Plankton Types

Why This Matters

Plankton might be microscopic, but they're the foundation of virtually everything happening in the ocean. Understanding how marine ecosystems function means understanding how energy flows from sunlight to apex predators, how carbon moves between atmosphere and ocean, and how nutrient cycling keeps the whole system running. Plankton sit at the center of all three processes. When exam questions ask about primary production, trophic transfer, biogeochemical cycles, or larval ecology, they're really asking: do you understand plankton?

Plankton aren't just "small floating things." They're classified in two completely different ways: by function (what they do in the ecosystem) and by size (which determines who eats whom). The functional categories tell you about ecological roles; the size categories tell you about food web structure. Don't just memorize names. Know which classification system each type belongs to and what it reveals about ocean dynamics.

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Functional Classification: What They Do

Functional categories group plankton by their ecological role: how they obtain energy and what they contribute to ecosystem processes.

Phytoplankton

• Primary producers that photosynthesize. These are the "plants" of the ocean, converting sunlight and $CO_2$ into organic matter via photosynthesis.
• Produce roughly 50% of Earth's oxygen, making them as important as all terrestrial forests combined for atmospheric composition.
• Drive the biological carbon pump by absorbing atmospheric carbon and sinking it to deep water when they die. This is one of the ocean's most important climate regulation mechanisms.

Zooplankton

• Heterotrophs that consume other organisms. They're the grazers and predators of the planktonic world, ranging from single-celled protozoans to large jellyfish.
• Critical link in trophic transfer, converting phytoplankton biomass into food accessible to fish, whales, and other large consumers.
• Include both holoplankton and meroplankton, meaning this functional group overlaps with the life-cycle classification described below.

Bacterioplankton

• Bacteria and archaea that decompose organic matter. They're the recyclers, breaking down dead material and dissolved waste products.
• Drive the microbial loop, a pathway that recovers energy from dissolved organic matter (DOM) that would otherwise be lost from the food web. In this loop, bacteria consume DOM, are then grazed by nanoflagellates, and that energy re-enters the food chain.
• Regulate nutrient availability by converting organic nutrients back into inorganic forms (like nitrate and phosphate) that phytoplankton can use. This process is called remineralization.

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Life-Cycle Classification: Permanent vs. Temporary Residents

Some organisms drift their entire lives; others are just passing through on their way to becoming something else.

Holoplankton

• Spend their entire life cycle as plankton. From birth to death, they drift with currents and never settle onto the seafloor.
• Include copepods, krill, and many jellyfish, organisms that have evolved specifically for planktonic existence.
• Form the stable, permanent base of planktonic food webs that larger consumers depend on year-round.

Meroplankton

• Planktonic only during larval stages. These are temporary drifters that eventually settle to become benthic (bottom-dwelling) or nektonic (free-swimming) adults.
• Include larvae of fish, crabs, sea urchins, barnacles, and mollusks, connecting planktonic and benthic ecosystems.
• Critical for population dispersal, allowing species to colonize new habitats and maintain genetic connectivity across distances. This is why many sessile (non-moving) adult organisms like barnacles still have wide geographic ranges.

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Size Classification: The Plankton Size Spectrum

Size determines who eats whom. Each size class occupies a different position in the food web, and energy transfers upward through successive size categories.

Picoplankton

• Smallest plankton at less than 2 micrometers ($<2 \, \mu m$). Invisible even under standard light microscopy.
• Dominated by cyanobacteria like Prochlorococcus, the most abundant photosynthetic organism on Earth, with estimated populations around $10^{27}$ cells globally.
• Fuel the microbial loop by being consumed by nanoplankton-sized grazers, keeping energy cycling within the smallest size classes.

Nanoplankton

• Range from 2 to 20 micrometers ($2\text{–}20 \, \mu m$). Includes small flagellates, small diatoms, and heterotrophic protists.
• Bridge picoplankton and microplankton, grazing on bacteria while being consumed by larger protozoans.
• Contribute significantly to primary production in nutrient-poor open ocean waters where larger phytoplankton can't thrive due to low nutrient concentrations.

Microplankton

• Range from 20 to 200 micrometers ($20\text{–}200 \, \mu m$). Visible under standard microscopy. Includes larger diatoms, dinoflagellates, and ciliates.
• Dominant primary producers during blooms, especially in nutrient-rich coastal and upwelling zones where nutrients like nitrate and silicate are abundant.
• First size class directly accessible to metazoan grazers like copepod nauplii, connecting microbial production to the classic food chain.

Macroplankton

• Range from 200 micrometers to 2 centimeters ($0.2\text{–}20 \, mm$). Visible to the naked eye. Includes adult copepods, krill, and small jellyfish.
• Primary food source for planktivorous fish, making them the critical link between plankton and fisheries.
• Perform diel vertical migration (DVM), moving to surface waters at night to feed and descending hundreds of meters by day to avoid visual predators. This daily behavior is one of the largest mass migrations on Earth and actively transports carbon to depth.

Megaplankton

• Exceed 2 centimeters in size ($>20 \, mm$). Includes large jellyfish, salps, and colonial organisms like siphonophores.
• Can dominate ecosystems during blooms, sometimes outcompeting fish for zooplankton prey.
• Increasingly important in overfished systems, where jellyfish populations expand to fill ecological roles vacated by fish. This pattern has been documented in ecosystems like the Black Sea and parts of the East Asian marginal seas.

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Quick Reference Table

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Self-Check Questions

1. What's the key difference between classifying plankton by function versus by size, and why do marine biologists use both systems?

2. Which two plankton types are both involved in carbon cycling but move carbon in opposite directions? Explain the mechanism for each.

3. Compare and contrast holoplankton and meroplankton. How does their life-cycle difference affect their ecological roles and population dynamics?

4. If you were sampling plankton in nutrient-poor open ocean versus a nutrient-rich coastal upwelling zone, which size classes would you expect to dominate in each location, and why?

5. An FRQ asks you to explain how energy from sunlight reaches a tuna. Trace the pathway through at least three plankton types, identifying each by both function and size classification.