The answer, published in the journal Regional Environmental Change, is a study in contrasts. The kelp itself, it turns out, is still there, in densities that statistically match those recorded more than five decades earlier. But the living community woven around those forests has been quietly rewritten. An invasive sea anemone that did not exist in Patagonian waters in 1972 now smothers hard surfaces across sheltered sites, covering up to 88.4 percent of the seafloor in places. The region’s commercially harvested sea urchin has crashed by as much as 90 percent, its remaining individuals measurably smaller than their ancestors. And one widespread, long-lived urchin species appears to have vanished almost entirely from the study region.

The comparison is possible because of an extraordinary scientific legacy. In 1972, ecologist Paul Dayton sailed aboard the research vessel Hero as part of one of the first quantitative expeditions to describe kelp forest ecology across the breadth of Patagonia. His work, published in the mid-1980s, established the region’s forests as functionally unique—shaped by exposure gradients and largely free of the human disturbance that had transformed kelp ecosystems in the Northern Hemisphere. Dayton’s coordinates, transect designs and written descriptions became a frozen snapshot of a baseline world. In February 2026, a team led by Albert Pessarrodona of the University of Western Australia, working with Rewilding Chile’s Patagonian Megatransect initiative, relocated five of those original forests and replicated the historical sampling with remarkable fidelity, laying the same 25-meter transects along the same isobaths in the shallow subtidal zone.

The expedition surveyed twelve sites in total, spanning a dramatic environmental gradient that begins inside the Aysén fjord, threads through the channel networks of the Chonos Archipelago, and ends at fully wave-exposed oceanic islands. At each site, divers counted and measured kelp holdfasts—the root-like anchors of the plants—while a second diver tallied every mobile invertebrate larger than 2.5 centimeters. Kelp were classified into recruits, juveniles and adults using the same holdfast-diameter criteria Dayton had applied. Herbivore densities were quantified across three depth strata using quadrats, and the cover of every sessile organism was estimated with point-intercept methods. The statistical machinery brought to bear on the comparison was thoroughly modern: generalized linear mixed models with Gamma and Tweedie distributions, site treated as a random effect, and a 5,000-resample bootstrap to generate empirical confidence intervals where historical raw data were incomplete.

The headline finding on the kelp itself was reassurance. Adult giant kelp densities did not differ significantly between 1972 and 2026, and the community patterns across the fjord-to-ocean gradient closely matched the historical descriptions. At offshore sites, the dynamics remained what ecologists call top-down: grazing by the red sea urchin Loxechinus albus, the dominant herbivore by biomass, shapes where kelp can persist, carving urchin barrens at depth and confining forests to shallow boulder tops where wave exposure limits urchin access. Toward the sheltered inner fjord, the rules change. There, abiotic stress—freshwater runoff, low salinity, summer nutrient depletion—suppresses kelp growth, producing stunted, dwarfed plants barely three meters long, and non-consumptive interactions such as competition for space take center stage. The fjord’s shallow surface layer, with salinity dipping as low as 0.6 practical salinity units, excludes grazers that cannot tolerate the brackish conditions, so forests exist near the fjord head entirely devoid of herbivores.

Yet the apparent stability of the kelp itself conceals a subtler warning. Mean adult holdfast diameter was significantly larger in 2026—21.6 centimeters versus 13.6 in 1972—suggesting that today’s forests are dominated by older cohorts, a demographic signature that in other regions has preceded collapse. Recruit densities, meanwhile, were higher in the historical surveys, though the authors caution this likely reflects seasonal timing differences between the November historical sampling and their February expedition rather than a genuine decadal decline in reproduction. In the sheltered inner fjord, kelp populations run on an annual or semi-annual cycle, vanishing each late summer and persisting through a microscopic phase, an adaptation to the nutrient-poor, stratified waters that follow summer warming. Offshore, bathed in nutrient-rich subantarctic water, populations are perennial and reproduce year-round. These life-history differences, the study argues, reveal the fundamental role of geomorphology and oceanography in structuring the entire ecosystem.

The clearest footprints of global change appeared in the animals living among the kelp. Loxechinus albus, the target of the world’s largest sea urchin fishery—roughly 30,000 tons per year in the Aysén region alone—declined by 59 to 90 percent across the depths examined at the historical sites. Depth-specific averages that once ranged from 0.6 to 101 individuals per square meter in 1972 now span just 0.8 to 20. Maximum body sizes have shrunk from more than 10 centimeters to 7.5, a pattern classically associated with intense fishing pressure that removes the largest, oldest individuals. Independent stock assessments indicate standing stocks have fallen 45 to 51 percent since the late 1990s, and the bootstrapping analysis showed that the overwhelming majority of historical density values—between 80 and 94 percent depending on depth—fell outside the contemporary range of variation. Intriguingly, the surveys suggest offshore kelp forests may now be more extensive than Dayton reported, plausibly because reduced urchin grazing has loosened the top-down control that once constrained them—a cascade with consequences no one yet fully understands.

Equally striking is the fate of Pseudechinus magellanicus, a small, long-lived, opportunistic urchin that was ubiquitous in the 1970s at average densities of 1.7 to 30.1 individuals per square meter. In 2026, it was recorded at only four of twelve sites, at densities of essentially zero. Because this species is not commercially harvested, the mechanism behind its disappearance remains a mystery, though the researchers speculate that reduced recruitment—or populations living at the edge of their geographic range—may be involved. Its functional role, grazing on drifting kelp fragments from cryptic refuges in holdfasts and crevices, may already be quietly absent from the system.

Then there is the anemone. Metridium senile, a plumose filter-feeder native to the North Atlantic and North Pacific, first appeared in Chilean Patagonia in 2005. Two decades later, it is common at all but three of the surveyed sites, colonizing gravel, kelp fronds and even the surfaces of mobile invertebrates, and reaching peak cover in the sheltered channels and inner fjord. Its distribution appears governed by physics: oscillatory wave turbulence at exposed sites deforms its tentacle crown and cripples its feeding efficiency, while the low-salinity surface layer of the Aysén fjord falls below its known tolerance of roughly 15 to 37.5 practical salinity units, excluding it from the shallowest inner-fjord habitats. Where conditions suit it, however, it dominates—likely competing with native filter feeders including mussels, barnacles and a rich diversity of indigenous anemones. The study found no simple relationship between anemone cover and kelp recruitment or urchin density, but the authors warn that impacts may be most severe in the annual, sheltered kelp populations that depend on microscopic life stages to persist, and they flag potential effects on commercially important resources.

The broader lesson is one that marine ecologists have learned repeatedly around the world: the canopy can look intact while the forest’s inner machinery is being dismantled. Patagonia’s kelp forests have persisted through five decades of intensifying pressures—industrial fisheries, the explosive expansion of salmon aquaculture, marine invasions, and increasingly extreme climatic events—and the region’s relative climatic stability has likely buffered them from the declines seen elsewhere. But persistence of the foundation species is not the same as health of the ecosystem. The authors argue that protecting these globally significant forests will require conserving not just the kelp itself, but the ecological processes, connectivity and environmental conditions that underpin their resilience. In a region where long-term monitoring has been nearly impossible due to sheer remoteness, this rare half-century resurvey delivers both reassurance and alarm: the forests are still standing, but the fingerprints of the Anthropocene are now pressed firmly into every square meter of the seafloor beneath them.

Subject of Research: Long-term ecological change in Patagonian giant kelp forests assessed by resurveying sites first studied in 1972

Article Title: Footprints of global change in Patagonian kelp forests after half a century

Article References: Pessarrodona, A., Palacios, M., Kaminsky, J., Mora-Soto, A., Dayton, P. K., & Hüne, M. (2026). Footprints of global change in Patagonian kelp forests after half a century. Regional Environmental Change, 26(4), Article 194. https://doi.org/10.1007/s10113-026-02680-1

Image Credits: AI Generated

DOI: 10.1007/s10113-026-02680-1

Keywords: giant kelp, Macrocystis pyrifera, Patagonia, kelp forests, sea urchins, invasive species, Metridium senile, Loxechinus albus, Chile, fjords, marine conservation, historical ecology

The numbers at the heart of the analysis are stark. By synthesizing published data on global gains and losses of seaweed forests, the team estimated that continued declines in kelp forests, the most widespread and productive seaweed ecosystems on Earth, could reduce natural carbon sequestration by somewhere between 1.5 and 30 million tons per year. To grasp the imbalance, consider that this projected loss is between 1,000 and 10,000 times greater than the entire annual carbon dioxide sequestration benefit achieved by all kelp forest restoration projects completed to date. In other words, for every ton of carbon that restoration efforts have managed to capture through replanting, climate-driven habitat loss is erasing one thousand to ten thousand tons of sequestration capacity somewhere else on the planet. No realistic scaling of current restoration activity could close a gap of that magnitude within the timeframe that climate targets demand.

The spatial arithmetic is equally sobering. The authors estimate that approximately 25 million hectares of wild kelp forests will be lost by 2050 as warming oceans, marine heatwaves, and shifting ecological pressures continue to degrade these habitats. That projected loss area is more than 6,000 times the total area of kelp forest that has been successfully restored since 1958, when records of restoration activity in this field essentially began. The comparison illustrates a fundamental asymmetry: destroying a kelp forest takes years at most, while rebuilding one demands sustained labor, favorable conditions, and enormous financial investment, and even then the restored patch may never recover the structural complexity and ecological function of the original.

This asymmetry is not hypothetical. The essay points to large-scale kelp restoration projects in Norway and California as case studies in what genuine restoration actually requires. These flagship efforts consumed enormous investments of time, money, and human effort to restore comparatively modest areas of seabed. Extrapolating from those real-world costs, the researchers calculate that tens of thousands of additional projects of similar scale would be needed merely to compensate for the kelp forest losses projected to occur as a consequence of climate change. The price tag for such a global undertaking would run to tens or even hundreds of billions of United States dollars, a sum that far exceeds the current budgets of all marine restoration programs combined. Even if the funding were somehow secured, the ecological and logistical constraints of growing kelp at that pace and scale remain essentially untested and likely unattainable in the required window.

Underlying all of this is a deeper structural problem in how natural climate solutions have been folded into global climate planning. Carbon sequestration by natural ecosystems has historically been so reliable that its continued performance has been implicitly embedded in climate projections and net-zero strategies. Forests, wetlands, seagrasses, and seaweed forests have simply been assumed to keep doing what they have always done: absorbing carbon dioxide and storing it. Climate change is now compromising that assumption from within, degrading the very ecosystems on which those projections depend. When the carbon sink itself is shrinking faster than human intervention can expand it, the accounting that underpins net-zero commitments becomes dangerously optimistic. The authors argue that this dynamic applies with particular force to seaweed, where restoration enthusiasm has outpaced a rigorous assessment of what restoration can realistically deliver.

There is also a subtler risk that the essay identifies: the psychological and political effect of promoting seaweed restoration as a carbon dioxide removal method. If governments, companies, and the public believe that replanting kelp can meaningfully offset emissions, the perceived need for the harder work of eliminating fossil fuel combustion diminishes. The authors warn that this narrative may offer false hope and could delay the phase-out of fossil fuels, locking in further warming that will, in turn, accelerate the loss of the very seaweed forests that restoration schemes claim to rescue. It is a feedback loop of misplaced confidence, in which optimism about technological and ecological fixes erodes the political will required to address the root cause of the problem.

The solution the authors advocate is a shift in emphasis from restoration to protection. Preventing the loss of threatened seaweed habitats, they contend, is a cheaper and more effective climate change mitigation strategy than attempting to rebuild what has already been destroyed. Conservation measures such as reducing local stressors, including pollution, overgrazing by herbivores whose predators have been overfished, and destructive coastal development, can maintain existing carbon sequestration capacity at a fraction of the cost of restoration. A hectare of kelp forest preserved today sequesters carbon immediately and continues supporting fisheries, buffering coastlines, and sheltering biodiversity, whereas a hectare of restored kelp may take years or decades to approach comparable function, if it survives at all. In climate terms, the cheapest ton of carbon is always the one never lost.

None of this means seaweed forests are unworthy of restoration, and the authors are careful to make that distinction. Restoring and conserving these ecosystems remains worthwhile for the countless ecological and economic benefits they provide, from nursery habitat for commercially valuable fish species to coastal protection against storm surge. The argument is about honesty in carbon accounting and realism about scale. As the authors state, conserving and restoring seaweed forests is worthwhile, but society needs to be realistic about what these efforts can achieve in terms of meaningful climate change mitigation through carbon dioxide removal. Climate-driven losses of seaweed forests are currently orders of magnitude greater than gains due to restoration, and restoring seaweed forests cannot deliver meaningful climate change mitigation at the scales and speeds required to offset growing emissions. That statement, grounded in the quantitative analysis of the PLOS Biology essay, should serve as a corrective to a decade of enthusiasm that has sometimes outrun the evidence.

The broader lesson extends well beyond kelp. Around the world, natural climate solutions are being marketed with carbon removal claims that have not been tested against the accelerating losses that climate change itself imposes on the ecosystems in question. Mangroves, seagrass meadows, peatlands, and forests all face the same dynamic: degraded sinks, expensive restoration, and policy frameworks that assume permanence where none is guaranteed. The seaweed analysis offers a template for evaluating these claims honestly, comparing projected losses against achieved gains and asking, bluntly, whether a proposed intervention can operate at the scale the carbon budget requires. For seaweed forests, the answer is currently no. Protecting what remains, cutting emissions at the source, and treating restoration as an ecological priority rather than a climate offset are, according to this analysis, the only strategies that align with both the biology of these ecosystems and the mathematics of the climate crisis. The underwater forests that still stand are worth far more than any forest we might hope to replant.

Subject of Research: Climate-driven declines in kelp and seaweed forests undermining restoration-based carbon dioxide removal

Article Title: Climate-driven losses in seaweed carbon sequestration outpace habitat restoration gains

Article References: Climate-driven losses in seaweed carbon sequestration outpace habitat restoration gains. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: kelp forests, seaweed, blue carbon, carbon sequestration, climate change, habitat restoration, natural climate solutions, carbon dioxide removal, marine ecosystems, PLOS Biology, ocean warming, net-zero