Zooplankton are a diverse group of small, often microscopic, aquatic animals that drift within water bodies, largely unable to swim against currents. They are a heterotrophic component of the plankton community, meaning they must consume other organisms to obtain energy, unlike phytoplankton which produce their own food through photosynthesis. This fundamental difference in how they acquire nutrients sets the stage for the varied feeding strategies observed across the many types of zooplankton.
The Varied Diets of Zooplankton
Zooplankton diets span all three categories: carnivores, herbivores, and omnivores, reflecting their immense diversity. Some zooplankton are primary consumers, feeding on phytoplankton, which are the microscopic plant-like organisms at the base of aquatic food webs. Many copepods, a common type of zooplankton, predominantly consume phytoplankton. Other examples of herbivorous zooplankton include certain cladocerans, often referred to as water fleas.
Some zooplankton are carnivores, preying on other zooplankton, small invertebrate larvae, or even fish larvae. Chaetognaths, also known as arrow worms, are notable carnivorous zooplankton that use grasping spines to capture smaller zooplankton like copepods. Certain species of copepods, while many are herbivorous, also exhibit predatory behavior, actively hunting other zooplankton. Jellyfish are predominantly carnivores, stunning prey with stinging cells on their tentacles.
Many zooplankton are omnivores, consuming a mixed diet of both plant and animal matter. This flexible strategy allows them to adapt their diet to available food sources. Krill, shrimp-like crustaceans, are a well-known example of omnivorous zooplankton, feeding on both phytoplankton and smaller zooplankton. Some cyclopoid copepods also display omnivorous feeding behaviors, consuming both plant and animal material.
Zooplankton’s Role in Aquatic Ecosystems
The diverse feeding habits of zooplankton position them centrally within aquatic food webs, linking primary producers to higher trophic levels. As they consume phytoplankton, zooplankton facilitate the transfer of energy from these microscopic plants to larger organisms, including fish, marine mammals, and other invertebrates. They are a fundamental component in the diets of many aquatic species, supporting ecosystem biodiversity and health.
Zooplankton also contribute significantly to nutrient cycling within aquatic ecosystems. Through processes like excretion and egestion, they release dissolved organic matter and other nutrients back into the water column, which can then fuel further primary production by phytoplankton. This nutrient recycling is important for maintaining ecosystem productivity.
Zooplankton also play a role in the biological carbon pump, a process that sequesters carbon dioxide from the atmosphere into the deep ocean. When zooplankton consume carbon-rich phytoplankton, they incorporate this carbon into their bodies. Carbon can then be transported to deeper waters through their daily vertical migrations or when their fecal pellets and dead bodies sink, contributing to long-term carbon storage. This mechanism helps regulate atmospheric carbon dioxide levels.