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.
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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).
“Our work shows that zooplankton are unsung heroes of carbon sequestration. Their seasonal migrations create a massive, previously unquantified carbon flux,” said Guang Yang, lead author of the study and a Marine Ecologist at the Institute of Oceanology at the Chinese Academy of Sciences.
Climate Crisis
2024 was the warmest year on record. The annual average sea surface temperature over the extra-polar ocean reached a record high of 20.87C, according to Corpenicus.
Rising sea surface temperature can stimulate algal bloom occurrence. Algal blooms happen when excess nutrients cause explosive algae growth in aquatic systems. According to the Columbia Climate School, climate change triggers algal blooms in three ways: drought followed by heavy rains increases nutrient runoff into water bodies, which also reduces water flow, leading to warming and stagnation that favors blooms; higher levels of CO2 in the atmosphere and water, especially toxic blue-green algae; and rising sea levels, which create shallower, more stable coastal waters that enhance algae growth.
Some toxic algal blooms will deplete oxygen in sunlit waters and cause oxygen deficiency that further endangers aquatic life. For example, a recent Karenia Mikimotoi bloom in Australia killed over 450 marine species.
Research at the University of Bristol last year found that plankton’s poor performance under the current speed of temperature rise is affecting “huge swathes” of marine life that depend on them as a food source. Other findings published in 2023 suggest that a warming climate can change the character of plankton from carbon sinks to carbon emitters.
“Plankton are the lifeblood of the oceans, if their existence is endangered, it will present an unprecedented threat that will disrupt the whole marine ecosystem with devastating, wide-reaching consequences for marine life and also human food supplies,” said Rui Ying, the lead author of the University of Bristol research.
“Because [plankton] can both capture and emit carbon dioxide, they are like switches that could either help reduce climate change or make it worse,” Holly Moeller, the co-author of the study, said. “These bugs are tiny, but their impacts can really scale up,” she added.
Research from the University of Plymouth in May found that between 2003 and 2022, the depth of the photic zone – the sunlit upper water layers where photosynthesis occurs – was reduced by over 50 metres (about 165 feet). Consequently, 21% of the global ocean, including vast areas of coastal regions and open ocean, have become darker. This will expose marine life to increased competition for resources like oxygen and nutrients, decreasing their chances of survival.
“There has been research showing how the surface of the ocean has changed colour over the last 20 years, potentially as a result of changes in plankton communities. But our results provide evidence that such changes cause widespread darkening that reduces the amount of ocean available for animals that rely on the sun and the moon for their survival and reproduction,” said Thomas Davies, Associate Professor of Marine Conservation at the University of Plymouth.
Featured image: NASA via Wikimedia Commons.
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