Coral skeletons are built from aragonite, a crystalline form of calcium carbonate, laid down in seasonal growth bands like tree rings. As the coral grows, it incorporates trace elements dissolved in the surrounding seawater, and the ratios of those elements to calcium locked into each band preserve a snapshot of environmental conditions at the time of deposition. Barium, yttrium, iron and manganese are largely carried to the coast by rivers and groundwater after being weathered from volcanic soils, so their concentrations in coral skeletons serve as proxies for sediment and terrigenous input. Phosphorus, recorded through the phosphorus-to-calcium ratio, is a well-established tracer of nutrient pollution. Strontium, meanwhile, varies with water temperature, giving researchers a thermometric handle on past climate.

The team drilled a core from a massive coral at Olowalu, on Maui’s leeward coast, and analyzed the skeleton along its growth axis. The record spans 253 years, from 1760 to 2013, and captures an extraordinary arc of human history: pre-colonial Hawaiian stewardship of the land, the rise and fall of the sugarcane plantation economy, large-scale diversion of stream water for irrigation, and modern coastal development. Using computed tomography-based methods, including the CoralCT platform developed by researchers at Tulane University, the team quantified annual extension rates, skeletal density, and the product of the two, calcification, which is the most direct measure of how much calcium carbonate the coral actually produces each year.

The geochemical data reveal a clear, persistent signature of watershed disturbance. Averaged over the record, the ratios of barium, phosphorus and iron relative to calcium increased by between 17 and 55 percent from the eighteenth century to the late twentieth century. The single largest jump occurred in the mid-1900s, precisely when Maui’s sugarcane plantations reached their maximum extent. Plantation agriculture stripped hillsides of vegetation, loosened volcanic soils, and diverted enormous volumes of freshwater, all of which conspired to push more sediment, more nutrients and more dissolved terrestrial metals toward the reef. The coral recorded every step of that transformation in its skeleton.

Superimposed on this long-term pollution trend, the researchers found a rhythmic pulse in the trace element record. Variations in the terrigenous proxies oscillated at annual to decadal frequencies that match the El Niño-Southern Oscillation and the Pacific Decadal Oscillation, the two great engines of Pacific climate variability. El Niño years in Hawai’i bring distinctive rainfall anomalies, and decadal shifts in Pacific climate modulate both precipitation and groundwater recharge across the islands. In other words, the reef’s chemical record does not simply track what humans did to the land; it also tracks how climate controlled when and how much of that land-based material reached the ocean, delivered by both storm runoff and the submarine groundwater discharge that seeps through volcanic aquifers along Maui’s coast.

The most dramatic finding, and the one with the most troubling implications for reef conservation, concerns coral calcification itself. From the pre-colonial baseline, calcification rates rose by roughly 30 percent until about 1970, a period that included the plantation era. That initial increase may reflect an early nutrient subsidy: moderate inputs of sediment and dissolved nutrients can fertilize reef food webs, and corals that feed more heterotrophically can sometimes build skeleton faster, even under rising carbon dioxide levels. But after approximately 1970, coinciding with peak sugarcane production and maximum terrigenous loading, the trend reversed and calcification declined. The coral’s own chemistry records the moment when land-based inputs crossed from helpful to harmful.

The statistical case for that reversal is strong. The researchers found a negative correlation between calcification rate and the terrigenous proxies, including barium-to-calcium, yttrium-to-calcium, phosphorus-to-calcium and iron-to-calcium. Years with the heaviest terrestrial inputs were years of depressed skeletal growth. Laboratory and field studies have long suggested mechanisms for this relationship: suspended sediment clouds reduce light available to the coral’s photosynthetic symbionts, phosphate interferes directly with the crystal chemistry of aragonite precipitation, and episodic smothering events damage tissue and divert energy away from skeleton building. What the Olowalu core adds is the century-scale confirmation that these mechanisms operate over decades, not just during individual floods.

Timing matters here, because the mid-twentieth century was also a period of warming oceans, and the global decline in coral calcification is often attributed primarily to ocean acidification and thermal stress. The Maui record complicates that picture. It shows that local land-use change produced measurable growth declines decades before modern bleaching crises, and that the trajectory of coral growth at any single reef may be as much a story about the watershed behind it as about the seawater in front of it. This supports a growing body of evidence that managing local stressors, particularly sediment and nutrient runoff, can buy reefs time against the global stressors they cannot escape. The paper’s authors and collaborators, including conservation groups working in Olowalu, have pointed to sediment-reduction projects in the watershed as concrete steps informed by exactly this kind of baseline.

Perhaps the most valuable contribution of the study is its baseline. Without a pre-colonial reference point, managers have had to guess what a healthy reef’s water chemistry should look like, and debates over causation devolve into arguments about which measured decline is natural and which is anthropogenic. The 253-year Olowalu record shows, unambiguously, that barium, phosphorus and iron levels at the reef were substantially lower before industrial agriculture, that the enrichment tracks the documented history of plantation expansion and water diversion, and that this enrichment coincides with a measurable suppression of coral growth. The reef has been keeping score all along; the researchers have simply learned to read the scorecard.

For the reefs of west Maui, whose importance to Hawai’i’s tourism economy, shoreline protection and Native Hawaiian cultural practice is difficult to overstate, the message of the core is clear. The geochemical imprint of land-use decisions made more than a century ago is still legible in the reef today, and the declines in coral calcification that began around 1970 are a warning recorded in stone. As climate change intensifies both extreme rainfall and drought across the Hawaiian Islands, the coupling between ridge and reef documented here will only tighten. The coral at Olowalu will keep writing its diary regardless; the study demonstrates that what it writes next depends, in large measure, on how the land above it is managed now.

Subject of Research: Historical coral core geochemistry linking land-use change and climate variability to reef calcification in west Maui

Article Title: Effect of land-use change and climate on coral calcification and geochemistry: a 253-year time series from west Maui

Article References: Kekuewa, S. A. H., Prouty, N. G., Nalley, E. M., Hawco, N. J., Nelson, C. E., & Kealoha, A. K. (2026). Effect of land-use change and climate on coral calcification and geochemistry: a 253-year time series from west Maui. Coral Reefs. https://doi.org/10.1007/s00338-026-02950-8

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02950-8

Keywords: coral reefs, coral cores, geochemistry, land-use change, sugarcane agriculture, sediment runoff, nutrient pollution, calcification, Maui, El Niño-Southern Oscillation, Pacific Decadal Oscillation, ridge-to-reef

Ciguatera fish poisoning is caused by ciguatoxins, potent neurotoxins produced by benthic dinoflagellates of the genus Gambierdiscus and related organisms that grow on dead coral and macroalgae in tropical reef ecosystems. The toxins accumulate in herbivorous reef fish and pass up the food web to large predatory species that are a staple of island diets. Disturbances that create fresh surfaces for dinoflagellate colonisation, such as cyclone damage to coral or sediment and nutrient pulses from land, can therefore increase the abundance of toxigenic organisms and raise the risk of poisoning. Because there is no routine test for ciguatoxin in fish sold or caught for local consumption, island communities depend heavily on their own environmental knowledge to judge when and where fish are safe to eat.

To capture that knowledge, the researchers conducted structured interviews with 37 long-term residents of Ambae, asking them to describe changes in rainfall, tropical cyclones, land use, runoff and reef conditions over their lifetimes. The interview responses were then compared with MERRA-2 reanalysis rainfall data, the Modern-Era Retrospective Analysis for Research and Applications, version 2, produced by NASA, along with tropical cyclone records for the southwest Pacific and provincial-level ciguatera case data in which Ambae cases dominate. This dual approach, pairing perceived change with measured change, is a recognised method for validating local observations and for identifying where memory, experience and instrumentation diverge.

The comparison produced a striking pattern of convergence on intensity and divergence on frequency. Residents overwhelmingly perceived that rainfall had become more intense and that cyclones had grown stronger, and these perceptions broadly matched the observed record, which shows increasing rainfall variability and rising cyclone intensity across the study period. When it came to how often cyclones strike, however, perceptions diverged from the data: community members tended to believe cyclones were becoming more frequent, while the observed records did not show a matching upward trend in frequency. The authors attribute this gap to the psychology of high-impact storms, in which memories of devastating events such as Cyclone Funa, which struck Ambae in January 2008, and Cyclone Pam in 2015, anchor perceptions of frequency around the most traumatic experiences rather than the full statistical record.

This memory effect has practical consequences. Risk perception research has repeatedly shown that experience of extreme events shapes how communities prepare, adapt and respond, and the Ambae findings reinforce the point that perceived frequency can be inflated by salient disasters even when the underlying hazard statistics remain more stable. For disaster risk managers in Vanuatu and other small island developing states, the message is that community early-warning knowledge is reliable for intensity and impact but should be complemented by objective monitoring when it comes to frequency-based planning, so that adaptation resources are allocated according to actual hazard profiles rather than recollection alone.

The second strand of the study turned to health. Using exploratory statistical analysis, the researchers examined whether annual ciguatera case counts were more closely associated with rainfall anomalies or with tropical cyclone activity. The results pointed clearly toward cyclones: years with greater cyclone activity were more strongly linked to elevated case counts than years with unusual rainfall. This is mechanistically plausible, since cyclones deliver a double disturbance, physically shattering coral structures that dinoflagellates readily colonise and dumping large pulses of terrestrial sediment, nutrients and organic material onto nearshore reefs through intensified runoff. Studies from other Pacific islands, including Rarotonga in the Cook Islands and reef systems in American Samoa and Palau, have documented similar ridge-to-reef pathways in which catchment disturbance and storm runoff degrade benthic habitat and alter the ecological conditions that favour toxigenic dinoflagellates.

The authors are careful to stress the limits of causal inference. The ciguatera case series is short, recorded only at annual resolution, and compiled at a coarse provincial scale dominated by Ambae cases, which means the analysis cannot isolate the exact timing and location of exposures or control for changes in reporting practice, health-seeking behaviour and island population movements. Ambae’s own recent history complicates the picture further: the Ambae volcano erupted in a Plinian episode during 2017 and 2018, forcing the evacuation of the entire island population and disrupting both land use and fishing patterns. Volcanic ashfall, deforestation and displacement all interact with climatic stressors in ways that a short annual case record cannot disentangle, and the researchers explicitly frame their ciguatera findings as indicative associations rather than proof of causation.

Even so, the study’s central contribution is methodological and quietly transformative. It demonstrates that community knowledge and satellite-based environmental monitoring can be systematically combined to identify plausible ridge-to-reef pathways linking climatic disturbance, land-use change, reef degradation and climate-sensitive disease risk. Earlier work by the same lead author, published in Communications Earth & Environment in 2024, showed that Indigenous knowledge in Vanuatu, combined with scientific observation, can function as an early warning system for ciguatera outbreaks. The new research strengthens that foundation by quantifying which components of community observation are most trustworthy, and by anchoring them in a 43-year environmental record that no single resident could hold in memory.

For the people of Ambae, where reef fish remain central to nutrition, livelihoods and cultural practice, the stakes of this research are immediate. Ciguatera is the most widely reported marine food-borne illness in the Pacific islands, with tens of thousands of cases recorded regionally over recent decades, and its burden falls disproportionately on communities with the least access to laboratory diagnostics. Understanding that cyclone years may seed conditions for later poisoning outbreaks gives health authorities a possible window for targeted public health messaging, and gives fishing communities a scientific echo of the traditional knowledge they already apply when deciding whether a reef is safe. As climate change continues to intensify extreme rainfall and cyclone behaviour across the southwest Pacific, integrating local observation with environmental records offers small island communities one of the few practical tools available for anticipating a hazard that hides inside an otherwise valuable food source.

The research also carries a broader message for climate adaptation science. Perception studies conducted across Africa, Asia and the Pacific have often found mismatches between what communities observe and what meteorological records show, and those mismatches are frequently used to question local knowledge. The Ambae study flips that framing, showing that local observation can be highly accurate where it matters most, for intensity and impact, and that apparent errors are themselves informative about how humans experience and remember environmental risk. In an era when monitoring infrastructure is sparse across much of the Pacific, the residents of small islands are, in effect, a distributed sensing network, and this study shows how their observations can be calibrated against, and combined with, the satellite record to protect both ecosystems and human health.

Subject of Research: Community perceptions of climate variability, land-use change and ciguatera fish poisoning risk on Ambae Island, Vanuatu, compared with observed rainfall and cyclone records

Article Title: Community perceptions and observed environmental records of climate variability, land-use change, and ciguatera fish poisoning risk on Ambae Island, Vanuatu

Article References: Rarai, A., Weber, E., Sagero, P. O., & Parsons, M. (2026). Community perceptions and observed environmental records of climate variability, land-use change, and ciguatera fish poisoning risk on Ambae Island, Vanuatu. Regional Environmental Change, 26(4), Article 193. https://doi.org/10.1007/s10113-026-02686-9

Image Credits: AI Generated

DOI: 10.1007/s10113-026-02686-9

Keywords: ciguatera fish poisoning, Vanuatu, Ambae Island, climate variability, tropical cyclones, rainfall, land-use change, coral reefs, community perception, MERRA-2, ridge-to-reef, Pacific Islands