Why Are Plankton So Crucial For Life on Earth – And How Is Climate Change Affecting Them? Oceans.Org Nansen Chen Aug 15th 2025 Plankton, the microscopic organisms that form the basis of the marine food web, produce half of the Earth’s oxygen and absorb up to 40% of the global carbon emissions. But with climate change-driven ocean warming, algae reproduction and shrinking photic zones are threatening plankton populations, affecting both biodiversity and humans. Plankton is defined as a creature that is carried by tides and currents and cannot swim well to resist those forces. They are microscopic, measuring no more than one inch (2.45 centimeters) , although they also include larger species like some crustaceans (crabs, shrimp) and jellyfish. Plankton are classified by their size, type, and drift time – phytoplankton (plants) and zooplankton (animals) are the most common basic categories.
Phytoplankton......As the smallest plankton, phytoplankton – also known as microalgae – need sunlight to survive and grow because they contain chlorophyll. For this reason, they are most commonly found floating in the photic zone of the ocean, where sunlight penetrates the water. Phytoplankton laid the foundation of the ocean food web. Given that phytoplankton can be photosynthetic under light to produce life-sustaining energy, they are regarded as autotroph – or self-feeding. Other autotrophs that can produce energy from light, water or carbon are collectively called primary producers, forming the bottom of the food web. The marine food chain begins with phytoplankton consumed by zooplankton and krill. These small organisms are then preyed upon by fish, and the chain of consumption extends upward to top predators like sharks, polar bears, and humans. Phytoplankton also produce oxygen as a by-product during the photosynthetic process. Approximately half of the oxygen production on Earth comes from the ocean, according to the US National Ocean Service.
Prochlorococcus, the smallest phytoplankton on Earth, produce up to 20% of the oxygen in the entire biosphere. Phytoplankton not only supply food and oxygen for marine life globally but also regulate atmospheric carbon through the biological pump, a process that moves carbon from the ocean surface to the deep sea. In photosynthesis, the energy-producing process where inorganic carbon is converted into organic compounds like glucose is called carbon fixation, a key part of the biological pump. Through photosynthesis, phytoplankton absorb carbon dioxide (CO2) and incorporate the carbon in the same way wood and the leaves of a tree do. While most of this carbon recycles back into surface waters when phytoplankton are eaten or decay, some of it sinks to the deep ocean, where it will be trapped for centuries. Phytoplankton are responsible for nearly half of the world’s primary production by converting sunlight into energy and organic matter, forming the base of the aquatic food web and contributing to the global carbon cycle. This is impressive considering they account for less than 1% of Earth’s photosynthetic biomass — the total mass of organisms that turn sunlight into food. What’s more, despite accounting for only 1-2% of the world’s total plant carbon biomass, marine phytoplankton are responsible for recycling 30 to 50 billion metric tonnes of carbon annually, nearly 40% of the total.
Zooplankton...... Zooplankton live in the ocean, lakes and ponds. They are heterotrophic, meaning they are secondary consumers in the food web, obtaining nutrients by feeding on primary producers such as phytoplankton in surface water. They are also an integral part of the biological carbon pump. According to a 2023 study, as a secondary consumer, the manure particles they discharge will sink rapidly and sequester carbon, capturing CO2 for long-term storage in the deep sea.Their seasonal, vertical migration enhances the sinking of fecal particles into deeper water. A study published in June revealed that zooplankton, including copepods, krill, and salps, transports nearly 65 million tonnes of carbon to depths below 500 meters undersea in the Southern Ocean, a key region for carbon storage. Mesozooplankton (mainly small crustaceans called copepods) account for 80% of this carbon flux, while krill and salps contribute 14% and 6%, respectively.The Southern Ocean absorbs roughly 40% of all human-made CO2 stored by the oceans. Vertically migrating zooplankton efficiently sequester carbon in the deep ocean while simultaneously containing nutrients (phytoplankton).......read on https://earth.org/why-are-plankton-so-crucial-for-life-on-earth-and-how-is-climate-change-affecting-them/