Hidden organic alkalinity in Arctic waters skews ocean acidification forecasts

The Arctic Ocean’s margins are acidifying faster than almost anywhere else in the global ocean, and the numbers scientists use to track that change may be systematically wrong. A new study published in Communications Earth & Environment shows that a long-overlooked component of seawater chemistry — organic alkalinity — is present at unexpectedly high concentrations in Arctic coastal waters, and that ignoring it introduces substantial errors into the two most widely used indicators of ocean acidification: pH and the aragonite saturation state. The finding, led by Shuzhen Song of the Ocean University of China and the Norwegian Institute for Water Research, together with Richard Bellerby and colleagues, carries immediate implications for how acidification is monitored across the Svalbard fjords, Tanafjorden, the Barents Sea Opening, and potentially the broader Atlantic sector of the Arctic margins.

To understand why this matters, it helps to recall how ocean acidification is actually measured. Marine chemists rarely determine pH or carbonate saturation directly across large regions. Instead, they measure total alkalinity and dissolved inorganic carbon, then calculate the remaining carbonate system variables using thermodynamic equations. Total alkalinity represents the seawater’s capacity to neutralize acid, and in the open ocean it is dominated by a small set of well-characterized inorganic species: bicarbonate, carbonate, borate, hydroxide, and hydrogen ions, with minor contributions from phosphate and silicate. The standard calculation packages assume that these inorganic species account for essentially all of the measured alkalinity. That assumption works reasonably well in the deep ocean, but it begins to fail in coastal waters, where dissolved organic molecules — humic substances, degradation products of terrestrial and marine organic matter — carry their own acid-neutralizing functional groups.

These organic molecules contribute what chemists call organic alkalinity. Carboxylate and phenolate groups on dissolved organic matter bind hydrogen ions, adding to the total alkalinity pool without being captured by the conventional speciation equations. In tropical and temperate estuaries, organic alkalinity has been recognized as a potentially substantial fraction of total alkalinity for years. What the new study demonstrates is that Arctic margins, fed by riverine inputs of organic-rich water, melting permafrost catchments, and intense seasonal production of organic matter, host elevated organic alkalinity concentrations that have been effectively invisible in regional acidification assessments. When the conventional equations are applied to water containing appreciable organic alkalinity, the missing organic component is misattributed to the inorganic species, and the calculated carbonate system variables come out biased.

The magnitude of the bias is striking. In the Svalbard fjords, Tanafjorden, and the Barents Sea Opening, the researchers found that failing to account for organic alkalinity introduces errors of 1 to 51 percent in the calculated hydrogen ion concentration — the quantity that defines pH — and errors of 1 to 117 percent in the calculated aragonite saturation state. Aragonite saturation state is the critical parameter for cold-water corals, pteropods, and other calcifying organisms, because it determines whether aragonite, the form of calcium carbonate their shells and skeletons are built from, will dissolve or precipitate. A saturation state above one means aragonite is stable; below one, it begins to dissolve. An error of up to 117 percent in that number means the difference between water that appears hospitable to shell-building life and water that is actually corrosive — or the reverse.

The direction of the bias depends on the local chemistry, which is precisely what makes the problem so insidious. In some Arctic margin waters, ignoring organic alkalinity makes the calculated conditions look more acidic than they really are; in others, it masks acidification that is actually underway. Because the organic component varies with river discharge, season, and the composition of the dissolved organic matter pool, there is no single correction factor that monitoring programs could apply. Each water mass, and each season within each water mass, carries its own organic alkalinity signature. The study’s authors argue that this variability, combined with the substantial error magnitudes, renders conventional acidification assessments for Arctic margins non-negligibly biased — a carefully worded but damning conclusion for a field that has built regional trend analyses on the standard equations.

The implications extend beyond the sampled fjords and basins. The Arctic Ocean’s Atlantic sector receives enormous fluxes of terrestrial organic carbon from Eurasian rivers, and the margins are regions where ocean acidification is progressing fastest, driven by the combined effects of rising atmospheric carbon dioxide, melting sea ice, and large inputs of fresh water. If organic alkalinity biases the carbonate calculations in the Svalbard fjords and the Barents Sea Opening, the same physics and chemistry likely apply across the broader Arctic margins, wherever riverine organic matter and coastal production inject dissolved organics into the carbonate system. The researchers suggest their findings have potential implications for the broader Atlantic sector of the Arctic margins, a region that includes some of the most intensively monitored acidification hotspots in the polar north.

Perhaps the most consequential part of the study is its forward-looking analysis. Using outputs from Earth system models, the team constructed regional projections of future acidification in the Arctic margins, first with the conventional treatment that ignores organic alkalinity, and then with organic alkalinity explicitly accounted for. The results indicate that when organic alkalinity is included, the projected pH decline becomes larger and faster. In other words, the conventional assessments may have been underestimating the pace of acidification in these waters. If that holds across the Arctic margins, the timeline for biological impacts — corrosive events, stress on calcifiers, shifts in community composition — could be shorter than current models suggest, and the margin waters may cross critical saturation thresholds earlier than monitoring programs anticipate.

The technical roots of the problem lie in the speciation equations themselves. The thermodynamic constants used to partition total alkalinity among inorganic species are exquisitely well determined for the inorganic system, but the equations have no term for the organic pool. When a titration measures total alkalinity in organic-rich water, the organic contribution is folded into the total, and the calculation then forces the inorganic species to explain alkalinity that they do not actually possess. The result is an overestimate of the inorganic carbonate and bicarbonate concentrations, which propagates into the calculated pH, the hydrogen ion concentration, and the saturation states. Correcting the problem requires either direct measurement of the organic alkalinity fraction — through techniques that quantify the proton-binding capacity of the dissolved organic pool — or empirical relationships linking organic alkalinity to measurable tracers such as dissolved organic carbon, colored dissolved organic matter, or salinity in river-influenced water masses.

For the Arctic monitoring community, the study arrives at a moment of reckoning. Regional acidification assessments, biological threshold analyses, and the growing network of coastal carbonate chemistry observatories across the Norwegian north have all relied on the conventional calculation packages. The new results indicate that these tools, applied without correction in organic-rich margin waters, produce non-negligible biases in the very quantities they are designed to track. The authors call for organic alkalinity to be accounted for in coastal waters and beyond — a phrase that hints at the wider relevance of the problem, since organic-rich margin waters exist not only in the Arctic but in river-influenced shelves, estuaries, and upwelling zones around the world. As the Arctic warms and permafrost thaw releases ever more organic carbon into the ocean, the organic alkalinity pool in these waters is likely to grow, making the bias larger over time rather than smaller.

The study, published open access on 30 September 2026, was supported by the Research Council of Norway, the National Natural Science Foundation of China, and the European Union’s Horizon Europe program, among others. Its message is deceptively simple: a component of seawater chemistry that most regional assessments have treated as negligible is, in the Arctic margins, anything but. Correcting the record will require new measurements, revised calculation frameworks, and a willingness to revisit acidification trends that have been reported for some of the fastest-changing waters on the planet. For the organisms that live in these waters — the cold-water corals of the Norwegian fjords, the pteropods of the Barents Sea, the shell-building communities of the Svalbard shelves — the difference between the calculated and the actual carbonate chemistry may determine whether their habitat remains habitable through the coming decades.

Subject of Research: Overlooked contribution of organic alkalinity in Arctic Ocean margins biases ocean acidification estimates

Article Title: Overlooked contribution of organic alkalinity in Arctic Ocean margins biases ocean acidification estimates

Article References: Overlooked contribution of organic alkalinity in Arctic Ocean margins biases ocean acidification estimates. (n.d.). https://doi.org/10.1038/s43247-026-04072-z

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04072-z

Keywords: ocean acidification, Arctic Ocean, organic alkalinity, carbonate chemistry, marine chemistry, climate change, Svalbard, Barents Sea, Overlooked, contribution, organic, alkalinity