Major Antarctic phytoplankton shifts occurred between 1997 and 2023, according to recent research published in Nature Climate Change (July 25, 2025). Climate change is causing these microscopic organisms, which are the foundation of the food web in the Southern Ocean, to change, primarily through altered sea ice regimes. The study, led by Alexander Hayward of the Danish Meteorological Institute, combined 14,824 in situ pigment samples with European Space Agency (ESA) satellite data to map long-term changes. Findings show declines in diatoms and increases in haptophytes and cryptophytes, with a marked regime shift after 2016 coinciding with accelerated sea ice loss.
A complementary Nature study on abrupt Antarctic changes confirms these as part of a broader ecosystem transformation. Such Antarctic phytoplankton shifts and climate change dynamics may weaken the ocean’s role in carbon storage while destabilizing food webs.
What Changes Have Been Observed in Antarctic Phytoplankton?
The research documents a restructuring of phytoplankton along the continental shelf and seasonal sea ice zones.
- Diatoms: Lost 31 percent of their climatological average, or 0.32 mg chl-a m⁻³.
- Haptophytes: Increased by 0.08 mg chl-a m⁻³.
- Cryptophytes: Rose by 0.23 mg chl-a m⁻³.
These changes were statistically significant in over 70% of the studied regions.
Figure: Antarctic phytoplankton trends (1997–2023) illustrating (a) the relative abundances of diatoms, haptophytes, and cryptophytes; (b) regional variations in chlorophyll a; (c) trends before and following 2017; and (d) the consequences of sea ice loss following 2016. Source: Nature
A turning point occurred around December 2016. Before this, diatoms declined at –0.03 mg chl-a m⁻³ yr⁻¹, while haptophytes steadily increased. After 2016, diatoms rebounded modestly, but cryptophytes expanded dramatically across nearly circumpolar waters. Diatom losses were especially pronounced in the Ross Sea and Prydz Bay, while West Antarctica saw sustained declines even post-2017.
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How Are These Shifts Connected to Sea Ice?
Sea ice concentration (SIC) emerged as the strongest driver of phytoplankton change, explaining 31% of diatom recovery variance and 21% of cryptophyte increases.
- Pre-2017: Rising sea ice limited light and nutrients, suppressing diatoms.
- Post-2016: Rapid sea ice loss created opportunities for smaller phytoplankton, while diatoms rebounded in localized regions.
Satellite data verify that diatoms were supported by high SIC (>75%), moderate SIC favored haptophytes, and regional variances influenced ecosystem dynamics. The August 2025 Nature update attributes these sea ice losses directly to human-driven warming, reinforcing the role of Antarctic phytoplankton shifts and climate change in shaping ecosystems.
Figure: Environmental trends across the Antarctic Shelf and seasonal sea ice zone (1997–2023). Source: Nature
| Phytoplankton Group | Overall Change (1997–2023, mg chl-a m⁻³) | Pre-2017 Trend (mg chl-a m⁻³ yr⁻¹) | Post-2017 Trend (mg chl-a m⁻³ yr⁻¹) | Shelf with Significant Change |
|---|---|---|---|---|
| Diatoms | –0.32 (33% decline) | –0.03 | +0.09 | 80% |
| Haptophytes | +0.08 | +0.031 | –0.02 | 82% |
| Cryptophytes | +0.23 | <0.01 | +0.07 | 70% |
Source: Hayward et al., 2025; updated with satellite data through mid-2025
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What Are the Consequences for Marine Food Webs?
Shifts in phytoplankton composition could disrupt the krill-centric ecosystem. Diatoms are the primary food source for Antarctic krill (Euphausia superba), which in turn sustain penguins, whales, and seals.
- Declining diatoms decrease krill availability.
- Microbial loops are favored by an increase in cryptophytes and haptophytes, which less effectively transfer energy up the food chain.
- Predator populations may become even more unstable as salp populations increase and krill populations decline.
The Australian Antarctic Division’s August 2025 update warns that continued phytoplankton changes could amplify biodiversity and fisheries risks across the Southern Ocean.
How Might Carbon Storage Be Affected?
Diatoms play an outsized role in carbon sequestration due to their silica shells, which sink rapidly and transport carbon to the deep ocean. The biological carbon pump is less efficient when it declines, whereas small phytoplankton have a greater chance of being recycled at the surface.
The general trend toward smaller species indicates decreased carbon export, even though diatom recoveries after 2016 offer some respite. The goal of ESA’s 2025-launched Phyto-CCI project is to monitor these dynamics and enhance calculations of the Southern Ocean’s contribution to global carbon sinks. This link between Antarctic phytoplankton shifts and climate change highlights the urgency of monitoring.
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What Environmental Factors Beyond Sea Ice Are Involved?
Sea ice is an important factor, but it’s not the only one:
- Iron availability: Declines to <0.4 nM l⁻¹ disadvantage diatoms, which have higher nutrient demands.
- Warming waters: Light and nutrient dynamics are changed by shifting mixed layer depths and rising surface temperatures.
- Atmospheric forcing: Stronger winds and nutrient upwelling compound phytoplankton responses.
Multivariate analyses show diatoms thriving under high iron and SIC, while haptophytes and cryptophytes succeed in warmer, low-iron conditions. These complex interactions reflect the broader influence of Antarctic phytoplankton shifts and climate change across multiple environmental variables.
Figure: Links between environmental conditions and Antarctic phytoplankton from 1997 to 2023. Source: Nature
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FAQ
1. What role do diatoms play in carbon storage?
They enhance long-term sequestration by sinking rapidly, unlike smaller phytoplankton, making them central to understanding Antarctic phytoplankton shifts and climate change.
2. How has sea ice loss since 2016 shaped communities?
It triggered diatom rebounds in some regions but allowed cryptophytes to expand across circumpolar waters.
3. Are these changes permanent?
Partial reversals post-2016 suggest recovery is possible, but ongoing warming may entrench dominance by smaller species.
4. What monitoring is underway?
ESA’s Phyto-CCI satellites now provide real-time tracking to project ecosystem and climate outcomes.
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