Zooplankton are tiny aquatic animals, many of which are microscopic, that drift in the vast expanses of oceans, lakes, and rivers. These diverse organisms range from single-celled protozoa to small crustaceans and the larval stages of various marine and freshwater animals. Despite their small size, zooplankton are incredibly abundant, forming a fundamental part of aquatic food webs by consuming even smaller organisms and, in turn, becoming a crucial food source for a wide array of aquatic life.
Tiny Eaters: Microscopic and Small Predators
Microscopic and very small aquatic organisms prey on zooplankton. Carnivorous zooplankton, such as certain copepods and cladocerans, hunt and consume other, smaller zooplankton. For example, predatory copepods like Euchaeta use specialized appendages to capture and ingest smaller copepods or nauplii.
Protozoa, single-celled organisms, also consume the smallest zooplankton. Ciliates and flagellates, for instance, feed on bacteria and very small zooplankton. Larval stages of various aquatic invertebrates and fish, too small to consume adult fish, often rely on zooplankton as their primary food source. These interactions occur at a microscopic scale.
Fish and Aquatic Invertebrate Predators
Many fish species, especially small to medium-sized ones, rely on zooplankton for food. Filter-feeding fish, such as herring, sardines, and anchovies, strain large quantities of water through specialized gill rakers to capture zooplankton. These schooling fish connect zooplankton to higher trophic levels in marine ecosystems.
Freshwater environments also have many fish species that prey on zooplankton, including minnows and certain perch. These fish may filter-feed or actively pursue individual zooplankton. Larger aquatic invertebrates, like jellyfish and comb jellies (ctenophores), also prey on zooplankton, using tentacles or sticky cells to capture them. Larval crustaceans and juvenile squid also feed on zooplankton.
Marine Mammals and Seabirds
Large marine mammals and seabirds rely on zooplankton, especially krill, a type of larger zooplankton. Baleen whales, including blue whales and humpback whales, are filter feeders that consume large amounts of zooplankton. They use baleen plates in their mouths to sieve krill and other small organisms from the water, often consuming tons daily.
Seabirds also feed on zooplankton. Penguins, petrels, and auks dive into the water to feed on dense aggregations of zooplankton. While seals primarily eat fish, many seal species, such as crabeater seals, consume large quantities of krill, particularly in polar regions where krill biomass is high.
The Foundation of Aquatic Food Webs
Zooplankton link primary producers with higher trophic levels in aquatic food webs. They graze on phytoplankton, microscopic plants that convert sunlight into energy, transferring this energy up the food chain. This role is important for the productivity of aquatic environments.
The abundance and distribution of zooplankton directly influence the populations of fish, marine mammals, and seabirds that depend on them. A healthy zooplankton population supports fisheries and provides food for many marine and freshwater animals. Disruptions to zooplankton populations, due to climate change, pollution, or other factors, can affect the entire ecosystem, impacting species that rely on this food source.