Zooplankton

Zooplankton are the drifting animals of the water column, from protozoan microzooplankton through copepods and krill to gelatinous salps and jellyfish, spanning the consumers between phytoplankton and fish. Copepods are the most abundant multicellular animals on Earth. They drive the biological pump: grazing repackages small algal cells into fast-sinking fecal pellets, and diel vertical migration actively transports carbon and nutrients hundreds of meters down each day. Their ammonium and DOC excretion fuels regenerated production, and their nightly migration is the source of the deep scattering layer seen on echosounders.