Zooplankton, or animal plankton, are a diverse collection of small, heterotrophic organisms that drift in oceans and freshwater bodies. This category includes single-celled protozoans, crustaceans like copepods and krill, and the larval stages of fish and invertebrates. Due to this vast diversity, zooplankton cannot be classified into a single dietary group; they exhibit a full spectrum of feeding strategies, including herbivores, carnivores, and omnivores. The trophic role of any given species is determined by its morphology, life stage, and the availability of food resources.
Zooplankton as Primary Consumers (Herbivores)
A significant portion of the zooplankton community functions as primary consumers, specializing in the consumption of plant matter and bacteria. These herbivorous species form the second trophic level in aquatic food webs, grazing mainly on phytoplankton, the microscopic organisms that use photosynthesis. Calanoid copepods are prime examples of this group and often dominate marine and freshwater ecosystems. They employ a sophisticated suspension-feeding mechanism, using specialized appendages to create water currents that filter small particles of phytoplankton and detritus.
Many rotifers also fall into this herbivorous category, utilizing their coronal cilia to generate a vortex that directs food particles toward their mouths. This grazing activity influences water clarity and the overall health of primary production. By consuming phytoplankton, these consumers convert solar energy into a form accessible to higher trophic levels.
Zooplankton as Secondary and Tertiary Consumers (Carnivores)
Certain zooplankton species are dedicated predators, occupying roles as secondary or even tertiary consumers by targeting other animals. These carnivores primarily feed on smaller zooplankton, protozoa, or the larval stages of fish and invertebrates. Chaetognaths, commonly known as arrow worms, are prominent examples of these specialized predators, recognized by their transparent bodies and grasping spines used to capture prey. They are effective hunters that exert substantial predatory pressure on populations of smaller copepods and micro-zooplankton.
Other predatory zooplankton include certain cyclopoid copepods and large, drifting cnidarians like jellyfish and ctenophores (comb jellies). These raptorial feeders actively search for and seize their prey. Ctenophores use sticky or stinging cells to passively ensnare organisms that drift into their path. The ability of these carnivores to control smaller consumer populations shapes the structure and diversity of the zooplankton community.
Zooplankton as Flexible Feeders (Omnivores)
The majority of zooplankton species exhibit dietary flexibility, classifying them as omnivores capable of consuming both plant and animal matter. This adaptive strategy allows them to switch their primary food source depending on resource availability, which is an advantage in fluctuating aquatic environments. For instance, many larger copepods, such as certain Calanoid species, graze on abundant phytoplankton but readily switch to preying on smaller zooplankton when plant matter is scarce.
Omnivorous behavior is also common among cladocerans like Daphnia, which filter-feed on algae and bacteria but also capture small protozoans or rotifers. The ability to consume food from multiple trophic levels is important for species that undergo life-history omnivory. Cyclopoid copepods, for example, may be herbivorous as larvae but become carnivorous as mature adults. This dietary plasticity provides resilience, ensuring energy transfer continues even when one food source declines.
Ecological Significance and Energy Transfer
Regardless of their specific classification as herbivores, carnivores, or omnivores, the collective role of zooplankton is central to the transfer of energy throughout the global aquatic food web. They serve as a bridge, taking the energy stored in primary producers like phytoplankton and bacteria and making it accessible to higher trophic levels, including fish, whales, and seabirds. This process is driven by their sheer abundance and their ability to package dispersed organic material into larger, faster-sinking units.
Zooplankton are integral to the “biological pump,” the mechanism by which carbon is exported from the surface ocean to the deep sea, where it can be stored for long periods. They contribute to this process both passively, by producing dense fecal pellets that sink rapidly, and actively, through diel vertical migration. During this daily migration, zooplankton feed in the surface waters at night and descend to deeper, darker layers during the day. There, they respire and excrete carbon at depth, effectively sequestering it away from the atmosphere. Their diverse feeding habits ensure that energy and nutrients are constantly recycled and transferred, providing stability to the entire aquatic ecosystem.