Antarctic phytoplankton communities restructure under shifting sea-ice regimes
- Hakan Sener
- Sep 3
- 3 min read
Antarctic diatoms down ~33% on shelves as sea ice collapses; cryptophytes surge—threatening krill and weakening the Southern Ocean carbon sink.
A new study led by Alexander Hayward (Danish Meteorological Institute) uses 26 years of observations and machine learning to show a major, recent reorganization of Antarctic summer phytoplankton: diatoms have declined by ~one-third on the continental shelf, while cryptophytes and haptophytes have increased, with a sharp, sea-ice-linked regime shift beginning in late 2016.
The authors warn that these changes could weaken the krill-centric food web and reduce biological carbon export, undermining a key global carbon sink.
Key Findings: Who’s Up, Who’s Down—and Why
Shelf diatoms down ~33%; cryptophytes and haptophytes up
Across the Antarctic continental shelf (austral summers, 1997–2023), modelled diatom chlorophyll-a fell by ~0.32 mg chl-a m⁻³ versus a 0.97 mg chl-a m⁻³ climatology—about a third lower. Cryptophytes rose ~0.23 mg chl-a m⁻³ and haptophytes ~0.08 mg chl-a m⁻³ (all P<0.05).
Meanwhile, shelf “hotspots” like Prydz Bay and the Ross Sea showed pronounced total chl-a declines, even as the broader Antarctic (shelf + seasonal sea-ice zone) saw a 41% summer chl-a increase since 1997.
A 2016 regime shift tied to sea-ice collapse
Trends flipped after December 2016, coincident with a dramatic drop in sea-ice concentration (SIC).
Before 2017: diatoms decreased (−0.03 mg m⁻³ yr⁻¹) and haptophytes increased (+0.031 mg m⁻³ yr⁻¹); cryptophytes were flat.
After 2017: diatoms rebounded (+0.09 mg m⁻³ yr⁻¹), cryptophytes surged (+0.07 mg m⁻³ yr⁻¹), and haptophytes leveled/slightly fell. SIC explained 31% of the post-2017 diatom recovery variance and 21% of cryptophyte increase; geography mattered—West Antarctica kept losing diatoms even after 2017, while cryptophytes expanded nearly circumpolarly.
Drivers under the hood: less iron, shifting stratification, warming, and freshening
Circumpolar iron (Fe) availability declined, disadvantaging Fe-hungry diatoms and favoring haptophytes/cryptophytes under low-Fe conditions.
Mixed layer depth (MLD) trends were patchy: diatoms thrived in shallower MLDs (<100 m); haptophytes tolerated deeper MLDs (>150 m); cryptophytes preferred very shallow layers (<50 m).
Sea-surface freshening in West Antarctica aligned with cryptophyte gains; diatoms correlated negatively with SST, while haptophytes and cryptophytes correlated positively, consistent with regional warming.
Food-web and carbon-cycle consequences
Diatoms are krill’s preferred prey and disproportionately power the biological carbon pump via fast-sinking, silica-ballasted cells.
Their decline, coupled with rising small flagellates, risks a shift toward salp-dominated grazing, weakening both higher-trophic support and carbon export efficiency (salp pellets often disaggregate).
Net effect: potential reduction in Southern Ocean CO₂ uptake—in a region already recognized as a major sink.
How They Did It
The team compiled 14,824 in-situ pigment samples (1997–2023, Dec–Feb) and trained random-forest models (10-member ensembles; R² = 0.81–0.92 for key groups) to partition total chlorophyll into diatoms, haptophytes, and cryptophytes.
They fused these with satellite products (OC-CCI chl-a, ESA SST CCI) and a data-assimilative ocean biogeochemistry model (ECCO-Darwin) to map trends at 9-km, monthly resolution, restricting predictions to the observed environmental envelope and quantifying uncertainty via bootstrapping and perturbation tests.
Why It Matters for Policy and Planning
Ecosystem services at risk: A move away from diatom dominance imperils krill, with cascading risks for penguins, whales, and fisheries management.
Carbon-sink fragility: Fewer diatoms can mean weaker particle export, diminishing a globally significant CO₂ sink just as emissions cuts lag.
Early-warning value of sea-ice: The post-2016 SIC crash tracks rapid community shifts—underscoring sea ice as a canary for Antarctic biogeochemistry.
Monitoring imperative: The authors call for sustained, year-round observations (e.g., NASA PACE) to determine whether the post-2017 diatom rebound persists or was a transient response to extreme sea-ice anomalies.
A Fast-Evolving Base of the Food Web
This study provides the clearest, circumpolar-scale evidence yet that Antarctic phytoplankton communities are restructuring in step with shifting sea-ice regimes, with long-term diatom declines on the shelf and a post-2016 surge in cryptophytes as sea ice plummeted.
Because diatoms anchor both krill and the biological carbon pump, these changes raise red flags for ecosystem resilience and the global carbon cycle.
The authors’ machine-learning approach, grounded in extensive in-situ pigments and data-assimilative physics/biogeochemistry, turns disparate observations into an actionable picture: protecting Antarctica’s climate buffer now means tracking—and planning for—rapid, sea-ice-driven shifts at the very base of the food web.
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