Posts Tagged 'Antarctic'

Molecular responses in an Antarctic bivalve and an ascidian to ocean acidification

Published 29 August 2023 Science Closed
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Highlights

  • The non-calcifying species Cnemidocarpa verrucosa sp. A showed a greater number of differentially expressed genes than the calcifying Aequiyoldia eightsii.
  • The Ocean Acidification caused an upregulation of genes involved in the immune system and antioxidant response in the ascidian Cnemidocarpa verrucosa sp. A.
  • The abundance of the key marine organisms (such as Cnemidocarpa verrucosa), could be affected by Ocean Acidification if pH predictions for polar regions come true.
  • Contrary to expected, Ocean Acidification could not affect the mollusk Aequiyoldia eightsii compared to the non-calcifying species.

Abstract

Southern Ocean organisms are considered particularly vulnerable to Ocean acidification (OA), as they inhabit cold waters where calcite-aragonite saturation states are naturally low. It is also generally assumed that OA would affect calcifying animals more than non-calcifying animals. In this context, we aimed to study the impact of reduced pH on both types of species: the ascidian Cnemidocarpa verrucosa sp. A, and the bivalve Aequiyoldia eightsii, from an Antarctic fjord. We used gene expression profiling and enzyme activity to study the responses of these two Antarctic benthic species to OA. We report the results of an experiment lasting 66 days, comparing the molecular mechanisms underlying responses under two pCO2 treatments (ambient and elevated pCO2). We observed 224 up-regulated and 111 down-regulated genes (FC ≥ 2; p-value ≤ 0.05) in the ascidian. In particular, the decrease in pH caused an upregulation of genes involved in the immune system and antioxidant response. While fewer differentially expressed (DE) genes were observed in the infaunal bivalve, 34 genes were up-regulated, and 69 genes were downregulated (FC ≥ 2; p-value ≤ 0.05) in response to OA. We found downregulated genes involved in the oxidoreductase pathway (such as glucose dehydrogenase and trimethyl lysine dioxygenase), while the heat shock protein 70 was up-regulated. This work addresses the effect of OA in two common, widely distributed Antarctic species, showing striking results. Our major finding highlights the impact of OA on the non-calcifying species, results that differ from the general trend, in which one remarks the higher impact on calcifying species. Our result proposes a deep discussion about the potential effect on non-calcifying species, such as ascidians, a diverse and abundant group, that form extended three-dimensional clusters in the shallow waters and shelf areas along the Southern Ocean.

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A synthesis of SNAPO-CO2 ocean total alkalinity and total dissolved inorganic carbon measurements from 1993 to 2022

Published 23 August 2023 Science Closed
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Total alkalinity (AT) and total dissolved inorganic carbon (CT) in the oceans are important properties to understand the ocean carbon cycle and its link with climate change (ocean carbon sinks and sources) or global change (ocean acidification). We present a data-base of more than 44 400 AT and CT observations in various ocean regions obtained since 1993 mainly in the frame of French projects. This includes both surface and water columns data acquired in open oceans, coastal zones and in the Mediterranean Sea and either from time-series or punctual cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ± 4 µmol kg-1 for both AT and CT. Given the lack of observations in the Indian and Southern Oceans, we added sea surface underway AT and CT data obtained in 1998–2018 in the frame of OISO cruises and in 2019 during the CLIM-EPARSES cruise measured onboard using the same technique. Separate datasets for the global ocean, and for the Mediterranean Sea are provided in a single format (https://doi.org/10.17882/95414, Metzl et al., 2023) that offers a direct use for regional or global purposes, e.g. AT/Salinity relationships, long-term CT estimates, constraint and validation of diagnostics CT–AT reconstructed fields or ocean carbon and coupled climate/carbon models simulations, as well as data derived from BG-ARGO floats. When associated with other properties, these data can also be used to calculate pH, fugacity of CO2 (fCO2) and other carbon systems properties to derive ocean acidification rates or air-sea CO2 fluxes.

Continue reading ‘A synthesis of SNAPO-CO2 ocean total alkalinity and total dissolved inorganic carbon measurements from 1993 to 2022’

Rising snow line: Ocean acidification and the submergence of seafloor geomorphic features beneath a rising carbonate compensation depth

Published 22 August 2023 Science Closed
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Highlights

  • Ocean acidification has caused the carbonate compensation depth (CCD) to rise by ~98 m.
  • Seafloor area below the CCD has increased by 3.6% in the last 200 years.
  • Risk of impact of rising CCD is greatest in the western equatorial Atlantic Ocean.
  • Different geomorphic features impacted by rising CCD in different ocean areas.

Abstract

Due to burning of fossil fuels, carbon dioxide is being absorbed by the ocean where its chemical conversion to carbonic acid has already caused the surface ocean to become more acidic than it has been for at least the last 2 million years. Global ocean modeling suggests that the carbonate compensation depth (CCD) has already risen by nearly 100 m on average since pre-industrial times and will likely rise further by several hundred meters more this century. Potentially millions of square kilometres of ocean floor will undergo a rapid transition in terms of the overlying water chemistry whereby calcareous sediment will become unstable causing the carbonate “snow line” to rise.We carried out a spatial analysis of seafloor geomorphology to assess the area newly submerged below the rising CCD. We found that shoaling of the CCD since the industrial revolution has submerged 12,432,096 km2 of ocean floor (3.60% of total ocean area) below the CCD. Further hypothetical shoaling of the CCD by 100 m increments illustrated that the surface area of seafloor submerged below the CCD has risen by 14% with 300 m of shoaling, such that 51% of the ocean area will be below the CCD. All categories of geomorphic feature mapped in one global database intersect the lysocline and will be (or already are) submerged below the CCD with much regional variation since the rise in CCD depth during the last 150 years varies significantly between different ocean regions. For seamounts, the highest percentages of increase in area submerged below the CCD occurred in the Southern Indian Ocean and the South West Atlantic regions (6.3% and 5.9%, respectively). For submarine canyons we found the South West Atlantic increased from 3.9% in pre-industrial times to 8.0% at the present time, the highest percentage of canyons found below the CCD in any ocean region.We also carried out a relative risk assessment for future submergence of ocean floor below the CCD in 17 ocean regions. In our assessment we assumed that the change in CCD from pre-industrial times to the present is an indicator of the likelihood and the change in percentage of seafloor submerged below the CCD due to a hypothetical 300 m rise in the CCD is an indicator of the consequences. We found that the western equatorial Atlantic is at high risk and 9 other Ocean Regions are at moderate risk. Overall, geomorphic features in the Atlantic Ocean and southern Indian Ocean are at greater risk of impact from a rising CCD than Pacific and other Indian Ocean regions.A separate analysis of the Arctic Ocean points to the possible submergence of glacial troughs incised on the continental shelf within a mid-depth (400–800 m) acidified water mass. We also found that the area of national Exclusive Economic Zones submerged below the rising CCD exhibits extreme variability; with 300 m of CCD shoaling we found a > 12% increase in area submerged below the CCD for 23 national EEZs, whereas there was virtually no change for other countries.

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Physiological response of an Antarctic cryptophyte to increasing temperature, CO2, and irradiance

Published 12 July 2023 Science Closed
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The Southern Ocean, a globally important CO2 sink, is one of the most susceptible regions in the world to climate change. Phytoplankton of the coastal shelf waters around the Western Antarctic Peninsula have been experiencing rapid warming over the past decades and current ongoing climatic changes will expose them to ocean acidification and high light intensities due to increasing stratification. We conducted a multiple-stressor experiment to evaluate the response of the still poorly studied key Antarctic cryptophyte species Geminigera cryophila to warming in combination with ocean acidification and high irradiance. Based on the thermal growth response of G. cryophila, we grew the cryptophyte at suboptimal (2°C) and optimal (4°C) temperatures in combination with two light intensities (medium light: 100 μmol photons m−2 s−1 and high light [HL]: 500 μmol photons m−2 s−1) under ambient (400 μatm pCO2) and high pCO2 (1000 μatm pCO2) conditions. Our results reveal that G. cryophila was not susceptible to high pCO2, but was strongly affected by HL at 2°C, as both growth and carbon fixation were significantly reduced. In comparison, warming up to 4°C stimulated the growth of the cryptophyte and even alleviated the previously observed negative effects of HL at 2°C. When grown, however, at temperatures above 4°C, the cryptophyte already reached its maximal thermal limit at 8°C, pointing out its vulnerability toward even higher temperatures. Hence, our results clearly indicate that warming and high light and not pCO2 control the growth of G. cryophila.

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Variations in the Southern Ocean carbonate production, preservation, and hydrography for the past 41, 500 years: evidence from coccolith and CaCO3 records

Published 15 June 2023 Science Closed
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Changes in ocean alkalinity affect atmospheric pCO2 (i.e., higher alkalinity lowers atmospheric pCO2). Ocean alkalinity is partly determined by sedimentary burial of carbonates, which is primarily controlled by carbonate flux and the degree of deep ocean carbonate saturation. In this study, we investigate the factors determining the coccolith burial in subantarctic sediments and the surface ocean changes in the subtropical South Indian Ocean. The downcore coccolith records from the subantarctic region (SK200/22a) of the Indian sector of the Southern Ocean display low coccolith concentration during the glacial period. A possible explanation for this is, 1) the low glacial production of coccolithophores due to the competition from diatoms and 2) dilution by biogenic silica in the glacial sediments. Additionally, reduced carbonate burial owing to the low carbonate saturation of the deep-water accounts for the decline in glacial coccolith concentration. This also explains the low coccolith dissolution index and enrichment of the large dissolution-resistant coccolith species, Coccolithus pelagicus subsp. braarudii in the glacial sediments. The low carbonate saturation is attributed to, 1) the replacement of carbonate saturated, North Atlantic Deep Waters by the undersaturated southern sourced water masses and 2) increased storage of dissolved CO2 in the deep glacial Southern Ocean. Our study suggests that changes in coccolith production and the deep ocean carbonate saturation determine their burial in subantarctic sediments for the last 41,500 years. Other than these changes, the study region also records the changes in the Agulhas Return Current via variation in the proportion of tropical-subtropical coccolith assemblage.

Continue reading ‘Variations in the Southern Ocean carbonate production, preservation, and hydrography for the past 41, 500 years: evidence from coccolith and CaCO3 records’

Severe 21st-century ocean acidification demands continuance and expansion of Antarctic Marine Protected Areas

Published 14 June 2023 Science Closed
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Antarctic coastal waters are home to several established or proposed Marine Protected Areas (MPAs) supporting exceptional biodiversity, which is threatened by anthropogenic climate change. Despite a particular sensitivity to ocean acidification (OA), little is known about the future carbonate chemistry of high-latitude Southern Ocean waters. Here, we use a high resolution ocean–sea ice–biogeochemistry model with realistic ice-shelf geometry to investigate 21st-century OA in Antarctic MPAs under four emission scenarios. By 2100, we project surface pH declines of up to 0.42 (total scale), corresponding to a 161% increase in hydrogen ion concentration relative to the 1990s. End-of-century aragonite undersaturation is ubiquitous across MPAs under the three highest emission scenarios. Vigorous vertical mixing of anthropogenic carbon on the continental shelves produces severe OA within the Weddell Sea, East Antarctic, and Ross Sea MPAs. Our findings call for continuity and expansion of Antarctic MPAs to reduce pressures on ecosystem integrity.

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Reduction in size of the calcifying phytoplankton Calcidiscus leptoporus to environmental changes between the Holocene and modern Subantarctic Southern Ocean

Published 5 June 2023 Science Closed
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The Subantarctic Zone of the Southern Ocean plays a disproportionally large role on the Earth system. Model projections predict rapid environmental change in the coming decades, including ocean acidification, warming, and changes in nutrient supply which pose a serious risk for marine ecosystems. Yet despite the importance of the Subantarctic Zone, annual and inter-annual time series are extremely rare, leading to important uncertainties about the current state of its ecosystems and hindering predictions of future response to climate change. Moreover, as the longest observational time series available are only a few decades long, it remains unknown whether marine pelagic ecosystems have already responded to ongoing environmental change during the industrial era. Here, we take advantage of multiple sampling efforts – monitoring of surface layer water properties together with sediment trap, seafloor surface sediment and sediment core sampling – to reconstruct the modern and pre-industrial state of the keystone calcifying phytoplankton Calcidiscus leptoporus, central to the global marine carbonate cycle. Morphometric measurements reveal that modern C. leptoporus coccoliths are 15% lighter and 25% smaller than those preserved in the underlying Holocene-aged sediments. The cumulative effect of multiple environmental drivers appears responsible for the coccolith size variations since the Last Deglaciation, with warming and ocean acidification most likely playing a predominant role during the industrial era. Notably, extrapolation of our results suggests a future reduction in cell and coccolith size which will have a negative impact on the efficiency of the biological pump in the Southern Ocean through a reduction of carbonate ballasting. Lastly, our results tentatively suggest that C. leptoporus coccolith size could be used as a palaeo-proxy for growth rate. Future culture experiments will be needed to test this hypothesis.

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A shift in the mechanism of CO2 uptake in the Southern Ocean under high emission-scenario

Published 25 May 2023 Science Closed
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The Southern Ocean is a major region of ocean carbon uptake, but its future changes remain uncertain under climate warming. Here we show the projected shift in the Southern Ocean CO2 sink using a suite of Earth System Models, revealing changes in the mechanism, position and seasonality of the carbon uptake. Dominant CO2 uptake shifts from the Subtropical to the Antarctic region under the high-emission scenario by the end of the 21st century. The warming-driven sea-ice melt, increased ocean stratification, mixed layer shoaling, and a weaker vertical carbon gradient will together reduce the winter outgassing in the future, which will trigger the switch from mixing-driven outgassing to solubility-driven uptake in the Antarctic region during the winter season. The future Southern Ocean carbon sink will be poleward-shifted, operating in a hybrid mode between biologically-driven summertime and solubility-driven wintertime uptake with further amplification of biological uptake by the increasing Revelle Factor.

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Contrasting life cycles of Southern Ocean pteropods alter their vulnerability to climate change

Published 16 May 2023 Science Closed
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Pteropods are a key part of biogeochemical cycling and epipelagic food webs in the Southern Ocean. However, shelled pteropods are vulnerable to climate change, due to their aragonite shells being particularly sensitive to ocean acidification. Currently our understanding of pteropod responses to environmental change is hindered by uncertainties surrounding their life cycles and population dynamics. In this study, we describe polar shelled pteropod diversity in the north-eastern Scotia Sea, inferring life history and population structures of the dominant pteropod species, Limacina rangii (formerly Limacina helicina antarctica) and Limacina retroversa. An annual timeseries of Limacina shell morphometrics was derived from individuals collected in a moored sediment trap at 400 m depth. We found that L. rangii and L. retroversa have contrasting life history strategies. L. rangii has a continuous spawning and recruitment period from November to March and can overwinter as juveniles and adults. L. retroversa has discrete spawning events from November to May, producing non–overlapping cohorts of juveniles and adults. Their development to the adult stage takes between two and five months, upon which they overwinter as adults. Our findings suggest different vulnerabilities of L. rangii and L. retroversa to a changing ocean. For example, since all life stages of L. rangii co-exist, vulnerability of one cohort is not detrimental to the stability of the overall population whereas, if one L. retroversa cohort fails to recruit, the entire population is threatened. Changes in pteropod populations could have cascading ramifications to Antarctic ecosystems and carbon cycling.

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Drivers of marine CO2-carbonate chemistry in the northern Antarctic Peninsula

Published 15 May 2023 Science Closed
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Abstract

The Bransfield Strait is a climate change hotspot at the tip of the northern Antarctic Peninsula (NAP). The region is marked by a mixture of relatively warm waters from the Bellingshausen Sea with cold shelf waters from the Weddell Sea. Additionally, its deep central basin (>800 m) preserves seawater properties from the north-western Weddell Sea continental shelf. This study assessed long-term changes in carbonate chemistry in the Bransfield Strait and found that the hydrographic setting (i.e., a mixture between modified-Circumpolar Deep Water with Dense Shelf Water [DSW]) drives temporal variability of carbonate parameters. The western basin has experienced decreases in pH (seawater scale) over the last three decades (1996–2019), varying from −0.003 to −0.017 pH units yr−1, while Ωar decreased from −0.01 to −0.07 yr−1 throughout the water column. The central basin was characterized by a high contribution of DSW with high carbon dioxide (CO2) content and the decomposition of organic matter produced and transported into its deep layer. With lower variability for all carbonate system variables, the eastern basin was likely regulated by internal mixing. Overall, the entire strait is almost reaching a CO2-saturated condition, highlighting how sensitive subpolar regions are to the effects of human-induced climate change.

Key Points

  • The western basin experiences steeper pH decreases than the surrounding areas at a rate of −0.017 pHsws units yr−1 due to Circumpolar Deep Water intrusions
  • Dense Shelf Water inflow into the deep layer of the central basin promoted a CT increase of about 50 μmol kg−1 in the 2010s relative to the 2000s
  • Internal mixing has likely reduced spatiotemporal variability of carbonate chemistry in the eastern basin since the 1990s
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Southern Ocean acidification revealed by Biogeochemical-Argo floats

Published 12 May 2023 Science Closed
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Abstract

Ocean acidification has potentially large impacts on calcifying organisms and ecosystems. Argo floats equipped with biogeochemical (BGC) sensors have been continuously measuring Southern Ocean pH since 2014. These BGC-Argo floats were deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project. Here we present a SOCCOM-era Objectively Mapped pH (SOM-pH) 2014-2019 climatology and explain the method for constructing this product. We show example SOM-pH fields demonstrating the spatial and temporal structure of Southern Ocean pH. Comparison with previous ship-based measurements reveals decreases in pH of up to 0.02 per decade, with a structure decaying with depth. An assessment of the trend structure reveals a pattern indicative of the meridional overturning circulation. Upwelling waters that have not been in recent contact with the atmosphere show negligible or small trends, while surface and downwelling waters that have had more exposure to the atmosphere show the strongest trends. Thus comparison of this new BGC-Argo mapped pH estimate to historic observations allows quantifying the structure of Southern Ocean acidification.

Key Points

  • We present a novel 12-month Southern Ocean pH mapped product, made possible by the Biogeochemical-Argo array initiated in 2014.
  • Comparing to ship-based measurements above 1500 m reveals a decrease in pH of up to 0.02 per decade.
  • pH changes are widespread with varying magnitudes reflecting the pattern of the meridional overturning circulation.
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Climate change and the sea: a major disruption in steady state and the master variables

Published 26 April 2023 Science Closed
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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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Long-term slowdown of ocean carbon uptake by alkalinity dynamics

Published 17 April 2023 Science Closed
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Oceanic absorption of atmospheric carbon dioxide (CO2) is expected to slow down under increasing anthropogenic emissions; however, the driving mechanisms and rates of change remain uncertain, limiting our ability to project long-term changes in climate. Using an Earth system simulation, we show that the uptake of anthropogenic carbon will slow in the next three centuries via reductions in surface alkalinity. Warming and associated changes in precipitation and evaporation intensify density stratification of the upper ocean, inhibiting the transport of alkaline water from the deep. The effect of these changes is amplified threefold by reduced carbonate buffering, making alkalinity a dominant control on CO2 uptake on multi-century timescales. Our simulation reveals a previously unknown alkalinity-climate feedback loop, amplifying multi-century warming under high emission trajectories.

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Carbonate system data tracing freshwater inflow into the Ross Sea through the eastern gate and along the Ross Ice Shelf (Antarctica)

Published 28 December 2022 Science Closed
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The eastern Ross Sea is a key area to understand the role of the Amundsen Sea inflow of freshwater that can influence the Ross Sea water properties and salt budget. A survey was carried out in the eastern Ross Sea during the austral summer 2019–20 to evaluate the contribution of the Amundsen Sea Water (ASW) to the salinity variability. A total of 248 seawater samples were collected f\or the analysis of total alkalinity (AT) and pH. The data collected were used together with temperature and salinity to obtain a full description of the carbonate system properties including total inorganic carbon (CT), CO2 partial pressure (pCO2), calcium carbonate saturation state of aragonite and calcite (Ω), and Revelle factor. Moreover, we estimated the anthropogenic carbon (Cant) throughout the TrOCA method to better understand the carbon cycle, also considering the effect of atmospheric CO2 uptake on ocean acidification. We used principal component analysis (PCA) to investigate the major controls on the carbonate system parameters with the aim of defining their sensitivity as chemical tracers. The changes in carbonate chemistry in surface waters were mainly due to the physical properties. AT and pH traced the entry of the ASW showing limited mixing between water masses on the shelf area. Shelf waters were enriched in Cant, which resulted lower than the estimated value for shelf waters produced in western Ross Sea.

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Impact of ocean acidification and ocean warming on the oxidation of dissolved Fe(II) in coastal and open Southern Ocean water

Published 8 December 2022 Science Closed
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The Southern Ocean is the largest region where major nutrients such as nitrate, silicate and phosphate are present in excess, yet the crucial micronutrient element iron (Fe) is scarce. It is well established that the Southern Ocean is key in exporting carbon to greater depths through biomass production by phytoplankton, but Fe is metabolically required for photosynthesis. Changes in uptake of carbon and heat to the ocean will impact ocean acidification and ocean warming. These anthropogenically linked processes are projected to lead to a drop in ocean pH by 0.2 units and an increase in the ocean’s temperature by 2°C by the end of the century and are already known to have tremendous ecological impacts on the ocean’s flora and fauna. However, little is known about how changes in ocean temperature and pH could alter the nutrient composition in future oceans.

Regarding nutrients, this work focuses on the dissolved (d) element Fe. It is essential for photosynthesis, but also a limiting element in the Southern Ocean due to limiting sources leading to low availability. Iron exists in two redox states in seawater. While the species dFe(III) is stable in seawater and occurs in relatively higher concentrations, its redox partner dFe(II) is tied to several physico-chemical processes impacting its oxidation time and overall presence. The importance of dFe(II) also lies with its accessibility for phytoplankton in its reduced oxidative state. The overall aim of this study was to investigate changes in concentration, speciation, and availability of the ‘more’ bioavailable, rapidly oxidizing Fe species dFe(II) under a changing Southern Ocean scenario.

Chapter 2 addressed the redox behaviour of dFe(II) and dFe(III), where several questions were explored for further experimental planning. The main question was how the coastal and open ocean systems differ in their dFe(II) concentrations and how ocean acidification and ocean warming impact Fe redox chemistry in both systems. I therefore performed controlled acidification and temperature alteration experiments in coastal and open ocean water taken from the Tasmanian coast and the Southern Ocean. This large dataset enabled us to project for future ocean dFe(II) concentrations and oxidation rates. I observed that a reduction in ocean pH by 0.2 units doubles the dFe(II) oxidation time in the open ocean and tripled in coastal water through model-based experiments. In contrast to these high impacts from pH, an increase in temperature by 1°C accelerated the oxidation by ~ 1.1 times (13% in coastal water and 8% in open ocean water). Therefore, realistic changes in temperature are likely to have small impacts on the oxidation of dFe(II) in both water systems compared to the proposed changes in pH.

For phytoplankton, these results pose contradicting outcomes, and studies display mixed results once parameters such as ocean warming, and acidification are combined. An increase in temperature might lead to less or no growth once a certain temperature threshold is crossed. Similarly, a decrease in pH is also thought to impact phytoplankton physiology. It also depends on the severity of acidification and the phytoplankton species itself. Ocean warming could reduce phytoplankton growth, despite increased Fe availability due to higher solubility in warmer water. Regarding ocean acidification, on the other hand, dFe(II) could become available for an extended time, therefore enabling further uptake of dFe(II) by phytoplankton for that time. When comparing mixed effects of ocean acidification and warming, a reduction in pH might have a greater impact on the dFe(II) oxidation than just temperature. Temperature changes, however, might be a greater concern in the near future before ocean acidification becomes relevant.

Due to this projection of temperature being a more imminent concern, I targeted the limiting element Fe in its less investigated form dFe(II). I observed how temperature alone impacts growth of two Southern Ocean phytoplankton species. I therefore ran an dFe(II)-enrichment incubation experiment in Chapter 3 with differing temperatures (3°C, 5°C, and 7°C) in coastal and open ocean water from the Southern Ocean using the well-studied haptophyte Phaeocystis antarctica and the diatom Fragilariopsis cylindrus. These enrichment experiments with altered temperatures overall confirmed that phytoplankton growth was elevated once 5 nM dFe(II) were added. In other words, freely available dFe(II) was present, almost regardless of the temperature increase from 3°C to 7°C. This could implicate that an increase in temperature has beneficial effects on growth in the case of higher concentrations of freely available dFe(II). However, these values of future dFe(II) concentrations and oxidation rates under acidified and warmer scenarios are only laboratory-based projections, to better understand the dFe(II) presence and demand by phytoplankton species in a future Southern Ocean.

In Chapter 4, a one-month field study onboard the RV Investigator was conducted east of the Australian continent along the East Australian Current (EAC) into nutrient-rich but Fe poor water in the Southern Ocean. I observed the overall distribution of dFe(II) and hydrogen peroxide in this understudied region. The findings suggest that dFe(II) concentrations are very low in the observed area of the open Southern Ocean (< 0.1 nM) compared to coastal waters (> 0.5 nM), likely driven by differences in terrestrial Fe inputs. Hydrogen peroxide was generally higher in the southern stations within the upper 200 m (~60 nM) while the dFe(II) : dFe ratios are 10 % higher than reported for previous Southern Ocean studies. High biological activity in the upper water extending to the frontal mixing zone where the two major currents meet (EAC and STF), may further have led to the observed low dFe concentrations and high H22O22 concentrations. Occasional higher dFe(II) peaks found in this area in surface water may be the result of several external sources such as rain or vertical transport from seamounts but also due to biological or physico-chemical impacts such as photochemical reduction or uptake by phytoplankton.

Overall, the work in this study advances our understanding of the coupled effects of the climate change parameters ocean acidification and ocean warming on the dFe(II) oxidation, with implications for its availability to phytoplankton and overall sources in the region east and south-east of Tasmania in coastal and open ocean water. The experimental approaches taken suggest a higher impact of ocean acidification compared to ocean warming and a potential benefit for phytoplankton species preferring dFe(II).

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Carbon and nutrient cycling in Antarctic landfast sea ice from winter to summer

Published 2 December 2022 Science Closed
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Seasonal cycling in carbon, alkalinity, and nutrients in landfast sea ice in Hangar Cove, Adelaide Island, West Antarctic Peninsula, were investigated during winter, spring, and summer 2014–2015. Temporal dynamics were driven by changes in the sea-ice physicochemical conditions, ice-algal community composition, and organic matter production. Winter sea ice was enriched with dissolved inorganic carbon (DIC) and inorganic nutrients from organic matter remineralization. Variations in alkalinity (Alk) and DIC indicated that abiotic calcium carbonate (ikaite) precipitation had taken place. Relative to other nutrients, low phosphate (PO4) concentrations potentially resulted from co-precipitation with ikaite. Seawater flooding and meltwater induced variability in the physical and biogeochemical properties in the upper ice in spring where nutrient resupply supported haptophyte productivity and increased particulate organic carbon (POC) in the interstitial layer. Rapid nitrate (NO3) and DIC (< 165 μmol kg−1) uptake occurred alongside substantial build-up of algal biomass (746 μg chlorophyll a L−1) and POC (6191 μmol L−1) during summer. Silicic acid drawdown followed NO3 depletion by approximately 1 month with a shift to diatom-dominated communities. Accumulation of PO4 in the lower ice layers in summer likely resulted from PO4 released during ikaite dissolution in the presence of biofilms. Increased Alk : DIC ratios in the lower ice and under-ice water suggested that ikaite dissolution buffered against meltwater dilution and enhanced the potential for atmospheric CO2 uptake. This study revealed strong seasonality in carbon and nutrient cycling in landfast sea ice and showed the importance of sea ice in biogeochemical cycling in seasonally ice-covered waters around Antarctica.

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Temperature as a likely driver shaping global patterns in mineralogical composition in bryozoans: implications for marine calcifiers under global change

Published 29 November 2022 Science Closed
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The Southern Ocean is showing one of the most rapid responses to human-induced global change, thus acting as a sentinel of the effects on marine species and ecosystems. Ocean warming and acidification are already impacting benthic species with carbonate skeletons, but the magnitude of these changes to species and ecosystems remains largely unknown. Here we provide the largest carbonate mineralogical dataset to date for Southern Ocean bryozoans, which are diverse, abundant and important as carbonate producers, thus making them excellent for monitoring the effects of ocean warming and acidification. To improve our understanding of how bryozoans might respond to ocean warming and acidification, we assess latitudinal and seafloor temperature patterns of skeletal mineralogy using bryozoan species occurrences together with temperature data for the first time. Our findings, combining new mineralogical data with published data from warmer regions, show that the proportions of high-Mg calcite and bimineralic species increase significantly towards lower latitudes and with increasing seawater temperature. These patterns are consistent with the hypothesis that seawater temperature is likely a significant driver of variations in bryozoan mineralogy at a global scale.

Continue reading ‘Temperature as a likely driver shaping global patterns in mineralogical composition in bryozoans: implications for marine calcifiers under global change’

Transcriptomic analysis reveals distinct mechanisms of adaptation of a polar picophytoplankter under ocean acidification conditions

Published 26 October 2022 Science Closed
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Graphical abstract.

Highlights

  • Increase of carbon dioxide emission to the atmosphere acidifies the ocean.
  • Ocean acidification drives the growth of a small green phytoplankter (picochlorophyte).
  • Picochlorophytes exhibit distinct metabolism compared to other polar phytoplankton.
  • Genes related to ribosomal proteins, amino acid synthesis, RNA post-transcriptional modification, nitrogen assimilation, molecular chaperones, light harvesting complexes, pigment synthesis, were found to be differentially expressed under future predicted CO2 levels.

Abstract

Human emissions of carbon dioxide are causing irreversible changes in our oceans and impacting marine phytoplankton, including a group of small green algae known as picochlorophytes. Picochlorophytes grown in natural phytoplankton communities under future predicted levels of carbon dioxide have been demonstrated to thrive, along with redistribution of the cellular metabolome that enhances growth rate and photosynthesis. Here, using next-generation sequencing technology, we measured levels of transcripts in a picochlorophyte Chlorella, isolated from the sub-Antarctic and acclimated under high and current ambient CO2 levels, to better understand the molecular mechanisms involved with its ability to acclimate to elevated CO2. Compared to other phytoplankton taxa that induce broad transcriptomic responses involving multiple parts of their cellular metabolism, the changes observed in Chlorella focused on activating gene regulation involved in different sets of pathways such as light harvesting complex binding proteins, amino acid synthesis and RNA modification, while carbon metabolism was largely unaffected. Triggering a specific set of genes could be a unique strategy of small green phytoplankton under high CO2 in polar oceans.

Continue reading ‘Transcriptomic analysis reveals distinct mechanisms of adaptation of a polar picophytoplankter under ocean acidification conditions’

Enhance seasonal amplitude of atmospheric CO2 by the changing Southern Ocean carbon sink

Published 19 October 2022 Science Closed
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The enhanced seasonal amplitude of atmospheric CO2 has been viewed so far primarily as a Northern Hemisphere phenomenon. Yet, analyses of atmospheric CO2 records from 49 stations between 1980 and 2018 reveal substantial trends and variations in this amplitude globally. While no significant trends can be discerned before 2000 in most places, strong positive trends emerge after 2000 in the southern high latitudes. Using factorial simulations with an atmospheric transport model and analyses of surface ocean Pco2 observations, we show that the increase is best explained by the onset of increasing seasonality of air-sea CO2 exchange over the Southern Ocean around 2000. Underlying these changes is the long-term ocean acidification trend that tends to enhance the seasonality of the air-sea fluxes, but this trend is modified by the decadal variability of the Southern Ocean carbon sink. The seasonal variations of atmospheric CO2 thus emerge as a sensitive recorder of the variations of the Southern Ocean carbon sink.

Continue reading ‘Enhance seasonal amplitude of atmospheric CO2 by the changing Southern Ocean carbon sink’

Temperature as a likely driver shaping global patterns in mineralogical composition in bryozoans: implications for marine calcifiers under global change

Published 12 October 2022 Science Closed
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The Southern Ocean is showing one of the most rapid responses to human-induced global change, thus acting as a sentinel of the effects on marine species and ecosystems. Ocean warming and acidification are already impacting benthic species with carbonate skeletons, but the magnitude of these changes to species and ecosystems remains largely unknown. Here we provide the largest carbonate mineralogical dataset to date for Southern Ocean bryozoans, which are diverse, abundant and important as carbonate producers, thus making them excellent for monitoring the effects of ocean warming and acidification. To improve our understanding of how bryozoans might respond to ocean warming and acidification, we assess latitudinal and seafloor temperature patterns of skeletal mineralogy using bryozoan species occurrences together with temperature data for the first time. Our findings, combining new mineralogical data with published data from warmer regions, show that the proportions of high-Mg calcite and bimineralic species increase significantly towards lower latitudes and with decreasing seawater temperature. These patterns are consistent with the hypothesis that seawater temperature is likely a significant driver of variations in bryozoan mineralogy at a global scale.

Continue reading ‘Temperature as a likely driver shaping global patterns in mineralogical composition in bryozoans: implications for marine calcifiers under global change’
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