Table of Contents

What Is Planktonology?

Planktonology is the scientific study of plankton --- the vast, diverse community of organisms that drift through the world’s oceans, lakes, and rivers. The name comes from the Greek planktos, meaning “wanderer” or “drifter,” and that’s the defining characteristic: plankton are organisms that can’t swim strongly enough to move against currents. They go where the water takes them.

That might make them sound passive and unimportant. They’re anything but. Plankton produce roughly half the oxygen in Earth’s atmosphere, form the foundation of nearly every aquatic food web, and cycle carbon on a scale that influences global climate. If plankton disappeared tomorrow, the consequences would be catastrophic for every living thing on the planet --- including you.

What Counts as Plankton?

Here’s what catches most people off guard: “plankton” isn’t a biological category like “mammal” or “insect.” It’s a lifestyle description. Any organism that drifts in water and can’t swim against currents qualifies as plankton, regardless of its size, species, or evolutionary history.

This means plankton include:

Bacteria (bacterioplankton)
Microscopic algae (phytoplankton)
Tiny animals (zooplankton)
Larvae of fish, crabs, and other animals (meroplankton --- organisms that are planktonic only during part of their life cycle)
Jellyfish (yes, even the large ones --- they drift with currents)
Viruses (virioplankton --- and there are an estimated 10^30 of them in the ocean)

Planktonologists categorize their subjects by size:

Femtoplankton (< 0.2 micrometers): viruses
Picoplankton (0.2-2 micrometers): bacteria and the smallest algae
Nanoplankton (2-20 micrometers): small algae and protists
Microplankton (20-200 micrometers): diatoms, dinoflagellates, small zooplankton
Mesoplankton (200 micrometers - 20 mm): copepods, krill larvae
Macroplankton (20 mm - 20 cm): krill, salps, jellyfish
Megaplankton (> 20 cm): large jellyfish, colonial organisms

The sheer range is staggering --- from viruses smaller than a wavelength of light to lion’s mane jellyfish with tentacles stretching 30 meters.

Phytoplankton: The Ocean’s Invisible Forests

Phytoplankton are microscopic organisms that photosynthesize --- they use sunlight to convert carbon dioxide and water into organic matter and oxygen, just like plants on land. But unlike trees, which are measured in meters, most phytoplankton are measured in micrometers. You could fit thousands of them on a pinhead.

Despite their size, their collective impact is staggering. Phytoplankton fix approximately 50 billion metric tons of carbon per year --- roughly equal to all terrestrial plants combined. They produce about 50% of Earth’s atmospheric oxygen. The cyanobacterium Prochlorococcus, discovered only in 1986, is the most abundant photosynthetic organism on Earth, with an estimated population of 3 x 10^27 cells. It alone produces about 20% of the oxygen in the biosphere.

Major Phytoplankton Groups

Diatoms are single-celled algae encased in exquisite glass (silicon dioxide) shells. They’re responsible for roughly 20% of global photosynthesis and dominate in nutrient-rich waters. When they die, their silica shells sink and accumulate on ocean floors, forming layers that geologists use to reconstruct past ocean conditions.

Coccolithophores are tiny algae covered in calcium carbonate plates (coccoliths). When they bloom, they can turn vast swaths of ocean milky white --- visible from space. The White Cliffs of Dover are made largely from ancient coccolithophore shells accumulated over millions of years. Coccolithophores are particularly sensitive to ocean acidification because lower pH dissolves their calcium carbonate armor.

Dinoflagellates are single-celled organisms with whip-like flagella. Some photosynthesize; others are predators. Some species cause harmful algal blooms (red tides) and produce toxins that can kill fish and sicken humans. Others are bioluminescent, creating the blue glow you sometimes see in ocean waves at night.

Cyanobacteria are photosynthetic bacteria that changed Earth’s history. About 2.4 billion years ago, their oxygen production transformed the atmosphere from anaerobic to aerobic, paving the way for complex life. Today, Prochlorococcus and Synechococcus dominate the open ocean’s phytoplankton community.

The Spring Bloom

In temperate and polar oceans, phytoplankton undergo dramatic seasonal blooms. During winter, storms mix nutrients from the deep ocean to the surface, but low light limits growth. In spring, increasing daylight triggers explosive population growth. Phytoplankton populations can double every day under favorable conditions, turning vast stretches of ocean green in a matter of weeks.

These blooms are visible from satellites and are among the largest biological events on Earth. The North Atlantic spring bloom covers millions of square kilometers and supports the entire marine food web for the rest of the year. The timing of the bloom matters enormously --- if it peaks before zooplankton populations are ready to graze on it, the energy transfer up the food web is disrupted, with consequences for fish stocks and everything that depends on them.

Zooplankton: The Ocean’s Grazers

Zooplankton are animal plankton that feed on phytoplankton, other zooplankton, or dissolved organic matter. They form the critical link between primary producers and higher trophic levels --- fish, marine mammals, and seabirds.

Copepods: The Most Important Animals You’ve Never Heard Of

Copepods are tiny crustaceans, typically 1-2 millimeters long, and they’re arguably the most abundant multicellular animals on Earth. The ocean contains an estimated 10^18 (a quintillion) copepods at any given time. They’re the primary grazers of phytoplankton and the primary food source for countless fish species, including many commercially important ones.

A single copepod can consume thousands of phytoplankton cells per day. They detect food using chemical sensors on their antennae and create feeding currents to draw particles toward their mouthparts. Some species can eat up to 50% of their body weight daily.

Copepods also perform the largest migration on Earth --- the diel vertical migration. Every night, they swim hundreds of meters from the deep ocean to the surface to feed on phytoplankton under cover of darkness. At dawn, they descend again to avoid visual predators. This nightly round trip, multiplied by trillions of individuals, transports enormous amounts of carbon from the surface to the deep ocean.

Krill

Antarctic krill (Euphausia superba) form one of the most abundant animal biomasses on Earth --- estimated at 300-500 million metric tons. They feed on phytoplankton and in turn feed whales, seals, penguins, and fish. Blue whales consume up to 3.6 metric tons of krill per day.

Krill swarms can be so dense they’re detected by satellites and fishing sonar. A single swarm observed in 1981 was estimated to contain 2.1 million metric tons of krill covering 450 square kilometers. The entire Antarctic ecosystem effectively runs on krill.

Jellyfish

Jellyfish are macroplankton that have become increasingly abundant in many oceans. Some scientists attribute this to overfishing (removing jellyfish competitors and predators), warming waters, and eutrophication (nutrient pollution that creates conditions jellyfish tolerate better than fish). Jellyfish blooms can clog fishing nets, block power plant cooling water intakes, and shut down beaches. A 2007 bloom in the Irish Sea was estimated to contain 10 billion jellyfish and devastated a salmon farm, killing over 100,000 fish.

The Biological Pump: Plankton and Climate

Plankton play a critical role in the global carbon cycle through what oceanographers call the “biological pump.”

Here’s how it works: Phytoplankton at the ocean surface absorb CO2 from the atmosphere during photosynthesis, converting it to organic carbon. Zooplankton eat the phytoplankton and produce fecal pellets that sink. Dead plankton also sink. This constant downward rain of organic material --- called “marine snow” --- transports carbon from the surface to the deep ocean, where it can remain sequestered for centuries to millennia.

The scale is enormous. The biological pump exports roughly 10 billion metric tons of carbon to the deep ocean each year. Without it, atmospheric CO2 concentrations would be about 200 parts per million higher than they are today --- enough to dramatically alter the climate.

This is why plankton matter for climate science. Changes in phytoplankton productivity, species composition, or distribution patterns can strengthen or weaken the biological pump, creating feedbacks that amplify or dampen climate change.

How Planktonologists Work

Studying organisms you mostly can’t see in an environment covering 361 million square kilometers of ocean requires creative approaches.

Sampling

Traditional plankton sampling uses nets towed behind research vessels. Mesh size determines what you catch --- fine mesh (64 micrometers) captures microplankton; coarser mesh (200-500 micrometers) targets mesozooplankton. Water samples collected with Niskin bottles capture pico- and nanoplankton too small for nets.

The Continuous Plankton Recorder (CPR), deployed since 1931, is towed behind ships of opportunity (cargo vessels, ferries) on regular routes, collecting plankton samples continuously. The CPR survey has accumulated over 7 million samples across the North Atlantic and North Pacific, creating one of the longest-running marine biological datasets in existence. It’s been critical for documenting how plankton communities have shifted over 90 years of changing climate.

Microscopy and Identification

Identifying plankton species requires expertise in microscopy. A single water sample might contain hundreds of species. Traditional identification relies on morphological features --- the shape of a diatom shell, the arrangement of a dinoflagellate’s plates, the number of segments on a copepod’s antenna.

Increasingly, molecular methods supplement microscopy. Environmental DNA (eDNA) analysis extracts and sequences genetic material directly from water samples, revealing species that microscopy might miss. Metabarcoding can characterize entire plankton communities from a single water sample.

Remote Sensing

Satellites detect phytoplankton from space by measuring ocean color. Chlorophyll-a (the primary photosynthetic pigment) makes water greener. NASA’s SeaWiFS, MODIS, and now PACE satellites have tracked global phytoplankton distribution since 1997. These data reveal seasonal bloom patterns, long-term trends, and the effects of climate variability on ocean productivity.

Experimental Approaches

Lab cultures allow planktonologists to study individual species under controlled conditions --- varying temperature, nutrient concentrations, light levels, pH, and CO2 to understand physiological responses.

Mesocosms --- large enclosed volumes of seawater (hundreds to thousands of liters) --- allow scientists to manipulate conditions while maintaining natural plankton communities. The KOSMOS mesocosm experiments off the coast of Norway have studied how ocean acidification affects entire plankton food webs, revealing that reduced pH shifts community composition in ways that weaken the biological pump.

Threats to Plankton

Climate Change

Ocean warming is shifting plankton distribution patterns poleward. Warm-water copepod species are replacing cold-water species in the North Atlantic --- a shift documented by the CPR survey over several decades. This matters because different species have different nutritional value. The cold-water copepod Calanus finmarchicus, a lipid-rich species critical for feeding cod larvae and herring, is being replaced by the smaller, less nutritious Calanus helgolandicus. The result: less energy available for fish, with cascading effects up the food web.

Ocean Acidification

As the ocean absorbs CO2, its pH drops. Since the Industrial Revolution, ocean pH has dropped from about 8.2 to 8.1 --- a 26% increase in acidity. This threatens shell-forming plankton (coccolithophores, foraminifera, pteropods) whose calcium carbonate structures dissolve in more acidic water. Pteropods --- small planktonic snails known as “sea butterflies” --- are already showing shell dissolution in the Southern Ocean.

Eutrophication

Excess nutrients from agricultural runoff and sewage stimulate phytoplankton blooms, some of which are toxic. When blooms die and decompose, the process consumes oxygen, creating “dead zones” where most marine life can’t survive. The Gulf of Mexico dead zone, fueled by nutrients from the Mississippi River, covers roughly 15,000 square kilometers annually.

Plastic Pollution

Microplastics (particles smaller than 5 mm) are now ubiquitous in ocean waters. Zooplankton ingest them, mistaking them for food. This reduces their feeding efficiency, can transfer toxic chemicals, and introduces plastic into the food web. Studies have found microplastics in plankton samples from every ocean basin, including the deep sea and polar regions.

Connections to Other Fields

Planktonology connects to marine biology as the foundation of ocean ecosystems, oceanography through ocean physics and chemistry, ecology through food web dynamics, climatology through the carbon cycle, and environmental science through pollution impacts.

If the broader marine ecosystem interests you, marine biology provides the wider context. For ocean physics and chemistry, oceanography goes deeper. If the carbon cycle and climate connections fascinate you, climatology expands the picture. And if the algal side of phytoplankton is your focus, phycology --- the study of algae --- is the specialized discipline.

Why This Field Needs Attention

Plankton are easy to ignore. They’re mostly invisible, they live in the ocean, and they don’t have the charisma of elephants or the majesty of redwood forests. But the numbers don’t lie: half the oxygen, the foundation of nearly every marine food chain, a critical component of the global carbon cycle.

When plankton communities change --- and they are changing, right now, in response to warming, acidification, and pollution --- the effects ripple through fisheries, climate systems, and ultimately, the breathability of our atmosphere.

Planktonology is the science that monitors these changes, understands their mechanisms, and predicts their consequences. It’s the study of organisms most people will never see, doing work that keeps the planet habitable. And frankly, that makes it one of the most important fields in modern biology, whether or not anyone outside the field has heard of it.

Frequently Asked Questions

What is the difference between phytoplankton and zooplankton?

Phytoplankton are plant-like plankton that photosynthesize, producing oxygen and forming the base of ocean food webs. Zooplankton are animal plankton that feed on phytoplankton or other zooplankton. Together, they form the foundation of nearly all marine ecosystems.

How much oxygen do plankton produce?

Phytoplankton produce approximately 50% of Earth's oxygen through photosynthesis — roughly the same amount as all terrestrial plants combined. A single genus, Prochlorococcus, is estimated to produce about 20% of the oxygen in the entire biosphere.

Can you see plankton with the naked eye?

Most plankton are microscopic, but some are visible. Jellyfish are technically plankton (they drift with currents rather than swimming against them). Some colonial organisms like Pyrosoma can be meters long. But the majority of plankton — and the most ecologically important ones — are invisible without a microscope.

Are plankton affected by climate change?

Significantly. Rising ocean temperatures shift plankton distribution patterns, with warm-water species moving poleward. Ocean acidification affects shell-forming plankton. Changing nutrient availability alters phytoplankton productivity. Since plankton underpin marine food webs, these changes cascade through entire ecosystems.