Posts Tagged 'Arctic'

Light, ammonium, pH, and phytoplankton competition as environmental factors controlling nitrification

The biogeochemical cycling of nitrogen (N) plays a critical role in supporting marine ecosystems and controlling primary production. Nitrification, the oxidation of ammonia (NH3) by microorganisms, is an important process in the marine N cycle, supplying nitrate (NO3−), the primary source of N that fuels new phytoplankton growth, and the primary substrate for the microbial process of denitrification. Understanding nitrification in the Chukchi Sea, the shallow sea overlying the continental shelf north of Alaska and the Bering Strait, is particularly important as phytoplankton growth there has been shown to be limited by N. However, the controls on nitrification in the water column and potential effects of climate change remain unknown. This study seeks to characterize the controls on nitrification in the Chukchi Sea. We found light to be a strong control on nitrification rates. Nitrification was undetectable at light levels above 23 μmol photons m−2 s−1. Subsequently, sea ice concentration was related to nitrification, with rates being higher at stations with high ice cover where light transmission to the water column was reduced. High ammonium (NH4+) concentrations also enhanced nitrification, suggesting that nitrifying organisms were substrate-limited, likely due to competition for NH4+ from phytoplankton. Unlike previous experimental studies, we found that nitrification rates were higher under low pH conditions. As the effects of ocean acidification and warming disproportionately impact the Arctic, nitrification rates will undoubtedly be affected. Our results will help guide future studies on potential implications of climate change on the biogeochemistry of N in the Chukchi Sea.

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Climate change and the sea: a major disruption in steady state and the master variables

Since the beginning of the industrial revolution, humans have burned enormous quantities of coal, oil, and natural gas, rivaling nature’s elemental cycles of C, N, and S. The result has been a disruption in a steady state of CO2 and other greenhouse gases in the atmosphere, a warming of the planet, and changes in master variables (temperature, pH, and pε) of the sea affecting critical physical, chemical, and biological reactions. Humans have also produced copious quantities of N and P fertilizers producing widespread coastal hypoxia and low dissolved oxygen conditions, which now threaten even the open ocean. Consequently, our massive alteration of state variables diminishes coral reefs, fisheries, and marine ecosystems, which are the foundation of life on Earth. We point to a myriad of actions and alternatives which will help to stem the tide of climate change and its effects on the sea while, at the same time, creating a more sustainable future for humans and ecosystems alike.

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High-frequency, year-round time series of the carbonate chemistry in a high-Arctic fjord (Svalbard)

The Arctic Ocean is subject to high rates of ocean warming and acidification, with critical implications for marine organisms as well as ecosystems and the services they provide. Carbonate system data in the Arctic realm are spotty in space and time and, until recently, there was no time-series station measuring the carbonate chemistry at high frequency in this region, particularly in coastal waters. We report here on the first high-frequency (1 h), multi-year (5 years) dataset of salinity, temperature, dissolved inorganic carbon, total alkalinity, CO2 partial pressure (pCO2) and pH at a coastal site (11 m) in a high-Arctic fjord (Kongsfjorden, Svalbard). We show that (1) the choice of formulations for calculating the dissociation constants of the carbonic acid remains unsettled for Arctic waters, (2) the water column is generally somewhat stratified despite the shallow depth, (3) the saturation state of calcium carbonate is subject to large seasonal changes but never reaches undersaturation (Ωa ranges between 1.4 and 3.0) and (4) pCO2 is lower than atmospheric CO2 at all seasons, making this site a sink for atmospheric CO2 (16.8 mol CO2 m−2 yr−1).

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How ocean warming and acidification affect the life cycle of six worldwide commercialised sea urchin species: a review

Ongoing global changes are expected to affect the worldwide production of many fisheries and aquaculture systems. Because invertebrates represent a relevant industry, it is crucial to anticipate challenges that are resulting from the current environmental alterations. In this review, we rely on the estimated physiological limits of six commercialised species of sea urchins (Loxechinus albusMesocentrotus franciscanusParacentrotus lividus, Strongylocentrotus droebachiensisStrongylocentrotus intermedius and Strongylocentrotus purpuratus) to define the vulnerability (or resilience) of their populations facing ocean warming and acidification (OW&A). Considering that coastal systems do not change uniformly and that the populations’ response to stressors varies depending on their origin, we investigate the effects of OW&A by including studies that estimate future environmental mutations within their distribution areas. Cross-referencing 79 studies, we find that several sea urchin populations are potentially vulnerable to the predicted OW&A as environmental conditions in certain regions are expected to shift beyond their estimated physiological limit of tolerance. Specifically, while upper thermal thresholds seem to be respected for L. albus along the SW American coast, M. franciscanus and S. purpuratus southern populations appear to be vulnerable in NW America. Moreover, as a result of the strong warming expected in the Arctic and sub-Arctic regions, the local productivity of S. droebachiensis is also potentially largely affected. Finally, populations of S. intermedius and P. lividus found in northern Japan and eastern Mediterranean respectively, are supposed to decline due to large environmental changes brought about by OW&A. This review highlights the status and the potential of local adaptation of a number of sea urchin populations in response to changing environmental conditions, revealing possible future challenges for various local fishing industries.

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The marine carbonate system variability in high meltwater season (Spitsbergen Fjords, Svalbard)


  • Spatial variability in hydrography and carbonate chemistry were investigated.
  • Lack of clear relationship between alkalinity and salinity in the surface water.
  • Effect of alkalinity fluxes from sediments on the bottom water was insignificant.
  • Freshening of the surface water reduces significantly saturation state of aragonite.


The spatial variability in hydrography (salinity and temperature) and carbonate chemistry (alkalinity – AT, total inorganic carbon concentration – CT, pH, CO2 partial pressure – pCO2, and the saturation state of aragonite – ΩAr) in high meltwater season (summer) was investigated in four Spitsbergen fjords – Krossfjorden, Kongsfjorden, Isfjorden, and Hornsund. It was found that the differences in hydrology entail spatial changes in the CO2 system structure. AT decline with decreasing salinity was evident, hence it is clear that freshwater input generally has a diluting effect and lowers AT in the surface waters of the Spitsbergen fjords. Significant surface water AT variability (1889–2261 µmol kg−1) reveals the complexity of the fjords’ systems with multiple freshwater sources having different alkalinity end-member characteristics and identifies the mean AT freshwater end-member of 595 ± 84 µmol kg−1 for the entire region. The effect of AT fluxes from sediments on the bottom water was rather insignificant, despite high AT values (2288–2666 μmol kg−1) observed in the pore waters. Low pCO2 results in surface water (200–295 μatm) points to intensive biological production, which can strongly affect the CT values, however, is less important for shaping alkalinity. It has also been shown that the freshening of the surface water in the fjords reduces significantly ΩAr (an increase in freshwater fraction contribution by 1% causes a decrease in ΩAr by 0.022). Although during the polar day, due to low pCO2, ΩAr values are still rather far from 1 (they ranged from 1.4 to 2.5), during polar night, when pCO2 values are much higher, ΩAr may drop markedly. This study highlights that the use of salinity to estimate the potential alkalinity can carry a high uncertainty, while good recognition of the surface water AT variability and its freshwater end-members is key to predict marine CO2 system changes along with the ongoing freshening of fjords waters due to climate warming.

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The history of chemical concepts and field studies of CO2 in seawater: a tribute to Kurt Buch (1881–1967)

This review of the research on the marine CO2 system spans the time between the mid-19th century and the first years after World War II. It covers the period from the first attempts to determine the amount of CO2 dissolved in seawater to the first complete physico-chemical characterization of the marine CO2 system. The development of the latter was significantly influenced by the theoretical and experimental work of the Finnish chemical oceanographer Kurt Buch (1881–1967) during the first half of the 20th century. To acknowledge his outstanding achievements in Chemical Oceanography, this review is dedicated to him.

The first part of our discussion is organized along the characteristic variables of the marine CO2 system. The analytical procedures that led successively to the definition of total CO2, alkalinity (“neutral carbonate”), the CO2 partial pressure (“CO2 tension”) and pH are briefly described. We trace the attempts to connect these variables quantitatively through the mass action law. After several failed attempts, CO2 dissociation constants were finally determined with the support of the International Council for the Exploration of the Sea (1931). Their results constituted the basis of the marine CO2 studies conducted after World War II.

The second focus of our review refers to the various field studies, including early measurements of total CO2 and alkalinity during Norwegian (1878) and Danish expeditions (1895/96) in the North Atlantic and Arctic Ocean and the first measurements of surface water pCO2 in the North Atlantic, in 1902. Furthermore, we acknowledge the achievements of the German Atlantic expedition (1925–1927) for the characterization of the vertical and horizontal distribution of pH, pCO2 and CaCO3 saturation in the Atlantic Ocean. Among Buch’s field studies of the CO2 system, we consider the Finnish monitoring program, in which pH and alkalinity were measured at over 70 stations in the northern Baltic Sea.

Whenever it is appropriate, we show the connection between past scientific ideas, concepts and knowledge with current efforts and developments concerning the understanding of the marine carbon cycle and its response to increasing atmospheric CO2.

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Impact of climate change on Arctic macroalgal communities

The Arctic region faces a warming rate that is more than twice the global average. Seaice loss, increase in precipitation and freshwater discharge, changes in underwater light, and amplification of ocean acidification modify benthic habitats and the communities they host. Here we synthesize existing information on the impacts of climate change on the macroalgal communities of Arctic coasts. We review the shortand long-term changes in environmental characteristics of shallow hard-bottomed Arctic coasts, the floristics of Arctic macroalgae (description, distribution, life-cycle, adaptations), the responses of their biological and ecological processes to climate change, the resulting winning and losing species, and the effects on ecosystem functioning. The focus of this review is on fucoid species, kelps, and coralline algae which are key ecosystem engineers in hard-bottom shallow areas of the Arctic, providing food, substrate, shelter, and nursery ground for many species. Changes in seasonality, benthic functional diversity, food-web structure, and carbon cycle are already occurring and are reshaping Arctic benthic ecosystems. Shallow communities are projected to shift from invertebrate-to algal-dominated communities. Fucoid and several kelp species are expected to largely spread and dominate the area with possible extinctions of native species. A considerable amount of functional diversity could be lost impacting the processing of land-derived nutrients and organic matter and significantly altering trophic structure and energy flow up to the apex consumers. However, many factors are not well understood yet, making it difficult to appreciate the current situation and predict the future coastal Arctic ecosystem. Efforts must be made to improve knowledge in key regions with proper seasonal coverage, taking into account interactions between stressors and across species.

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Seasonal peak in Arctic Ocean acidity could shift to the summer

Figure 1 | Simulations of seasonal variation in acidity in the Arctic Ocean. Orr et al. assessed the seasonal cycle of the partial pressure of CO2 (pCO2, which correlates with seawater acidity) in the Arctic Ocean, using simulations from a set of Earth-system models. The simulated data are plotted as the monthly anomaly — the difference between average monthly pCO2 and the annual average, measured in microatmospheres. Currently, pCO2 peaks around April, but declines when sea ice melts, reaching a minimum in the summer months when marine phytoplankton consume dissolved CO2 to grow; darker lines indicate periods of peak growth. Future global warming (simulated data are for 2091 to 2100) causes early melting of sea ice and blooming of phytoplankton, resulting in an earlier seasonal minimum of pCO2. However, pCO2 then reaches a maximum in the summer months, as a consequence of the high summer ocean temperatures. The combination of high temperatures and high acidity in the summer could be devastating for marine ecosystems.

The global ocean is gradually acidifying on multidecadal timescales. This acidification occurs when carbon dioxide generated by human activities is absorbed by the ocean, and produces conditions in which many marine organisms cannot thrive. Writing in Nature, Orr et al.1 present global simulations suggesting that future warming in the Arctic Ocean will cause CO2 levels to peak seasonally in surface waters in the summer, implying that climate change will further accelerate ocean acidification. The resulting increase in acidification would double down on the already heat-stressed ecosystem, with effects that could creep up the food web — further challenging the food security, culture and well-being of Indigenous peoples in the Arctic.

Ocean acidification varies depending on local environmental conditions and processes. For example, acidification of Arctic waters is enhanced by the freshwater input from melting sea ice, precipitation and rivers2. The partial pressure of CO2 (pCO2, which quantifies the pressure generated by CO2 dissolved in seawater, but which can be used as a broad measure of how much CO2 is dissolved) also varies naturally across days, seasons, years and even decades because it depends on a mixture of biological and physical processes.

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Arctic Ocean annual high in pCO2 could shift from winter to summer

Long-term stress on marine organisms from ocean acidification will differ between seasons. As atmospheric carbon dioxide (CO2) increases, so do seasonal variations of ocean CO2 partial pressure (pCO2), causing summer and winter long-term trends to diverge1,2,3,4,5. Trends may be further influenced by an unexplored factor—changes in the seasonal timing of pCO2. In Arctic Ocean surface waters, the observed timing is typified by a winter high and summer low6 because biological effects dominate thermal effects. Here we show that 27 Earth system models simulate similar timing under historical forcing but generally project that the summer low, relative to the annual mean, eventually becomes a high across much of the Arctic Ocean under mid-to-high-level CO2 emissions scenarios. Often the greater increase in summer pCO2, although gradual, abruptly inverses the chronological order of the annual high and low, a phenomenon not previously seen in climate-related variables. The main cause is the large summer sea surface warming7 from earlier retreat of seasonal sea ice8. Warming and changes in other drivers enhance this century’s increase in extreme summer pCO2 by 29 ± 9 per cent compared with no change in driver seasonalities. Thus the timing change worsens summer ocean acidification, which in turn may lower the tolerance of endemic marine organisms to increasing summer temperatures.

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Climate change drives rapid decadal acidification in the Arctic Ocean from 1994 to 2020

The Arctic Ocean has experienced rapid warming and sea ice loss in recent decades, becoming the first open-ocean basin to experience widespread aragonite undersaturation [saturation state of aragonite (Ωarag) < 1]. However, its trend toward long-term ocean acidification and the underlying mechanisms remain undocumented. Here, we report rapid acidification there, with rates three to four times higher than in other ocean basins, and attribute it to changing sea ice coverage on a decadal time scale. Sea ice melt exposes seawater to the atmosphere and promotes rapid uptake of atmospheric carbon dioxide, lowering its alkalinity and buffer capacity and thus leading to sharp declines in pH and Ωarag. We predict a further decrease in pH, particularly at higher latitudes where sea ice retreat is active, whereas Arctic warming may counteract decreases in Ωarag in the future.

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Factors affecting the subsurface aragonite undersaturation layer in the Pacific Arctic region


  • Freshwater content and mixing of Pacific water with Atlantic water determined the boundaries of aragonite undersaturation.
  • The upper boundary deepened inside the Beaufort Gyre when anticyclonic circulation enhanced gyre intensity.
  • The lower boundary was significantly lifted following an Atlantic-origin cold saline water intrusion event in 2017.


This study evaluated interannual variation in the subsurface aragonite undersaturation zone (ΩAr<1 layer) in the Pacific Arctic Ocean, using data from the 2016–2019 period. The upper boundary (DEPΩ<1UB) of the ΩAr<1 layer generally formed at a depth where the contribution of corrosive Pacific water was approximately 98 %. The intensity of the Beaufort Gyre associated with freshwater accumulation mainly determined interannual variation in DEPΩ<1UB, but the direction of its effect was opposite west and east of ~166°W. The lower boundary (DEPΩ<1LB) of the ΩAr<1 layer was generally found at a depth range where equal contributions of Pacific and Atlantic water were expected. An Atlantic-origin cold saline water intrusion event in 2017 caused by an anomalous atmospheric circulation pattern significantly lifted the DEPΩ<1LB, thus the thickness of the ΩAr<1 layer decreased.

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Membrane lipid sensitivity to ocean warming and acidification poses a severe threat to Arctic pteropods

Ocean warming and acidification will be most pronounced in the Arctic. Both phenomena severely threaten thecosome pteropods (holoplanktonic marine gastropods) by reducing their survival (warming) and causing the dissolution of their aragonitic shell (acidification). Lipids, particularly phospholipids, play a major role in veligers and juveniles of the polar thecosome pteropod Limacina helicina comprising more than two-thirds of their total lipids. Membrane lipids (phospholipids) are important for the temperature acclimation of ectotherms. Hence, we experimentally investigated ocean warming and acidification effects on total lipids, lipid classes, and fatty acids of Arctic early-stage L. helicina. The temperature and pCO2 treatments chosen resembled Representative Concentration Pathway model scenarios for this century. We found a massive decrease in total lipids at elevated temperatures and at the highest CO2 concentration (1,100 μatm) of the in situ temperature. Clearly, temperature was the overriding factor. Total lipids were reduced by 47%–70%, mainly caused by a reduction of phospholipids by up to 60%. Further, based on pHT development in the incubation water of pteropods during the experiment, some evidence exists for metabolic downregulation in pteropods at high factor levels of temperature and pCO2. Consequently, the cell differentiation and energy balance of early-stage larvae were probably severely compromised. Comparison of our experimental with ‘wild’ organisms suggests phospholipid reduction to values clearly outside natural variability. Based on the well-known significance of phospholipids for membranogenesis, early development, and reproduction, negative warming effects on such a basal metabolic function may be a much more immediate threat for pteropods than so far anticipated shell dissolution effects due to acidification.

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Marine pelagic ecosystem responses to climate variability and change

The marine coastal region makes up just 10% of the total area of the global ocean but contributes nearly 20% of its total primary production and over 80% of fisheries landings. Unicellular phytoplankton dominate primary production. Climate variability has had impacts on various marine ecosystems, but most sites are just approaching the age at which ecological responses to longer term, unidirectional climate trends might be distinguished. All five marine pelagic sites in the US Long Term Ecological Research (LTER) network are experiencing warming trends in surface air temperature. The marine physical system is responding at all sites with increasing mixed layer temperatures and decreasing depth and with declining sea ice cover at the two polar sites. Their ecological responses are more varied. Some sites show multiple population or ecosystem changes, whereas, at others, changes have not been detected, either because more time is needed or because they are not being measured.

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Pelagic and ice-associated microalgae under elevated light and pCO2: contrasting physiological strategies in two Arctic diatoms

Sea ice retreat, changing stratification, and ocean acidification are fundamentally changing the light availability and physico-chemical conditions for primary producers in the Arctic Ocean. However, detailed studies on ecophysiological strategies and performance of key species in the pelagic and ice-associated habitat remain scarce. Therefore, we investigated the acclimated responses of the diatoms Thalassiosira hyalina and Melosira arctica toward elevated irradiance and CO2 partial pressures (pCO2). Next to growth, elemental composition, and biomass production, we assessed detailed photophysiological responses through fluorometry and gas-flux measurements, including respiration and carbon acquisition. In the pelagic T. hyalina, growth rates remained high in all treatments and biomass production increased strongly with light. Even under low irradiances cells maintained a high-light acclimated state, allowing them to opportunistically utilize high irradiances by means of a highly plastic photosynthetic machinery and carbon uptake. The ice-associated M. arctica proved to be less plastic and more specialized on low-light. Its acclimation to high irradiances was characterized by minimizing photon harvest and photosynthetic efficiency, which led to lowered growth. Comparably low growth rates and strong silification advocate a strategy of persistence rather than of fast proliferation, which is also in line with the observed formation of resting stages under low-light conditions. In both species, responses to elevated pCO2 were comparably minor. Although both diatom species persisted under the applied conditions, their competitive abilities and strategies differ strongly. With the anticipated extension of Arctic pelagic habitats, flexible high-light specialists like T. hyalina seem to face a brighter future.

Continue reading ‘Pelagic and ice-associated microalgae under elevated light and pCO2: contrasting physiological strategies in two Arctic diatoms’

Enhanced silica export in a future ocean triggers global diatom decline

Diatoms account for up to 40% of marine primary production and require silicic acid to grow and build their opal shell3. On the physiological and ecological level, diatoms are thought to be resistant to, or even benefit from, ocean acidification. Yet, global-scale responses and implications for biogeochemical cycles in the future ocean remain largely unknown. Here we conducted five in situ mesocosm experiments with natural plankton communities in different biomes and find that ocean acidification increases the elemental ratio of silicon (Si) to nitrogen (N) of sinking biogenic matter by 17 ± 6 per cent under pCO2 conditions projected for the year 2100. This shift in Si:N seems to be caused by slower chemical dissolution of silica at decreasing seawater pH. We test this finding with global sediment trap data, which confirm a widespread influence of pH on Si:N in the oceanic water column. Earth system model simulations show that a future pH-driven decrease in silica dissolution of sinking material reduces the availability of silicic acid in the surface ocean, triggering a global decline of diatoms by 13–26 per cent due to ocean acidification by the year 2200. This outcome contrasts sharply with the conclusions of previous experimental studies, thereby illustrating how our current understanding of biological impacts of ocean change can be considerably altered at the global scale through unexpected feedback mechanisms in the Earth system.

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Author correction: contrasting drivers and trends of ocean acidification in the subarctic Atlantic

The Original Article was published on 07 July 2021

Correction to: Scientific Reports, published online 07 July 2021

The original version of this Article contained errors.

In Table 2 legend, the symbol of “picomol” was incorrectly given as “nanomol”.

“Average trends obtained with the seasonally detrended data the in situ temperature (T in °C yr−1), salinity (S in yr−1), Total Alkalinity (TA in µmol kg−1 yr−1), salinity-normalized alkalinity (nTA in µmol kg−1 yr−1), total dissolved inorganic carbon (DIC in µmol kg−1 yr−1), salinity-normalized dissolved inorganic carbon (nDIC in µmol kg−1 yr−1), in situ pH in total scale (pHT yr−1), total hydrogen ion concentrations ([H+]T in nanomol kg−1 yr−1), ion carbonate concentration excess over aragonite saturation (exCO3 = in µmol kg−1 yr−1), and anthropogenic CO2.”

now reads:

“Average trends obtained with the seasonally detrended data the in situ temperature (T in °C yr−1), salinity (S in yr−1), Total Alkalinity (TA in µmol kg−1 yr−1), salinity-normalized alkalinity (nTA in µmol kg−1 yr−1), total dissolved inorganic carbon (DIC in µmol kg−1 yr−1), salinity-normalized dissolved inorganic carbon (nDIC in µmol kg−1 yr−1), in situ pH in total scale (pHT yr−1), total hydrogen ion concentrations ([H+]T in picomol kg−1 yr−1), ion carbonate concentration excess over aragonite saturation (exCO3 = in µmol kg−1 yr−1), and anthropogenic CO2.”

Additionally, the article contains a repeated error where the symbol for “pmol” was incorrectly given as “nmol” in the Results section, under the subheading ‘Acidifcation drivers’, in Figure 6 legend, and in the Conclusions.

Furthermore, in Figure 6A and Supplementary Figure S5A “pmol” was incorrectly given as “nmol” in the y-axis. The original Figure 6 and accompanying legend, and Supplementary Information file appear below.

Acidification trends and drivers decomposition (T,S, nDIC and nTA) for the seasonally detrended average time series of total hydrogen ions concentration in pmol/kg/yr (Δ[H+]TA) and for excess of [CO3= ] over the [CO3= ] at aragonite saturation in µmol/kg/yr (Δex[CO3=]B). The nDIC driver trends is split in natural (nCnat) and anthropogenic components (nCanth). The colour code is shown on both panels.

The original Article and accompanying Supplementary Information file have been corrected.

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Ichnodiversity in the eastern Canadian Arctic in the context of polar microbioerosion patterns

Studies of marine microbioerosion in polar environments are scarce. They include our recent investigations of bioerosion traces preserved in sessile balanid skeletons from the Arctic Svalbard archipelago and the Antarctic Ross Sea. Here, we present results from a third study site, Frobisher Bay, in the eastern Canadian Arctic, together with a synthesis of our current knowledge of polar bioerosion in both hemispheres. Barnacles from 62 to 94 m water depth in Frobisher Bay were prepared using the cast-embedding technique to enable visualization of microboring traces by scanning electron microscopy. In total, six ichnotaxa of traces produced by organotrophic bioeroders were found. All recorded ichnotaxa were also present in Mosselbukta, Svalbard, and most in the Ross Sea. Frobisher Bay contrasts with Mosselbukta in that it is a siliciclastic-dominated environment and shows a lower ichnodiversity, which may be accounted for by the limited bathymetrical range and a high turbidity and sedimentation rate. We evaluate potential key ichnotaxa for the cold-temperate and polar regions, of which the most suitable are Flagrichnus baiulus and Saccomorpha guttulata, and propose adapted index ichnocoenoses for the interpretation of palaeobathymetry accordingly. Together, the three studies allow us to make provisional considerations about the biogeographical distribution of polar microbioerosion traces reflecting the ecophysiological limits of their makers.

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Regional sensitivity patterns of Arctic Ocean acidification revealed with machine learning

Ocean acidification is a consequence of the absorption of anthropogenic carbon emissions and it profoundly impacts marine life. Arctic regions are particularly vulnerable to rapid pH changes due to low ocean buffering capacities and high stratification. Here, an unsupervised machine learning methodology is applied to simulations of surface Arctic acidification from two state-of-the-art coupled climate models. We identify four sub-regions whose boundaries are influenced by present-day and projected sea ice patterns. The regional boundaries are consistent between the models and across lower (SSP2-4.5) and higher (SSP5-8.5) carbon emissions scenarios. Stronger trends toward corrosive surface waters in the central Arctic Ocean are driven by early summer warming in regions of annual ice cover and late summer freshening in regions of perennial ice cover. Sea surface salinity and total alkalinity reductions dominate the Arctic pH changes, highlighting the importance of objective sub-regional identification and subsequent analysis of surface water mass properties.

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Editorial: acidification and hypoxia in marginal seas

Editorial on the Research Topic
Acidification and Hypoxia in Marginal Seas

Ocean acidification and hypoxia (dissolved oxygen <2 mg L−1 or <62 μmol L−1) are universal environmental concerns that can impact ecological and biogeochemical processes, including element cycling, carbon sequestration, community shifts, contributing to biodiversity reduction, and reducing marine ecosystem services (Riebesell et al., 2000Feely et al., 20042009Andersson et al., 2005Doney, 2006Cohen and Holcomb, 2009Doney et al., 20092020Kleypas and Yates, 2009Ekstrom et al., 2015Gattuso et al., 2015). While the stressors are global in their occurrence, local and regional impacts might be enhanced and even more accelerated, thus requiring even greater and faster consideration (Doney et al., 2020).

The driving mechanisms of acidification and hypoxia are inextricably linked in near-shore and coastal habitats. Along coastal shelf and its adjacent marginal seas, where the natural variability of multiple stressors is high, human-induced eutrophication is additionally enhancing both local acidification and hypoxia. For example, the well-known eutrophication of surface waters in the northern Gulf of Mexico caused hypoxic conditions that result in a pH decrease by 0.34 in the oxygen-depleted bottom water, which is significantly more than the pH decrease via atmospheric CO2 sequestration alone (pH decrease by 0.11; Cai et al., 2011). Similar changes in coastal conditions involving biological respiration and atmospheric CO2 invasion have also been observed in other marginal seas, urbanized estuaries, salt marshes and mangroves (Feely et al., 200820102018Cai et al., 2011Howarth et al., 2011). Other natural and anthropogenic processes, such as increased wind intensity and coastal upwelling, enhanced stratification due to global warming, along with more intense benthic respiration, more frequent extreme events, oscillation of water circulations, and variations in the terrestrial carbon and/or alkalinity fluxes, etc., all influence the onset and maintenance of acidification and/or hypoxia. For example, coastal upwelling brings both low pH and hypoxic water from below and enhances acidification and hypoxia in the coastal regions (Feely et al., 2008). Although acidification and hypoxia in the open oceans have received considerable attention already, the advances in our understanding of the driving mechanisms and the temporal evolution under global climate change is still poorly understood, particularly with respect to the region-specific differences, various scales of temporal and spatial variability, predictability patterns, and interactive multiple stressor impacts. Therefore, coastal ecosystems have a much broader range of rates of change in pH than the open ocean does (Carstensen and Duarte, 2019). The importance of understanding acidification and hypoxia for the biogeochemical and ecosystem implications in marginal seas is essential for climate change mitigation and adaptation strategy implementations in the future.

The scope of this Research Topic is to cover the most recent advances related to the status of acidification and hypoxia in marginal seas, the coupling mechanisms of multi-drivers and human impacts, ecosystem responses, prediction of their evolution over space and time, and under future climate change scenarios. The authors of this Research Topic contributed a total of 35 papers covering a wide variety of subjects spanning from acidification and/or hypoxia (OAH) status, the carbonate chemistry baseline and trends, the impacts of OAH on the habitat suitability and ecosystem implications, and the long-term changes and variability of OAH in marginal seas.

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Impact of sea ice melting on summer air-sea CO2 exchange in the East Siberian Sea

The role of sea ice melting on the air-sea CO2 flux was investigated at two ice camps in the East Siberian Sea of the Arctic Ocean. On average, sea ice samples from the two ice camps had a total alkalinity (TA) of ∼108 and ∼31 μmol kg–1 and a corresponding salinity of 1.39 and 0.36, respectively. A portion (18–23% as an average) of these sea ice TA values was estimated to exist in the sea ice with zero salinity, which indicates the excess TA was likely attributed to chemical (CaCO3 formation and dissolution) and biological processes in the sea ice. The dilution by sea ice melting could increase the oceanic CO2 uptake to 11–12 mmol m–2 d–1 over the next 21 days if the mixed layer depth and sea ice thickness were assumed to be 18.5 and 1.5 m, respectively. This role can be further enhanced by adding TA (including excess TA) from sea ice melting, but a simultaneous release of dissolved inorganic carbon (DIC) counteracts the effect of TA supply. In our study region, the additional impact of sea ice melting with close to unity TA:DIC ratio on air-sea CO2 exchange was not significant.

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