International researchers have reconstructed the composition of phytoplankton communities around Antarctica over nearly three decades, constituting the most comprehensive study of its kind to date.
The study documents a significant shift in marine phytoplankton species—the microscopic unicellular algae that form the first link in the oceans’ food chain.
Conducted by the Danish Meteorological Institute (DMI), the study shows that energy-rich diatoms—preferred by krill—are declining across vast areas of Antarctica as they are overtaken by smaller, less nutritious phytoplankton species.
The study published in the journal Nature Climate Change documents a significant shift in marine phytoplankton species—the microscopic unicellular algae that form the first link in the oceans’ food chain.
“We may be witnessing a fundamental reorganization of life in Antarctica,” says the lead author, Alexander Hayward, climate scientist at the National Centre for Climate Research, DMI.
“The tiny algae at the base of Antarctica’s food web are changing in a way that could reverberate through the entire ecosystem—from krill to whales—and alter how the ocean helps regulate our climate,” he adds.
The implications of a significant change mean less food for krill, which would affect penguins, seals, and baleen whales that depend on krill.
Phytoplankton, like plants, absorb carbon dioxide through photosynthesis. Diatoms—with silica-rich skeletons—sink rapidly and drag carbon to the depths of the ocean. Golden/brown algae and cryptophytes do not sequester carbon to the same extent.
Satellite images, machine learning, and NASA models
The research was based on a dataset of 14,824 field samples of phytoplankton pigments collected mainly during the Antarctic summer months between 1997 and 2023.
“This study highlights the value of routine and opportunistic field sampling—taking a water sample from time to time and seeing what it contains,” says co-author Simon Wright, marine biologist at the Institute of Marine and Antarctic Studies.
“Over time, a valuable database is built,” he notes.
Using advanced machine learning, this database was analyzed to calculate the proportions of major algal groups based on their known pigment markers.
These results were combined with satellite data (such as ocean color during algal blooms, sea ice concentration, and sea surface temperature), environmental conditions (using NASA’s ECCO-Darwin biogeochemical model that includes the carbon cycle, nutrients, oxygen and alkalinity) and field measurements to model phytoplankton groups in the Antarctic Ocean over the 26-year period.
“Our analysis showed that, from 1997 to 2016, there were large reductions in diatom populations as sea ice increased,” says co-author Pat Wongpan, sea ice scientist with the Australian Antarctic Program Partnership at the University of Tasmania.
“Diatoms were replaced by golden/brown algae and cryptophytes that are more effectively grazed by gelatinous salps, which are a poorer food source for fauna and less efficient at carbon transport,” he adds.
Over the study period, the iron content (an important micronutrient for phytoplankton) of surface waters declined and temperatures rose—a cocktail that particularly affected diatoms, which require iron. Cryptophytes and golden/brown algae are less iron-dependent and, consequently, fare better under environmental changes.
Changes in plankton communities became more pronounced after 2016, when Antarctica recorded a drastic reduction in sea-ice extent. The trends reversed, with diatoms recovering and cryptophytes growing sharply, indicating a regime shift tied to sea ice, iron supply, and warming.
Although phytoplankton is fundamental to Antarctica’s iconic marine food web and to the carbon pump, long-term changes in the composition of its community are poorly understood. This new study aims to change that.
“Our investigation documents a shift in the south‑ern Ocean’s ecological system caused by climate change, which could, in turn, influence the climate through a feedback mechanism.”
“The carbon dioxide that would otherwise be stored in the depths of the ocean could now be released back into the atmosphere,” says Hayward.
“The observed correlation between changes in phytoplankton communities and the regime shift associated with trends in sea-ice cover highlights the sensitivity of the Antarctic marine ecosystem to climate change,” the paper concludes.
