Near Antarctica, the seafloor is shifting quickly. Spots that once held patches of red algae are now crowded with dense brown seaweed, all within just four years.
This is happening right now along the Western Antarctic Peninsula, where sea ice has plummeted to levels unseen in the satellite era. Between 2019 and 2023, researchers documented macroalgae — the kelps and seaweeds of polar waters — surging across the seafloor with startling speed. Total plant cover increased by an average of 17% across all study sites. But the real shock came from what type of algae took over.
Brown macroalgae, the massive perennial species that form underwater canopies, increased by 72% on average. These are not delicate organisms. Species like Himantothallus grandifolius and various Desmarestia grow blades over two meters long, layering atop one another in thick mats. Their biomass dwarfs that of smaller red algae. When researchers converted percent cover to actual weight, the results were staggering: estimated biomass increases ranged from 20% to over 100% depending on location, averaging more than 60%.
The driving force? Sea ice concentration plummeted across the same period. After peaking in 2014, Antarctic sea ice has shattered record lows repeatedly — in 2017, 2022, 2023, and again in 2024. Some scientists now speak of a regime shift, a fundamental change in how the Southern Ocean behaves. If this trend continues, projections of 15–50% ice loss by 2100 may prove conservative.
Light governs everything for Antarctic macroalgae. Unlike terrestrial plants that might struggle with nutrients or water, these seaweeds have ample dissolved minerals. What they lack is sunlight. Sea ice blocks it. When ice vanishes, photons reach the seafloor, and algae respond.
The research team surveyed four sites near Anvers Island using underwater video transects between 5 and 40 meters depth. In 2019, they established baseline measurements. They returned in 2020 to two sites, finding conditions largely unchanged — consistent with sea ice concentrations that had remained stable. But 2023 told a different story.
Every single transect showed increased macroalgal cover. At some depth intervals, seaweed blanketed over 90% of the rocky bottom. One transect at the northernmost site reached 91% cover, approaching what researchers suspect may be the upper limit imposed by iceberg scouring, which periodically scrapes the seafloor clean in shallow waters.
The dominance of brown algae was unexpected. Previous research along a sea ice gradient in 2019 found strong correlations between ice cover and total macroalgal abundance, but nothing predicted this dramatic shift toward large, heavy overstory species. Smaller red algae actually decreased in visible cover at three of four sites by an average of 29%, though this may be misleading. Many red algae now grow hidden beneath the brown canopy, invisible to cameras but still present.
Why does this matter beyond Antarctic ecology?
Macroalgae are carbon pumps. They fix carbon dioxide through photosynthesis, building tissue from atmospheric carbon dissolved in seawater. When they die or shed blades, that organic matter fuels detrital food webs throughout coastal Antarctica. Amphipods, small crustaceans abundant in these forests, feed on decomposing algae. Larger animals feed on amphipods. The system builds upward from dead seaweed.
More significantly, Antarctic macroalgae travel. Currents and storms rip plants from rocks and transport them into deep water, sometimes hundreds or thousands of meters down. There, in cold, oxygen-poor conditions, decomposition slows. Carbon gets sequestered, locked away from the atmosphere for centuries or longer. This is blue carbon — carbon captured by marine organisms and stored in ocean sediments.
Scientists only recently began quantifying macroalgae's contribution to Antarctic blue carbon. If biomass is indeed increasing by 60% or more in just four years, the implications cascade. More seaweed means more carbon fixed. More biomass means more material exported to depth. As sea ice continues declining, these underwater forests could sequester substantially more carbon than current models predict.
But there's nuance. The researchers caution that their biomass estimates, while clearly showing large increases, carry uncertainty. Converting percent cover from video to actual wet weight requires assumptions about how thickly seaweeds layer. In areas of high brown algae cover, plants stack atop one another in dense mats impossible to measure precisely without destructive sampling. The team collected only 22 quarter-square-meter quadrats, cutting and weighing everything inside, to develop conversion factors. More data would strengthen confidence in absolute numbers.
The pattern, however, remains robust. Brown macroalgae are expanding disproportionately as ice retreats.
One site revealed something else entirely: a massive bloom of filamentous colonial diatoms, microscopic algae forming chains visible to the naked eye. In 2020, at depths between 25 and 35 meters, these diatoms covered up to 75% of the seafloor, smothering invertebrates and macroalgae alike. By 2023, they had vanished.
Similar blooms have been reported elsewhere on the Antarctic Peninsula, often near retreating glaciers. The hypothesis: glacial melt reduces salinity and possibly changes nutrient delivery, creating conditions favorable for these opportunistic diatoms. Summer salinity measurements from Palmer Station's seawater intake showed 2019–2020 had significantly lower values than subsequent years, with 20% of days dropping below a salinity of 33. In contrast, summer 2022–2023 saw zero such days. The pattern fits.
If glacial melt triggers these diatom blooms, they represent another dimension of rapid ecological change. Unlike perennial macroalgae that persist for years, diatom blooms appear and disappear on seasonal to annual timescales, potentially disrupting established communities when they arrive.
Macroinvertebrate communities showed no significant changes across the study period despite dramatic shifts in plant cover. Sponges, sea stars, urchins, and other conspicuous animals remained stable. This suggests the benthos may respond to environmental change on different timescales, with mobile consumers perhaps tracking resources more slowly than primary producers can colonize new space.
The four-year window observed here is the shortest documented for quantitative change in Antarctic subtidal communities. Previous studies compared decades-apart snapshots or tracked succession on artificial surfaces. This work shows the Southern Ocean responding in ecological real-time.
What happens next depends on ice. If current trends reflect a genuine regime shift rather than multidecadal oscillation, these underwater forests will continue expanding. Sites farther south, currently ice-locked most of the year, could see similar transitions as they become ice-free. The researchers' 2019 regression models, which predicted macroalgal cover based on sea ice concentration, accurately forecast 2023 observations when ice declined further. The relationship holds.
But no one predicted the brown algae boom. That detail emerged from revisiting sites, from noticing which species filled newly available space. It illustrates how ecosystems surprise us, how secondary effects compound, how we cannot simply extrapolate trends without watching what actually happens.
The researchers are candid about what remains unknown. Is the disproportionate increase in brown macroalgae a general feature of low-ice Antarctic communities, or specific to these particular sites and years? Does it represent a temporary colonization advantage that will equalize over time, or a new stable state? How much of this biomass reaches deep water versus decomposing locally?
Answering these questions matters for global carbon budgets. If Antarctic macroalgae double their carbon sequestration as ice retreats, that's a negative feedback on climate change — more plant growth pulling CO₂ from the atmosphere. But feedbacks interact. Increased glacial melt might favor diatom blooms that smother macroalgae. Warmer water could change species composition. Iceberg calving might intensify as ice shelves collapse, increasing disturbance.
The Southern Ocean is not a passive victim of climate change. It's responding, reorganizing, finding new configurations. Seaweed forests expanding at 20–100% in four years qualify as rapid. They represent biological momentum, ecosystems shifting into unfamiliar territory with consequences we're only beginning to measure.
Antarctic sea ice extent in 2024 finished second-lowest on record. The lowest was 2023. Before that, 2022. The pattern is clear and accelerating. Along the Western Antarctic Peninsula, the ocean floor is already different than it was half a decade ago. What it will look like in another five years is an experiment running in real-time, watched by scientists with underwater cameras and a growing sense that the changes coming may outpace our ability to predict them.
Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.3354/meps14840
