Zooplankton are a diverse group of small animals that drift in oceans, seas, and freshwater bodies. Their name comes from the Greek words for “animal” and “to drift,” defining them by their ecological niche rather than a shared genetic lineage. Unlike phytoplankton, which are microscopic plants, zooplankton are consumers. They inhabit aquatic environments globally, from the sunlit surface to the deep sea, making them a fundamental component of these ecosystems.
The Microscopic Menagerie: Types of Zooplankton
Zooplankton are broadly divided into two categories based on their life cycles. The first group, holoplankton, consists of organisms that are planktonic for their entire lives. This group includes some of the most numerous animals on Earth, such as copepods, krill, and gelatinous salps. It also includes microscopic freshwater rotifers and cladocerans, often called water fleas.
The second category is meroplankton, which includes animals that spend only their earliest stages as plankton. These are the larval forms of larger creatures such as crabs, barnacles, sea stars, clams, and many fish. This planktonic phase allows for wide dispersal before they mature and settle into their adult habitats on the seafloor or as free-swimming animals.
The diversity of zooplankton includes single-celled organisms like foraminifera and multicellular animals. While many are microscopic, some species of jellyfish and colonial siphonophores can be quite large. This range means zooplankton represents a cross-section of nearly the entire animal kingdom.
Keystones of Aquatic Ecosystems: Ecological Significance
Zooplankton are a primary link in aquatic food webs, connecting phytoplankton with larger animals. As primary consumers, they graze on phytoplankton, becoming a food source for organisms including small fish, the young of commercial species, and filter-feeding giants like baleen whales. This transfer of energy sustains a vast array of marine life.
Zooplankton also play a part in nutrient cycling. As they consume phytoplankton, they excrete waste products like nitrogen and phosphorus. This remineralization releases nutrients back into the sunlit zone of the water, where they can be used again by phytoplankton for growth, sustaining primary production, especially in nutrient-limited ocean regions.
These organisms contribute to the biological carbon pump, which moves carbon from the surface to the deep sea. Zooplankton consume carbon-rich phytoplankton at the surface. This carbon is transported to deeper waters when the zooplankton migrate, or through the sinking of their fecal pellets and bodies, sequestering it in the deep ocean.
The health of zooplankton communities can serve as an indicator of environmental conditions. Many species are sensitive to changes in water temperature, chemistry, and pollution. Shifts in their populations can signal broader ecological changes, so scientists monitor them to assess water quality.
The Daily Lives of Drifters: Behavior and Life Cycles
Many zooplankton species perform a diel vertical migration (DVM), the largest daily migration of biomass on Earth. At dusk, they ascend from deeper waters to the surface to feed on phytoplankton under the cover of darkness. As dawn approaches, they descend back into the depths to avoid visual predators, a daily trade-off between finding food and safety.
Feeding strategies among zooplankton are highly varied. Some, like copepods and krill, are filter feeders that strain small particles from the water. Others are active predators, such as the arrow worm, which is an ambush predator that uses bristles around its mouth to snatch prey.
Reproductive strategies are geared toward rapid population growth when conditions are favorable, leading to massive blooms. Some freshwater zooplankton, like cladocerans and rotifers, have adaptations to survive harsh conditions. They produce durable resting eggs that can withstand drying and freezing, allowing populations to rebound later.
Zooplankton in a Changing World: Threats and Resilience
Zooplankton face numerous threats from environmental change. Rising water temperatures can alter their metabolism, growth rates, and life cycles, and force shifts in their geographic distribution. Ocean acidification, from the absorption of excess carbon dioxide, poses a risk to zooplankton that build calcium carbonate shells, like pteropods and foraminifera, by hindering their ability to form these structures.
Pollution is another challenge. Microplastics are often mistaken for food by zooplankton, and ingestion can cause internal damage and reduce their feeding capacity. Chemical contaminants also accumulate in zooplankton and are then transferred up the food web to fish, marine mammals, and humans.
These pressures have consequences for entire ecosystems. A decline in zooplankton can reduce the food available for commercial fisheries and marine megafauna. Changes in the community can also alter the efficiency of the biological carbon pump. While their rapid reproduction offers some resilience, the pace of environmental change presents a formidable challenge.