in aquatic food webs. By grazing on phytoplankton, they convert the microscopic primary production into a form of biomass that larger organisms can consume. This makes them a fundamental food source for small fish, filter-feeding whales, and numerous other marine invertebrates.
The volume of consumption by this group is substantial; microzooplankton alone consume between 59% and 75% of the total daily marine primary production. This grazing activity directly regulates the abundance and species composition of phytoplankton, influencing ecosystem productivity and overall health. The transfer of energy sustains commercial fisheries and marine megafauna.
Biogeochemical Cycling
Zooplankton are deeply involved in global biogeochemical cycles. Through their feeding and excretion, they facilitate the recycling of essential nutrients, such as nitrogen and phosphorus, back into the water. This nutrient regeneration supports the growth of new phytoplankton, fueling the aquatic ecosystem’s base.
The Biological Pump
Zooplankton play a substantial part in the biological pump, a process that transfers carbon from the surface ocean to the deep sea. When they consume carbon-rich phytoplankton, they package this carbon into dense fecal pellets that sink rapidly to the ocean floor. This mechanism sequesters atmospheric carbon dioxide into the deep ocean over long timescales.