zooplankton

Zooplankton are heterotrophic, mostly small aquatic organisms that drift or swim weakly in water. They occupy the second trophic level, feeding on phytoplankton, bacteria, and detritus, and in turn they are prey for many fishes, seabirds, and marine mammals. The term includes protozoans such as foraminiferans and radiolarians; crustaceans such as copepods and krill; and gelatinous zooplankton like hydromedusae and scyphomedusae.

Zooplankton are broadly categorized as holoplankton, which spend their entire lives in the plankton (for example copepods, krill, many jellyfish), and meroplankton, whose larvae are planktonic but later become non-planktonic adults (such as fish, mollusk, and some crustacean larvae).

Ecological roles are central to aquatic ecosystems. They link primary production to higher trophic levels and contribute to carbon cycling through fecal pellets and mucus, aiding the biological carbon pump. Many zooplankton species perform diel vertical migration, rising to the surface at night to feed and descending during the day, which helps transport carbon to deeper waters.

Distribution and habitat range widely, from oceans and seas to some freshwater systems, occupying surface to deep pelagic zones and under sea ice. Abundances vary with season, nutrient supply, temperature, and predation pressure. Climate change, ocean warming, acidification, and deoxygenation can alter their distributions, phenology, and community composition.

Research methods include net tows, optical and acoustic sensing, imaging systems, and molecular tools to identify and quantify groups, alongside laboratory studies to understand feeding, reproduction, and behavior.