The life of the most basic organisms in the sea – phytoplankton – reveals to us dramatic climatic shifts in the icy world of Antarctica. A new research paper, created in collaboration between scientists from Denmark, Australia, the USA, Spain, and New Zealand, opens a view into almost three decades of changes in the composition of plankton communities in the "South", revealing that tiny algae are slowly replacing the current king of the ecosystem – diatoms – in terms of surface presence and importance.

Ministry at the lowest level

From 1997 to 2023, a significant decrease in the share of diatoms (siliceous algae) was observed, alongside a parallel increase in smaller plankton – haptophytes and cryptophyte protozoa. The decrease in diatoms is estimated at approximately 0.32 mg Chl-a/m³ (about one third of their average concentration), while haptophytes and cryptophytes recorded increases of 0.08 and 0.23 mg Chl-a/m³ respectively. These changes have significantly impacted biological processes and the importance of the Antarctic sea in carbon storage, suggesting a reorganization at the very core of the food webs.

Regeneration after 2016 – new opportunity or temporary shift?

An intriguing phase occurred after 2016, during which the first signs of diatom recovery were recorded, especially on the Antarctic continental shelf. At the same time, cryptophytes grew exponentially, coinciding with the sudden retreat of sea ice. Whether this is just a temporary adaptation to disturbed conditions – such as increased light and variable nutrient availability – or the beginning of a more permanent return of large phytoplankton communities remains an open question.

Climate triggers: ice, heat, iron

The reasons for these biological oscillations are quite clear: less and less sea ice, rising surface temperatures, and reduced iron – a vital nutrient for diatoms – have created an environment more favorable for smaller, more flexible organisms such as haptophytes and cryptophytes. The loss of sea ice is not only an indicator of global warming – it changes the structure of the ecosystem and the functioning of the biological "pump" that drives carbon into the depths of the oceans.

Extended implications – from krill to dolphins

Diatoms not only dominated quantitatively – they fed krill, a key link in the food chain that includes penguins, seals, and whales. With the suppression of diatoms, the food machine becomes noticeably weaker: less food for krill means less energy for larger organisms, which can have far-reaching consequences for Antarctic life communities.

Climate model and earthly confirmation

To understand these processes, scientists combined satellite observations – surface temperatures, ocean color, and ice cover – with pigment analysis from over 14 thousand samples taken from the sea. Machine learning models converted pigment data into spatial estimates of phytoplankton, creating the first reliable depiction of temporal and spatial changes in phytoplankton groups in Antarctica.

How much has the system changed?

From 1997 to 2023, the total concentration of chlorophyll (Chl-a) in parts of the Antarctic shelf and the seasonal ice zone increased by about 41% compared to the reference value (0.78 mg Chl-a/m³). Interestingly, within the shelf itself, the trend of change is not significant, but the wider system records dramatic fluctuations. Changes were statistically confirmed in 80% of locations, with a high level of reliability.

Open questions and continued monitoring needs

Although the changes are clearly documented, the key question remains: will the recovery of diatoms after 2016 be stable and long-term or just an episodic adaptation? Further satellite missions, such as PACE (Plankton, Aerosol, Cloud and ocean Ecosystem), will be crucial for monitoring these trends and answering this dilemma. Moreover, the survival of the biological carbonate pump – a process that so greatly influences Earth’s climate regulation – is at stake.

The bigger picture of global changes

This research into Antarctic microbiological dynamics illustrates a broader phenomenon – that climate change is altering ecosystems from their basic biological components. Reduction in sea ice, rising temperatures, and changes in nutrient availability have a domino effect: they change phytoplankton, which in turn changes the krill population, and thus the entire food chain up to the largest whales. Changes at the very bottom of the chain can have consequences in all directions – including our oceans’ ability to store CO₂.

Why all this needs to be monitored now

The climate sensitivity of Antarctic ecosystems makes them both vulnerable and crucial for global processes. Any change in phytoplankton affects not only ocean biology, but also the carbon cycle, climate, and life on continents. Therefore, constant monitoring, a combination of satellite and field data, and sophisticated models are essential to understand and, where possible, mitigate the consequences.