Posts Tagged 'Arctic'

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

Highlights

  • 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.

Abstract

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.

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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 https://doi.org/10.1038/s41598-021-93324-3, 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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A seamless ensemble-based reconstruction of surface ocean pCO2 and air–sea CO2 fluxes over the global coastal and open oceans

We have estimated global air–sea CO2 fluxes (fgCO2) from the open ocean to coastal seas. Fluxes and associated uncertainty are computed from an ensemble-based reconstruction of CO2 sea surface partial pressure (pCO2) maps trained with gridded data from the Surface Ocean CO2 Atlas v2020 database. The ensemble mean (which is the best estimate provided by the approach) fits independent data well, and a broad agreement between the spatial distribution of model–data differences and the ensemble standard deviation (which is our model uncertainty estimate) is seen. Ensemble-based uncertainty estimates are denoted by ±1σ. The space–time-varying uncertainty fields identify oceanic regions where improvements in data reconstruction and extensions of the observational network are needed. Poor reconstructions of pCO2 are primarily found over the coasts and/or in regions with sparse observations, while fgCO2 estimates with the largest uncertainty are observed over the open Southern Ocean (44 S southward), the subpolar regions, the Indian Ocean gyre, and upwelling systems.

Our estimate of the global net sink for the period 1985–2019 is 1.643±0.125 PgC yr−1 including 0.150±0.010 PgC yr−1 for the coastal net sink. Among the ocean basins, the Subtropical Pacific (18–49 N) and the Subpolar Atlantic (49–76 N) appear to be the strongest CO2 sinks for the open ocean and the coastal ocean, respectively. Based on mean flux density per unit area, the most intense CO2 drawdown is, however, observed over the Arctic (76 N poleward) followed by the Subpolar Atlantic and Subtropical Pacific for both open-ocean and coastal sectors. Reconstruction results also show significant changes in the global annual integral of all open- and coastal-ocean CO2 fluxes with a growth rate of  PgC yr−2 and a temporal standard deviation of 0.526±0.022 PgC yr−1 over the 35-year period. The link between the large interannual to multi-year variations of the global net sink and the El Niño–Southern Oscillation climate variability is reconfirmed.

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Particulate iron bioavailability to phytoplankton in Antarctic and Arctic waters: effects of ocean acidification and the organic ligand EDTA

Particulate iron (PFe) usually is not considered as a bioavailable iron fraction to phytoplankton. In this study we tested the bioavailability of one PFe species, goethite (α-FeO(OH)), to phytoplankton community in Southern Ocean under the effect of ocean acidification (OA) (pHT ca. 7.5) and representative concentration pathways (RCP) 8.5 condition (pCO2 ca. 1300 µatm), and to an Arctic diatom species, Nitzschia frigida, under the effect of the organic ligand, EDTA (using the commercially available salt disodium ethylenediaminetetraacetate dihydrate), as a chelator, respectively.

In March 2019, a natural phytoplankton community was sampled and used for the deck incubation experiment in the Southern Ocean. The sampling site was 68.10°S, 6.00° W, which was in the region of Queen Maud Land (Norwegian: Dronning Maud Land, DML). We observed marine biogeochemical performance of the phytoplankton community under OA. Different chemical and biological parameters during the incubation were determined, including dissolved iron (DFe), total acid leachable iron (TaLFe), macronutrients including nitrate (NO3-), phosphate (PO43-) and silicate, total pH (pHT), dissolved inorganic carbon (DIC), the concentration & fugacity of carbon dioxide (fCO2), chlorophyll a (Chla) concentration & in vivo fluorescence. The results show that the tested phytoplankton assemblage was more severely influenced by OA than iron bioavailability, especially under severe OA. Goethite, as one type of PFe, is insoluble under the tested OA scenarios. There could be PO43- remineralization in all treatments but species shift to diatoms only in ambient pH treatments (mild OA), which coincides with the judgement that OA impact is predominant in comparison to iron enrichment in this experiment. We should analyze phytoplankton species to test this hypothesis. OA can result in that phytoplankton launches Hv channel-mediated H+ efflux mechanism, carbon concentration mechanism (CCM) down-regulation of phytoplankton and the thriving of more tolerant species with more efficient CCM.

In April 2021, using an Arctic diatom species, Nitzschia frigida, we investigated the possibility of EDTA increasing goethite bioavailability to phytoplankton and photosynthetic performance by measuring relative electron transport rate (rETR) in the experiment performed at Trondheim Biological Station (Norwegian: Trondheim Biologiske Stasjon, TBS). The results show that elevating EDTA concentration can increase the bioavailability of goethite while decrease that of ferric chloride (FeCl3). This is inconclusive according to possibly negatively biased α (the slope of a typical P/E (photosynthesis/irradiance) curve), because it results in underestimation of goethite bioavailability under the influence of EDTA.

Further research regarding the combined effect of OA and EDTA on PFe bioavailability to phytoplankton is recommended.

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Rapid acidification of the Arctic Chukchi Sea waters driven by anthropogenic forcing and biological carbon recycling

Abstract

The acidification of coastal waters is distinguished from the open ocean because of much stronger synergistic effects between anthropogenic forcing and local biogeochemical processes. However, ocean acidification research is still rather limited in polar coastal oceans. Here, we present a 17-year (2002-2019) observational dataset in the Chukchi Sea to determine the long-term changes in pH and aragonite saturation state (Ωarag). We found that pH and Ωarag declined in different water masses with average rates of -0.0047 ± 0.0026 year-1 and -0.017 ± 0.009 year-1, respectively, and are ∼2-3 times faster than those solely due to increasing atmospheric CO2. We attributed the rapid acidification to the increased dissolved inorganic carbon owing to a combination of ice melt-induced increased atmospheric CO2 invasion and subsurface remineralization induced by a stronger surface biological production as a result of the increased inflow of the nutrient-rich Pacific water.

Plain Language Summary

Anthropogenic CO2 absorbed by the ocean leads to a lower pH and the calcium carbonate saturation state (Ω) and threatens the marine ecosystems state of healthiness via a process called ocean acidification (OA). The Arctic Ocean is particularly sensitive to OA because more CO2 can be dissolved in cold water. This study used the observations collected over 17 years from 2002 to 2019 to estimate long-term trends of Ωarag and pH in the Chukchi Sea. The results show that rapid acidification occurred throughout all water masses from 2002-2019, leading to or approaching aragonite undersaturation. The rapid acidification is attributed to the enhanced increasing concentration of dissolved inorganic carbon. While sea ice melt induced uptake of anthropogenic CO2 partly explains the long-term acidification, the remainder is due to the increased nutrient-rich Pacific inflow water which promotes the high biological CO2 utilization in the surface waters but leads to stronger subsurface acidification due to the regenerated CO2. We suggest that the acidity in Chukchi Arctic Shelf waters will increase in the future if the increased inflow of Pacific water continues.

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The distribution of pCO2W and air-sea CO2 fluxes using FFNN at the continental shelf areas of the Arctic Ocean

A feed-forward neural network (FFNN) was used to estimate the monthly climatology of partial pressure of CO2 (pCO2W) at a spatial resolution of 1° latitude by 1° longitude in the continental shelf of the European Arctic Sector (EAS) of the Arctic Ocean (the Greenland, Norwegian, and Barents seas). The predictors of the network were sea surface temperature (SST), sea surface salinity (SSS), the upper ocean mixed-layer depth (MLD), and chlorophyll-a concentration (Chl-a), and as a target, we used 2 853 pCO2W data points from the Surface Ocean CO2 Atlas. We built an FFNN based on three major datasets that differed in the Chl-a concentration data used to choose the best model to reproduce the spatial distribution and temporal variability of pCO2W. Using all physical–biological components improved estimates of the pCO2W and decreased the biases, even though Chl-a values in many grid cells were interpolated values. General features of pCO2W distribution were reproduced with very good accuracy, but the network underestimated pCO2W in the winter and overestimated pCO2W values in the summer. The results show that the model that contains interpolating Chl-a concentration, SST, SSS, and MLD as a target to predict the spatiotemporal distribution of pCO2W in the sea surface gives the best results and best-fitting network to the observational data. The calculation of monthly drivers of the estimated pCO2W change within continental shelf areas of the EAS confirms the major impact of not only the biological effects to the pCO2W distribution and Air-Sea CO2 flux in the EAS, but also the strong impact of the upper ocean mixing. A strong seasonal correlation between predictor and pCO2W seen earlier in the North Atlantic is clearly a yearly correlation in the EAS. The five-year monthly mean CO2 flux distribution shows that all continental shelf areas of the Arctic Ocean were net CO2 sinks. Strong monthly CO2 influx to the Arctic Ocean through the Greenland and Barents Seas (>12 gC m−2 day−1) occurred in the fall and winter, when the pCO2W level at the sea surface was high (>360 µatm) and the strongest wind speed (>12 ms−1) was present.

Continue reading ‘The distribution of pCO2W and air-sea CO2 fluxes using FFNN at the continental shelf areas of the Arctic Ocean’

Upper environmental pCO2 drives sensitivity to ocean acidification in marine invertebrates

Minimizing the impact of ocean acidification requires an understanding of species responses and environmental variability of population habitats. Whereas the literature is growing rapidly, emerging results suggest unresolved species- or population-specific responses. Here we present a meta-analysis synthesizing experimental studies examining the effects of pCO2 on biological traits in marine invertebrates. At the sampling locations of experimental animals, we determined environmental pCO2 conditions by integrating data from global databases and pCO2 measurements from buoys. Experimental pCO2 scenarios were compared with upper pCO2 using an index considering the upper environmental pCO2. For most taxa, a statistically significant negative linear relationship was observed between this index and mean biological responses, indicating that the impact of a given experimental pCO2 scenario depends on the deviation from the upper pCO2 level experienced by local populations. Our results highlight the importance of local biological adaptation and the need to consider present pCO2 natural variability while interpreting experimental results.

Continue reading ‘Upper environmental pCO2 drives sensitivity to ocean acidification in marine invertebrates’

Bioindicators of severe ocean acidification are absent from the end-Permian mass extinction

The role of ocean acidification in the end-Permian mass extinction is highly controversial with conflicting hypotheses relating to its timing and extent. Observations and experiments on living molluscs demonstrate that those inhabiting acidic settings exhibit characteristic morphological deformities and disordered shell ultrastructures. These deformities should be recognisable in the fossil record, and provide a robust palaeo-proxy for severe ocean acidification. Here, we use fossils of originally aragonitic invertebrates to test whether ocean acidification occurred during the Permian–Triassic transition. Our results show that we can reject a hypothesised worldwide basal Triassic ocean acidification event owing to the absence of deformities and repair marks on bivalves and gastropods from the Triassic Hindeodus parvus Conodont Zone. We could not, however, utilise this proxy to test the role of a hypothesised acidification event just prior to and/or during the mass extinction event. If ocean acidification did develop during the mass extinction event, then it most likely only occurred in the latest Permian, and was not severe enough to impact calcification.

Continue reading ‘Bioindicators of severe ocean acidification are absent from the end-Permian mass extinction’

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