Zooplankton are tiny animals and animal-like organisms that drift in oceans, lakes, and rivers, forming one of the most critical links in aquatic food webs. They range from single-celled creatures smaller than a grain of sand to jellyfish several meters across, and despite their often microscopic size, they collectively shape the chemistry of the ocean, regulate algal blooms, feed most of the world’s fish, and move billions of tons of carbon from the surface to the deep sea every year. Understanding what they are and what they do reveals how profoundly aquatic ecosystems depend on organisms most people never see.
What Counts as Zooplankton
The word “plankton” comes from the Greek for “wanderer,” and it refers to any organism that drifts with water currents rather than swimming strongly enough to determine its own path. Zooplankton are the animal members of that drifting community, as opposed to phytoplankton, which are the plant-like, photosynthesizing members. The definition is ecological rather than taxonomic: zooplankton include crustaceans like copepods and krill, the larvae of crabs and sea urchins, arrow worms, tiny snails called pteropods, jellyfish, and even single-celled predators like certain ciliates and dinoflagellates. A jellyfish several meters wide qualifies as zooplankton because, despite its size, it cannot swim effectively against ocean currents.
Scientists commonly divide zooplankton by size. The smallest, nanoplankton, measure just 2 to 20 micrometers. Microplankton range from 20 to 200 micrometers, mesoplankton from 0.2 to 20 millimeters, macroplankton from 2 to 20 centimeters, and megaplankton from 20 to 200 centimeters.1Research Starter. Zooplankton These size classes matter because organisms of different sizes eat different food, sink at different rates, and play different roles in nutrient cycling.
Another useful distinction is between holoplankton and meroplankton. Holoplankton spend their entire lives drifting: copepods, krill, and jellyfish fall into this group. Meroplankton are temporary visitors, the larval stages of animals that eventually settle on the seafloor as adults, including barnacles, sea stars, clams, and worms. A study using DNA barcoding in the Barents Sea identified 72 meroplankton taxa from eight phyla, with larvae present year-round and peaking in abundance not during the spring phytoplankton bloom, as traditionally expected, but in August and November.2Frontiers in Marine Science. Meroplankton Diversity, Seasonality and Life-History Traits Across the Barents Sea Polar Front Revealed by High-Throughput DNA Barcoding That finding hints at how much we still have to learn about when and where zooplankton appear.
Controlling Algal Blooms From the Top Down
One of the most immediate things zooplankton do is eat phytoplankton. That grazing pressure is not a minor footnote; it is one of the main forces determining whether microscopic algae grow steadily or explode into harmful blooms. When zooplankton graze actively, they keep phytoplankton populations in check. When grazing pressure drops, algae can proliferate unchecked.
Modeling work has shown that zooplankton grazing can delay the start of the spring phytoplankton bloom by about three weeks in the Southern Ocean and controls the bloom’s peak size.3Journal of Geophysical Research: Biogeosciences. The Role of Zooplankton Grazing and Nutrient Recycling for Global Ocean Biogeochemistry and Phytoplankton Phenology The selectivity of that grazing also matters. When zooplankton are not picky about which algae they consume, their own populations tend to decline, giving phytoplankton more room to bloom.4Ecological Modelling. Selective grazing of zooplankton on phytoplankton defines rapid algal succession and blooms in oceans
This relationship plays out dramatically during harmful algal blooms. A study of blooms caused by the toxic dinoflagellate Dinophysis in New York estuaries found that low grazing pressure by tiny protozooplankton at the start of bloom season allowed the harmful algae to take hold. Once blooms peaked, grazing rates exceeded the algae’s growth rate and helped bring the bloom down. Interestingly, juvenile copepods sometimes promoted blooms by eating the protozooplankton that would otherwise have consumed the harmful algae, creating a cascading effect.5PubMed. Zooplankton grazing can facilitate and control the proliferation of harmful algal blooms caused by Dinophysis acuminata in NY, USA, estuaries The takeaway is that zooplankton do not simply suppress blooms; their community composition and feeding behavior determine whether blooms start, how bad they get, and when they end.
Within the zooplankton community itself, microzooplankton, the smallest grazers like ciliates and heterotrophic dinoflagellates, exert outsized pressure on primary producers compared to the larger mesozooplankton.6Science of The Total Environment. Global change alters coastal plankton food webs by promoting the microbial loop In the Arabian Sea, these microzooplankton dominated in both numbers and biomass, accounting for up to 99% of individuals in their size class.7Deep Sea Research Part II: Topical Studies in Oceanography. The structure of zooplankton communities, in the 2 to 2000 μm size range, in the Arabian Sea during and after the SW monsoon, 1994 They are the unsung workhorses of grazing.
Moving Carbon to the Deep Ocean
Beyond their role in food webs, zooplankton are major players in the ocean’s carbon cycle. Every day, vast numbers of zooplankton undertake what is known as diel vertical migration: they rise from depths of hundreds of meters to feed near the surface at night, then sink back down during the day to avoid visual predators. This daily commute is the largest synchronized animal migration on Earth.
The climate significance of this behavior is substantial. When zooplankton feed on carbon-rich phytoplankton at the surface and then metabolize that food at depth, they effectively shuttle carbon from the sunlit upper ocean into the twilight zone and beyond. Three main pathways accomplish this: the production of fast-sinking fecal pellets, the excretion of dissolved organic matter, and respiration of carbon dioxide at depth.8PubMed. Active Carbon Transport by Diel Vertical Migrating Zooplankton: Calculated and Modeled, but Never Measured Carbon deposited below roughly 1,000 meters can remain sequestered for decades to millennia, effectively removing it from the atmosphere.
Fecal pellets alone represent a meaningful fraction of the ocean’s carbon export. A meta-analysis found that zooplankton fecal pellets contributed between about 5% and 22% of the total particulate organic carbon sinking out of the surface ocean, with the proportion varying by ecosystem productivity.9ICES Journal of Marine Science. Meta-analysis of the role of zooplankton faecal pellets in ocean carbon export flux In certain high-productivity regions, fecal pellet carbon accounted for as much as 91% of the particulate organic carbon sinking to depth.10Biogeosciences. The contribution of zooplankton faecal pellets to deep-carbon transport in the Scotia Sea (Southern Ocean)
Climate change may actually intensify some of these carbon export mechanisms in certain regions. Modeling of the North Pacific suggests that even as the total biomass of migrating zooplankton declines under warming, the efficiency of carbon transfer through the twilight zone can increase, driven by shifts in particle size and reduced breakdown of sinking material in oxygen-depleted waters.11Limnology and Oceanography Letters. Zooplankton diel vertical migration enhances carbon export via distinct mechanisms in a warming North Pacific The relationship between zooplankton and carbon export is not a simple one, and researchers are actively exploring whether artificial light cues at the ocean surface could manipulate migration patterns to boost carbon sequestration.12PubMed Central. The biological carbon pump, diel vertical migration, and carbon dioxide removal
Zooplankton grazing also recycles nutrients back into the surface ocean. Fast nutrient recycling by zooplankton sustains higher concentrations of chlorophyll, the pigment phytoplankton use for photosynthesis, during summer and autumn.13Journal of Geophysical Research: Biogeosciences. The Role of Zooplankton Grazing and Nutrient Recycling for Global Ocean Biogeochemistry and Phytoplankton Phenology So zooplankton simultaneously move carbon down and keep surface productivity going, a dual role that makes them indispensable to the ocean’s chemistry.
Jellyfish, Salps, and the Gelatinous Carbon Express
Not all zooplankton are small crustaceans. Gelatinous zooplankton, a group that includes jellyfish, comb jellies, and pelagic tunicates like salps, turn out to have a surprisingly large footprint in the carbon cycle. Their watery, low-density bodies might seem like poor vehicles for carbon transport, but the opposite is true. When gelatinous organisms die, their carcasses sink rapidly and can reach the seafloor largely intact.
A global modeling study estimated that gelatinous zooplankton consume roughly 8 to 13 billion metric tons of carbon per year from phytoplankton and other zooplankton. Their combined fecal matter and carcass export at 100 meters depth amounted to about 1.6 to 5.2 billion metric tons of carbon per year, equivalent to roughly a third to 40% of total global particulate organic carbon export. The transfer efficiency of that sinking material was remarkably high, with 38 to 62% of it reaching 1,000 meters and 25 to 40% making it all the way to the seafloor.14Global Biogeochemical Cycles. Gelatinous Zooplankton‐Mediated Carbon Flows in the Global Oceans: A Data‐Driven Modeling Study Jelly-falls at depths below 50 meters may be largely unaccounted for in current estimates of how much carbon reaches the ocean bottom, meaning the real contribution could be even larger.
Salps, barrel-shaped tunicates that form chains and occasionally bloom in enormous numbers, are particularly efficient. In the subarctic Northeast Pacific, salp fecal pellets made up as much as 82% of all the particulate organic carbon produced as fecal pellets by the entire surface zooplankton community. Those pellets sank at speeds of 400 to 1,200 meters per day, and microbial breakdown was slow, less than 1% of pellet carbon respired per day, meaning most of the material reached the deep ocean intact.15PubMed Central. The Outsized Role of Salps in Carbon Export in the Subarctic Northeast Pacific Ocean When salp blooms occur, they can briefly dominate the biological carbon pump in a region.
Keystone Species Everyone Depends On
Certain zooplankton species punch far above their weight in terms of ecosystem importance. Antarctic krill, a shrimp-like crustacean only a few centimeters long, sustains whales, seals, penguins, and numerous fish and seabird species across the Southern Ocean.16PubMed Central. Driving forces of Antarctic krill abundance Krill also support a commercial fishery, mostly for aquaculture feed and nutritional supplements. Climate change is altering krill habitat, and shifts in krill abundance ripple through the entire food web.
In the Arctic, a group of copepods in the genus Calanus occupy a similarly pivotal role. These copepods store large quantities of energy-rich lipids in their bodies, making them an exceptionally high-calorie food source. Warming in the Arctic has been predicted to shift dominance from the larger, lipid-rich Arctic species toward the smaller boreal species Calanus finmarchicus, which could reduce the energy available to predators like fish, seabirds, and marine mammals. However, research has shown that lipid content tracks body size across all three species rather than being a fixed species-specific trait, and there is considerable size overlap between species, which could provide some resilience against the worst-case scenarios.17ICES Journal of Marine Science. Pelagic food-webs in a changing Arctic: a trait-based perspective suggests a mode of resilience
What Climate Change Is Doing to Zooplankton
Warming oceans are already reshaping zooplankton communities in three consistent ways documented across long-term monitoring programs worldwide: shifts in seasonal timing, poleward movement of geographic ranges, and a trend toward smaller body sizes in warmer conditions.18Nature Communications. Monitoring and modelling marine zooplankton in a changing climate Spring species tend to appear earlier in the year, autumn species later, and warm-water species are expanding toward the poles. This matters because zooplankton are the bridge between phytoplankton and fish. If zooplankton shift their timing but the phytoplankton they eat or the fish larvae that eat them do not shift at the same rate, the result is a trophic mismatch, a gap in the food web that can cause recruitment failures in fish populations.19Journal of Plankton Research. Marine plankton phenology and life history in a changing climate: current research and future directions
Ocean acidification poses a different kind of threat, especially to zooplankton that build calcium carbonate shells. Pteropods, small swimming snails sometimes called “sea butterflies,” are among the most vulnerable. In the California Current Ecosystem, about 53% of nearshore pteropod individuals and 24% of offshore individuals showed severe shell dissolution damage linked to acidified water. Researchers estimated that the incidence of severe dissolution has already doubled compared to pre-industrial conditions in nearshore habitats and is on track to triple by 2050.20PubMed Central. Limacina helicina shell dissolution as an indicator of declining habitat suitability owing to ocean acidification in the California Current Ecosystem Pteropods are not just canaries in the coal mine; they are a significant food source for commercially important fish, and their loss would cascade through food webs.
Some pteropod populations do show signs of coping. In northern high-latitude waters, researchers found that under declining aragonite saturation, pteropods tended to build flatter and thicker shells, a form of structural adaptation that may buffer them against dissolution to a degree.21Frontiers in Marine Science. Integrated Assessment of Ocean Acidification Risks to Pteropods in the Northern High Latitudes Whether that plasticity is enough to keep pace with the rate of acidification remains an open question.
Microplastics and Zooplankton
Pollution adds another layer of stress. Microplastics, tiny fragments of synthetic material now ubiquitous in the world’s waters, are readily ingested by zooplankton. A review of the literature documented microplastic ingestion in 39 zooplankton species spanning 28 taxonomic orders, including both permanent and temporary members of the plankton community. Nearly half of the studies that looked for negative effects found them, reporting impacts on feeding behavior, growth, development, reproduction, and lifespan.22PubMed. Bioavailability and effects of microplastics on marine zooplankton: A review Because zooplankton sit near the base of the food web, contaminants they accumulate can be passed up the chain to fish, marine mammals, and ultimately to human seafood.
Freshwater Zooplankton and Why They Matter Too
Zooplankton are not exclusively marine. Lakes, rivers, and reservoirs have their own zooplankton communities, and these play similar roles in freshwater food webs: grazing on algae, recycling nutrients, and feeding fish. The tiny crustacean Daphnia, sometimes called the water flea, is probably the most studied zooplankton genus on Earth. It has been used as a standard model organism for ecotoxicity testing since the 1980s because it is well characterized, easy to culture, and sensitive to pollutants, making it an essential indicator species for environmental stress.23PubMed Central. Daphnia as a model organism to probe biological responses to nanomaterials-from individual to population effects via adverse outcome pathways
Freshwater zooplankton face their own set of threats. Road salt, widely used for winter de-icing, introduces chloride into waterways at concentrations that can harm Daphnia even below current water quality guidelines. Laboratory experiments showed decreased reproduction and increased mortality in six Daphnia species at chloride concentrations between 5 and 40 milligrams per liter.24PubMed. Road Salt Impacts Freshwater Zooplankton at Concentrations below Current Water Quality Guidelines That finding suggests regulatory thresholds may need revisiting.
Eutrophication, the enrichment of water with excess nutrients from agriculture and sewage, is another growing problem. In nutrient-loaded lakes, toxic cyanobacteria blooms become more frequent and intense. While some Daphnia species can graze on cyanobacteria and help control blooms, increasingly severe conditions tend to favor smaller, less effective grazers. In a high-altitude lake, rising temperatures and eutrophication shifted the dominant zooplankton genus from the larger Daphnia to the smaller Bosmina, weakening the zooplankton community’s capacity to regulate algae.25PubMed Central. Eutrophication strengthens the response of zooplankton to temperature changes in a high‐altitude lake Researchers predict that under increasing temperatures and nutrient loading, zooplankton will face intense selection pressure to tolerate cyanobacteria, and short generation times may allow rapid evolution of traits that improve tolerance, though this would likely lead to zooplankton that coexist with toxic blooms rather than suppressing them.26Freshwater Biology. Understanding cyanobacteria‐zooplankton interactions in a more eutrophic world
Zooplankton as Windows Into the Past
Some zooplankton leave permanent records. Planktonic foraminifera, single-celled zooplankton that build tiny calcium carbonate shells, are among the most important tools in paleoceanography. When these organisms die, their shells accumulate in ocean sediments over millions of years. The chemical composition of those shells, including oxygen isotopes and the ratio of magnesium to calcium, records information about the seawater temperature at the time the shell formed. Carbon and boron isotope ratios in the same shells carry signals about past changes in ocean chemistry.27ICES Journal of Marine Science. Linking zooplankton time series to the fossil record Much of what we know about ocean temperatures and carbon cycles over the past few hundred million years comes from analyzing zooplankton fossils drilled from the seafloor.
Bioluminescence in the Open Ocean
If you have ever seen the ocean glow blue-green at night, there is a good chance copepods were involved. Among calanoid copepods, the most abundant group of zooplankton globally, bioluminescent species make up anywhere from 5% to 59% of individuals depending on latitude, with higher proportions in tropical and subtropical waters where predation pressure is greater.28Journal of Plankton Research. A light in the dark: ecology, evolution and molecular basis of copepod bioluminescence These copepods produce a light-emitting compound called coelenterazine, which they secrete into the water as a flash when threatened. The flash likely startles predators or draws the attention of the predator’s own predators, a “burglar alarm” defense. Coelenterazine produced by copepods is passed up the food chain and may supply the raw material for bioluminescence in other marine organisms that cannot synthesize it themselves.
How Scientists Are Tracking Zooplankton Now
Traditionally, studying zooplankton meant towing fine-mesh nets through the water, hauling the catch to a lab, and identifying organisms under a microscope, a process that is slow, labor-intensive, and depends on taxonomic expertise that is in decline worldwide. Newer methods are changing the field. Environmental DNA metabarcoding, which identifies species from the genetic material they shed into the water, can capture a broad picture of plankton diversity from a simple water sample. In the southern North Sea, researchers combined eDNA metabarcoding with automated imaging tools like FlowCAM and ZooScan to characterize both well-studied groups and lesser-known taxa in a single survey.29ICES Journal of Marine Science. Environmental DNA metabarcoding captures plankton diversity in the southern North Sea
Portable imaging systems are also being tested for field use. PlanktonScope, a compact imaging platform, has been compared against eDNA metabarcoding along pollution gradients to evaluate whether automated visual identification can serve as a faster alternative to genetic methods for coastal water quality monitoring.30PubMed. Zooplankton as indicators of coastal water quality: a comparative study of PlanktonScope imaging and DNA metabarcoding These tools are not yet replacements for traditional net sampling, but they are steadily closing the gap, offering the prospect of near-real-time zooplankton monitoring at scales that were impossible a decade ago. For a group of organisms whose importance far outweighs its public profile, that improved surveillance may be exactly what ocean management needs.