Plankton sit at the foundation of aquatic food webs, and what they eat and what eats them forms a chain that stretches from dissolved nutrients and sunlight all the way up to the largest animals on Earth. Phytoplankton harvest energy from the sun much like plants on land, while zooplankton graze on phytoplankton, on bacteria, and on each other. Those zooplankton then feed fish larvae, forage fish, jellyfish, and filter-feeding giants like baleen whales and whale sharks. The reality is messier and more interesting than a neat textbook ladder, though, because many plankton blur the line between plant and animal, the food web recycles itself through sinking debris, and the whole system is sensitive to shifts in temperature, timing, and pollution.
What Phytoplankton Eat
Phytoplankton are not “eating” in the way most people picture it. They are microscopic photosynthesizers, drifting in sunlit surface waters and using light energy to convert carbon dioxide and dissolved nutrients into organic matter. Their essential raw materials are nitrogen, phosphorus, iron, and silica (for diatoms that build glass-like shells). In nutrient-rich upwelling zones or river outflows, phytoplankton bloom explosively; in the nutrient-poor open ocean, they persist in lower numbers, limited by scarce iron or nitrogen.
But a surprising number of these “plant-like” organisms cheat. Many phytoplankton species also engulf bacteria or other tiny cells, a feeding strategy called mixotrophy. These organisms photosynthesize when conditions are good and switch to eating prey when nutrients run low, or they do both at once. Mixotrophic phytoflagellates and dinoflagellates are often the dominant plankton during seasonal stratification, when the water column settles into layers and nutrients at the surface become scarce. Eating bacteria can relieve that nutrient stress and actually boost primary production in otherwise unproductive waters.1PubMed. Mixotrophy in the Marine Plankton Researchers now recognize that a substantial fraction of protistan plankton species engage in this dual strategy, though our understanding of how they fit into food webs and nutrient cycling still lags behind what we know about “pure” phytoplankton and zooplankton.2PubMed Central. Mixoplankton and mixotrophy: future research priorities
Bacteria and the Microbial Loop
Not everything phytoplankton produce gets eaten by larger grazers. A large share of the organic matter phytoplankton release, whether as dissolved compounds leaked during growth or as dead cells breaking apart, feeds bacteria. This recycling pathway is called the microbial loop. Bacteria consume dissolved organic carbon, grow, and are then eaten by tiny flagellates and ciliates, which are in turn eaten by larger zooplankton. The net effect is that carbon that might otherwise be lost as dissolved waste gets packaged back into living cells and funneled up the food chain.
Laboratory co-culture experiments illustrate this neatly. When a marine bacterium was grown alongside a diatom, the bacterium showed dramatically increased growth compared to controls, fueled by the organic compounds the diatom released. Even dead diatom biomass turned out to be an excellent carbon source for bacteria.3PubMed Central. Scaling down the microbial loop: data‐driven modelling of growth interactions in a diatom–bacterium co‐culture In the open ocean, this relationship runs at massive scale and means that bacterial production is tightly linked to phytoplankton production. The microbial loop is not a dead end; it is a second conveyor belt running alongside the classical food chain.
Microzooplankton, the Hungriest Grazers
If you had to pick the single most important group of plankton consumers in the ocean, it would not be fish or whales. It would be microzooplankton, the tiny protists (mostly ciliates and dinoflagellates) that graze on phytoplankton. Estimates suggest microzooplankton consume roughly 60 to 70 percent of daily oceanic primary production.4PubMed Central. On the ecological duality between ciliates and dinoflagellates across marine ecosystems That is a staggering share. Most of the photosynthetic output of the ocean passes through these organisms before it ever reaches a copepod, let alone a fish.
Ciliates and dinoflagellates divide the labor in interesting ways. Ciliates use hair-like cilia to sweep bacteria and small flagellates into their mouths, generally sticking to small prey. Dinoflagellates are more flexible predators, consuming everything from tiny algae to large diatoms and even ciliates themselves. Some larger dinoflagellates use specialized feeding structures to engulf diatom chains and protists several times their own size, putting them in direct competition with copepods rather than with their ciliate neighbors.4PubMed Central. On the ecological duality between ciliates and dinoflagellates across marine ecosystems Many of these microzooplankton grazers are themselves mixotrophic, retaining functional algal organelles or hosting algal symbionts that let them photosynthesize while also eating. That photosynthetic boost may increase their growth efficiency and ultimately transfer more carbon up to higher levels.1PubMed. Mixotrophy in the Marine Plankton
Copepods, Krill, and Other Mesozooplankton
Copepods are the most abundant multicellular animals in the ocean, and they are the classic “middle link” between phytoplankton and fish. But copepods are not simple grazers that vacuum up whatever drifts past. They are selective feeders. Research has shown that younger copepod stages, which have low nitrogen-to-phosphorus ratios in their body tissue, preferentially select phosphorus-rich food. Older stages, with higher nitrogen demands, shift to nitrogen-rich food.5Oikos. Zooplankton eat what they need: copepod selective feeding and potential consequences for marine systems Copepods are, in a real sense, choosing their meals based on their current nutritional needs.
In polar waters, krill take center stage. Antarctic krill feed primarily on algae, which on a year-round average make up about 71 percent of their stomach contents, with protozoans accounting for roughly 17 percent and small animals about 12 percent. In open water, both diatom and copepod consumption rise when phytoplankton are abundant. Under sea ice, diatom feeding drops off sharply, but krill keep eating copepods at a steady rate.6Limnology and Oceanography. Feeding and overwintering of Antarctic krill across its major habitats: The role of sea ice cover, water depth, and phytoplankton abundance When fresh algae are unavailable, krill can also survive on detritus. Stomach analyses of krill in the Scotia Sea found lithogenic particles, diatom debris, and bacterial fatty acids but low proportions of fresh-diatom indicators, suggesting these animals were scraping by on sinking organic refuse rather than fresh food.7Deep Sea Research Part II: Topical Studies in Oceanography. Early spawning of Antarctic krill in the Scotia Sea is fuelled by “superfluous” feeding on non-ice associated phytoplankton blooms Krill are versatile feeders, which partly explains why they can sustain such enormous populations in harsh polar environments.
Jellyfish and the Gelatinous Web
Jellyfish, ctenophores, salps, and pyrosomes are often treated as footnotes in plankton food web diagrams, but they span a remarkable range of feeding roles. Stable isotope analysis of gelatinous zooplankton in the eastern tropical Atlantic revealed that this “jelly web” covered most of the isotopic niche space of the entire planktonic food web, spanning more than three trophic levels. At one end, pyrosomes feed like herbivores, filtering phytoplankton. At the other end, ctenophores occupy the position of higher predators.8Limnology and Oceanography. Tackling the jelly web: Trophic ecology of gelatinous zooplankton in oceanic food webs of the eastern tropical Atlantic assessed by stable isotope analysis
Jellyfish and other gelatinous predators also interact with benthic filter feeders like mussels and sea squirts. In shallow coastal systems such as Danish fjords, density-driven currents created by mixing between saltier and fresher water determine how much contact plankton have with both pelagic predators like jellyfish and bottom-dwelling filter feeders. The grazing pressure from these benthic invertebrates can rival the impact of pelagic grazers in controlling plankton abundance.9ScienceDirect (Elsevier / Journal of Environmental Management). Filter feeding and plankton dynamics in a Danish fjord: a review of the importance of flow, mixing and density-driven circulation
Forage Fish and the Link to Fisheries
Sardines, anchovies, herring, and other small schooling fish are the primary route through which plankton energy reaches commercially important species and marine predators. In the Humboldt Current system off Peru, both sardines and anchovies feed predominantly on zooplankton, but they partition the resource by prey size. Sardines eat smaller copepods and fewer euphausiids than anchovies do, which reduces direct competition between the two species.10Progress in Oceanography. Diet of sardine (Sardinops sagax) in the northern Humboldt Current system and comparison with the diets of clupeoids in this and other eastern boundary upwelling systems This prey-size partitioning has been reported in other eastern boundary upwelling systems as well, suggesting it is a recurring pattern wherever these fish coexist.
For fish larvae, the plankton food web is even more critical. Most marine fish start life as tiny larvae that depend on eating appropriately sized plankton at just the right time. This is the basis of the match-mismatch hypothesis: if larvae hatch when their plankton prey is abundant, survival is high; if the timing is off, entire year-classes of fish can fail. Research on North Atlantic cod shows that the duration of overlap between larval fish and their prey matters more than whether their peak abundances coincide perfectly. Warmer years tend to trigger earlier spring phytoplankton blooms, which prolongs the window of food availability and leads to more larvae surviving to recruitment.11PubMed Central. Recruitment variability in North Atlantic cod and match-mismatch dynamics
Climate change threatens to disrupt this timing. The environmental processes controlling bloom timing, like water column stratification, differ from those controlling fish spawning, like temperature-linked reproductive maturation. Under a high-emissions scenario, models project that spring and summer phytoplankton blooms at higher latitudes could start about 16 days earlier on average by the century’s end, while temperature-linked spawning in some fish shifts at twice the rate of bloom timing. For fish species whose spawning grounds are tied to fixed geographic features like estuaries or reefs, extreme mismatches of more than 30 days could increase tenfold in many areas.12PubMed. Climate change impacts on mismatches between phytoplankton blooms and fish spawning phenology
Whales and Whale Sharks
At the top of the plankton food chain sit some of the largest animals that have ever lived. Blue whales feed almost exclusively on krill, and they are far more strategic about it than their reputation as simple “gulpers” suggests. Research tracking blue whale foraging shows they adjust their feeding behavior based on prey density: at low krill densities, they reduce their feeding rate to conserve oxygen, while at high densities, they increase lunge frequency to maximize energy intake.13PubMed Central. Blue whales (Balaenoptera musculus) optimize foraging efficiency by balancing oxygen use and energy gain as a function of prey density They are not indiscriminate grazers but rather sophisticated foragers balancing the cost of each dive against what they stand to gain.
Even so, filter feeding at this scale has physical limits. Modeling work on baleen whale feeding suggests that accumulating krill can clog the baleen plates, and emptying the mouth of an average gulp could take around 16 minutes if krill builds up against the filter, roughly twice the duration of a typical foraging dive. Dense patches of prey are not always a pure advantage.14PubMed Central. Constraints on lunge feeding: krill can clog the baleen of filter-feeding whales
Whale sharks, the largest fish alive, take a different approach. Their filtering apparatus consists of 20 unique pads that completely seal the throat cavity, with a mesh averaging 1.2 millimeters in diameter. During surface feeding, whale sharks swim at about 1.1 meters per second with their mouths open, spending an average of roughly seven and a half hours a day feeding on dense plankton swarms dominated by small shrimp-like sergestids, copepods, arrow worms, and fish larvae. They use cross-flow filtration, a mechanism that lets particles smaller than the mesh get captured while reducing clogging.15PubMed. Feeding anatomy, filter-feeding rate, and diet of whale sharks Rhincodon typus during surface ram filter feeding off the Yucatan Peninsula, Mexico Manta rays and basking sharks use structurally different filters but serve the same ecological role, channeling enormous quantities of zooplankton into large-bodied animals.16PubMed. Comparison of the structure and composition of the branchial filters in suspension feeding elasmobranchs
How Plankton Defend Themselves
Plankton are not passive meals. Phytoplankton have evolved a wide array of defense mechanisms that span physiological, morphological, and behavioral strategies. Some produce toxins. Some generate bioluminescence that can startle predators or attract the predator’s own enemies. Diatoms build silica shells that make them harder to digest. Others form colonies too large for small grazers to handle.17PubMed. Phytoplankton defence mechanisms: traits and trade-offs
Chemical defense is particularly creative at the single-cell level. Some algae use compartmented or activated reactions that release defensive compounds only when the cell is damaged or contacted by a predator. A well-studied example involves DMSP, a sulfur compound that certain algae cleave into toxic byproducts when grazed upon, deterring protist predators. Bioluminescence and the rapid discharge of extrusomes (small ejectable structures) are visible examples of these fast-acting defenses, and researchers suspect there are many more chemical reactions happening at the microscale that we simply lack the technology to detect.18PubMed. The chemical defense ecology of marine unicellular plankton: constraints, mechanisms, and impacts
Marine Snow and the Biological Pump
Not all plankton food webs play out in the sunlit surface layer. Dead phytoplankton, fecal pellets, mucus webs, and other organic debris clump together into sinking aggregates called marine snow, which feeds a whole community of deep-sea organisms. In situ observations of deep pelagic feeding have documented crustaceans like munnopsid isopods grazing on marine snow, along with gelatinous species such as ctenophores and narcomedusae that were previously assumed to be strict carnivores.19PubMed Central. Deep pelagic food web structure as revealed by in situ feeding observations
Zooplankton themselves accelerate this downward carbon transport through diel vertical migration, the largest daily animal migration on Earth. Enormous numbers of zooplankton swim to the surface at night to feed on phytoplankton, then retreat to depths of hundreds of meters during the day, where they respire, excrete, and defecate. This shuttles carbon from the surface to the deep ocean through three main pathways: fecal pellet production, excretion of dissolved organic matter, and respiration of carbon dioxide at depth.20PubMed. Active Carbon Transport by Diel Vertical Migrating Zooplankton: Calculated and Modeled, but Never Measured Global models estimate this migration-driven export adds roughly 14 percent to the total carbon flux leaving the sunlit ocean, a substantial supplement to the passive sinking of marine snow.21Global Biogeochemical Cycles. Modeling the Impact of Zooplankton Diel Vertical Migration on the Carbon Export Flux of the Biological Pump In this way, what eats plankton, and where those consumers poop, has direct consequences for how much carbon the ocean pulls out of the atmosphere.
Freshwater Versus Marine Plankton Food Webs
If you are familiar with lake ecology, you might assume marine plankton food webs work the same way. They do not, and the difference comes down to the dominant grazers. In lakes, cladocerans like Daphnia (water fleas) are often the key herbivores. Daphnia are relatively unselective filter feeders that can exert powerful top-down control on phytoplankton, keeping algal blooms in check when fish predation on Daphnia is low. This is why trophic cascades, where removing or adding a top predator ripples down to change algal abundance, are a well-documented phenomenon in freshwater systems.22PubMed. Cladocerans versus copepods: the cause of contrasting top-down controls on freshwater and marine phytoplankton
In the ocean, copepods dominate. Because copepods are selective feeders that consume other zooplankton as well as phytoplankton, the marine food chain has more links of unequal length, which tends to dampen trophic cascades. The energy from phytoplankton passes through more intermediate steps before reaching fish, and the signal from a predator at the top gets diluted along the way.23Ecology Letters. Copepods act as a switch between alternative trophic cascades in marine pelagic food webs In high-latitude marine communities where krill and copepods coexist, trophic cascades can still occur, but in temperate and tropical ocean waters they are genuinely rarer.24International Review of Hydrobiology. Trophic Cascades in Marine and Freshwater Plankton The identity of the dominant grazer, in other words, shapes the whole architecture of the food web.
Microplastics in the Plankton Food Web
Zooplankton do not distinguish between a tiny food particle and a tiny plastic particle. Laboratory studies have demonstrated that microplastics are readily ingested by a wide range of zooplankton groups, and this ingestion is associated with negative effects on biological processes including feeding, growth, and reproduction.25PubMed. Bioavailability and effects of microplastics on marine zooplankton: A review Because zooplankton are eaten by so many larger animals, this creates a direct route for plastic to move up the food chain. Experiments have demonstrated this transfer explicitly: when zooplankton that had ingested fluorescent microspheres were fed to mysid shrimps, microscopy confirmed the microspheres inside the shrimps’ intestines within three hours.26PubMed. Ingestion and transfer of microplastics in the planktonic food web That was the first direct evidence of plastic transfer via plankton from one trophic level to a higher one, and it suggests the plankton food web acts as a highway for microplastic contamination reaching fish, seabirds, and marine mammals.27PubMed Central. Microplastics in the Food Chain
Climate Change and the Shifting Plankton Menu
Ocean warming and acidification are reshaping plankton communities in ways that could ripple through the entire food web. Experimental work on a Mediterranean plankton community found that warming favored small species like cyanobacteria, while ocean acidification had limited direct impact under nutrient-depleted conditions. The concern is that a shift toward smaller phytoplankton could reduce energy transfer to higher trophic levels and weaken the export of carbon to the deep ocean.28ICES Journal of Marine Science. Effect of ocean warming and acidification on a plankton community in the NW Mediterranean Sea
Warming also affects the zooplankton that eat those phytoplankton. Experimental studies show that higher temperatures shrink copepod body size, while elevated carbon dioxide levels tend to increase body size slightly; the two effects work against each other but do not cancel out. In terms of nutritional quality, warming combined with acidification altered the fatty acid composition of copepods, increasing the ratio of saturated fatty acids by about 45 percent compared to ambient conditions.29PLoS ONE. Combined Effects of Ocean Warming and Acidification on Copepod Abundance, Body Size and Fatty Acid Content Smaller copepods with altered fat profiles are a poorer meal for the fish larvae, forage fish, and other predators that depend on them. The evolutionary establishment of modern copepod lineages was itself shaped by changes in phytoplankton communities, particularly the rise of diatoms, so there is deep precedent for shifts in plankton composition cascading through higher trophic levels over time.30Academic Press. Evolution of Primary Producers in the Sea