Phytoplankton are eaten by an enormous range of organisms, from microscopic animals drifting in the same water column to shellfish anchored on the seafloor. As the base of nearly every aquatic food web, phytoplankton feed the ocean and freshwater ecosystems alike, transferring solar energy up through the food chain to fish, whales, and eventually humans. The most important consumers, by sheer volume, are tiny animals collectively called zooplankton.
Zooplankton: The Primary Consumers
The biggest share of phytoplankton is consumed by zooplankton, a broad category that includes both microscopic grazers and slightly larger crustaceans. These animals drift through the water column feeding on phytoplankton cells the way cattle graze on grass. The main groups include copepods, cladocerans (water fleas), and rotifers, each targeting different sizes and types of phytoplankton.
Copepods are small crustaceans found in virtually every body of water on Earth, and they are likely the single most abundant multicellular animals in the ocean. They filter or actively capture phytoplankton cells using specialized feeding appendages. Cladocerans, particularly the genus Daphnia, are among the most efficient phytoplankton grazers in freshwater lakes. They pull water through a fine mesh of bristles on their legs, trapping algal cells in the process. Rotifers, which are even smaller, handle the tiniest phytoplankton that larger grazers miss. In some nutrient-poor lakes, rotifers and copepods can exert as much grazing pressure on phytoplankton as the larger cladocerans.
These zooplankton don’t eat indiscriminately. Feeding experiments in Lake Constance found that all zooplankton species tested preferred small, motile phytoplankton cells (called phytoflagellates) over round, nonmotile green algae, regardless of how abundant either type was. Certain phytoplankton species were consistently avoided altogether. Meanwhile, only specific zooplankton could handle larger prey: filamentous algae were efficiently consumed only by one Daphnia species, and large diatom colonies were grazed well only by a type of copepod called Cyclops. This selectivity means the composition of the zooplankton community directly shapes which phytoplankton species thrive and which get eaten.
Filter-Feeding Shellfish
Mussels, oysters, clams, and other bivalves are powerful phytoplankton consumers in coastal and estuarine waters. These animals pump water through their gills, trapping phytoplankton cells on sticky mucus before directing them to the mouth. A single adult oyster can filter roughly 190 liters (50 gallons) of water per day, stripping out phytoplankton as it goes.
Bivalves show their own feeding preferences. Blue mussels, for instance, filter medium and larger phytoplankton cells far more efficiently than the smallest ones (those under about 2 micrometers). This means the tiniest phytoplankton often pass right through their gills. In bays with dense mussel or oyster beds, this constant filtration can dramatically reduce phytoplankton concentrations, improving water clarity and reshaping the local ecosystem.
Krill and the Link to Marine Mammals
Krill are shrimp-like crustaceans, typically 1 to 6 centimeters long, that swarm in enormous numbers in cold ocean waters. They feed directly on phytoplankton and represent one of the most important steps in the marine food chain. In the Southern Ocean, krill concentrate iron and other trace elements from phytoplankton into their tissue at roughly three times the concentration found in the phytoplankton itself.
Baleen whales, including blue whales and fin whales, don’t eat phytoplankton directly. Instead, they consume massive quantities of krill, making them secondary consumers. Before commercial whaling, blue and fin whales were the dominant krill consumers in the Southern Ocean. Their feeding and defecation cycle plays a surprisingly important role in recycling nutrients like iron back into surface waters, which in turn fuels more phytoplankton growth. It’s a loop: phytoplankton feed krill, krill feed whales, and whale waste fertilizes phytoplankton.
Seafloor Organisms
Phytoplankton don’t only feed animals that swim or drift. Organisms anchored to the seafloor, rocks, or other hard surfaces also capture phytoplankton from the water column. Sponges and sea squirts (ascidians) are dedicated filter feeders that draw water through their bodies and extract phytoplankton cells. In some coastal food webs, these bottom-dwelling filter feeders are significant enough that predators like reef crabs preferentially target them as a food source, creating another indirect link in the phytoplankton food chain.
Viruses: A Hidden Killer
Not all phytoplankton loss comes from being eaten. Marine viruses infect and burst open phytoplankton cells in a process called viral lysis, and the scale is staggering. In the Southern Ocean, viral lysis accounted for roughly 58% of seasonal phytoplankton carbon losses for small phytoplankton, making viruses an even larger source of mortality than grazing by zooplankton during certain periods. When viruses destroy phytoplankton, the cell contents spill into the water as dissolved organic matter, feeding bacteria rather than zooplankton. This diverts energy away from the traditional food chain and into what scientists call the “microbial loop.”
Why Phytoplankton Grazing Matters
The transfer of energy from phytoplankton to the animals that eat them powers fisheries, supports marine biodiversity, and influences the global carbon cycle. The efficiency of this transfer varies depending on conditions. Under low light and high nutrient availability, phytoplankton tend to be more nutritious (better cell composition and nutrient content), and energy moves more efficiently up to herbivores and then to fish. Under less favorable conditions, the transfer is leakier, and less energy reaches higher levels of the food chain.
The classic rule of thumb is that about 10% of the energy at one level of a food chain reaches the next, but real ecosystems are messier. In some experimental setups, zooplankton appeared to gain more energy than the phytoplankton alone could account for, likely because they supplemented their diet with bacteria, other zooplankton, or algae growing on surfaces. The point remains: phytoplankton are the essential first course. Without them, the entire aquatic food web collapses from the bottom up.