Pteropods (pelagic snails) are ubiquitous zooplankton in the Southern Ocean and abundant along the Western Antarctic Peninsula (WAP), one of the most rapidly warming regions on the planet. They are important prey for higher trophic levels, grazers of phytoplankton, and contribute to particulate organic and inorganic carbon export. Pteropods are heralded as bioindicators of ecosystem health due to the vulnerability of their aragonitic shells under ocean acidification conditions, which could greatly affect their abundances in the future. Despite their importance within Antarctic food webs, few studies have analyzed the effects of climate change on pteropod physiology and biogeography in the Southern Ocean. I utilized zooplankton net tows and sediment trap samples collected as part of the Palmer Antarctica Long Term Ecological Research (PAL LTER) program to determine long-term changes in pteropod biogeography and phenology (life history). I also conducted shipboard experiments on PAL LTER research cruises to analyze the effects of shifting temperature and food conditions on pteropod metabolism. Lastly, to examine WAP pteropod feeding ecology, I utilized high-throughput sequencing techniques and analyzed pteropod gut contents at an unprecedented taxonomic resolution. Pteropod populations along the WAP from 1993-2017 either remained stable (shelled pteropods) or increased (non-shelled pteropods) and were most strongly controlled by La Niña conditions the year prior, which led to warmer, ice-free waters. There was a weak relationship between pteropod abundance and carbonate chemistry, and no detectable long-term trend in carbonate chemistry parameters (i.e., aragonite saturation), thus ocean acidification is not presently a factor influencing WAP pteropod abundance. More open-water areas the year prior also increased growth rates of the shelled pteropod, Limacina helicina antarctica, and caused earlier time of appearance in the PAL LTER sediment trap. There was considerable interannual variability in the time of appearance of a new pteropod cohort, which ranged from year day 22 to 255, but no long-term, directional change in time of appearance or growth rate. The effects of warming seawater temperatures and shifting food availability on L. h. antarctica metabolism revealed that highest respiration and usually highest excretion rates occurred under higher temperatures, but the effect of food concentration was more limited. The proportion of dissolved organic matter to total organic and inorganic dissolved constituents was high and the metabolic ratios of C, N, and P were all below the canonical Redfield ratio, which can directly affect phytoplankton growth and bacterial production in the WAP. Analysis of L. h. antarctica gut contents revealed its microbiome for the first time with Mollicutes bacteria the most abundant prokaryote. Pteropods were mainly herbivorous in summer, consuming predominantly diatoms but also supplementing their diet with microzooplankton such as ciliates. My dissertation shows that pteropods along the WAP are sensitive to changes in the environment from daily to interannual time scales. These insights into the metabolic and ecologic responses of pteropods to ocean variability increase our understanding of the role of zooplankton in biogeochemical cycles and help predict future responses to climate change.
Posts Tagged 'Antarctic'
Environmental controls on pteropod ecology and physiology along the Western Antarctic Peninsula
Published 31 August 2020 Science ClosedTags: abundance, Antarctic, biological response, chemistry, crustaceans, field, otherprocess, physiology, respiration, zooplankton
Impact of climate change on the primary production and related biogeochemical cycles in the coastal and sea ice zone of the Southern Ocean
Published 26 August 2020 Science ClosedTags: Antarctic, biogeochemistry, chemistry
Highlights
• Changes in primary production in the Southern Ocean are analyzed in a comprehensive manner.
• General production increase is the sign of climate change in the Southern Ocean.
• Associated biogeochemical variables show regionally heterogenous signals of climate change.
• Sea ice affecting iron supply and stratification is a primary factor for CCSZ production change.
• Production change in SIZ is sensitive to change in iron supply arising from various mechanisms.
Abstract
Climate change in the Southern Hemisphere has exerted impact on the primary production in the Southern Ocean (SO). Using a recently released reanalysis dataset on global biogeochemistry, a comprehensive analysis was conducted on the complex biogeochemical seasonal cycle and the impact of climate change with a focus in areas within the meridional excursion of the sea ice boundary—coastal and continental shelf zone (CCSZ) and seasonal sea ice zone (SIZ). The seasonal cycles of primary production and related nutrients are closely linked with the seasonal changes in sea ice and sea surface temperatures. As sea ice retreats and allows energy and gas exchange across the sea surface, phytoplankton growth is initiated, consuming accumulated nutrients within the shallow depth of ~40 m. The seasonal evolutions of physical, biological and chemical variables show both spatial and temporal consistency with each other. Climate change has altered the timing and amplitude of the seasonal cycle. While primary production has generally increased along with an intensified uptake of CO2, some areas show a reduction in production (e.g., Prydz Bay, eastern Indian Ocean). In the CCSZ, increased iron utilization and light availability allowed production to be increased. However, the mechanism by which these factors are altered varies from one location to another, including changes in sea ice cover, surface stratification, and downwelling/upwelling. In the SIZ, where iron is generally a limiting factor, iron supply is a key driver of changes in primary production regardless of other nutrients. There is a clear influence of climatic change on the biogeochemical cycle although the signal is still weak.
Understanding the metabolic capacity of Antarctic fishes to acclimate to future ocean conditions
Published 26 August 2020 Science ClosedTags: Antarctic, biological response, fish, physiology, review
Antarctic fishes have evolved under stable, extreme cold temperatures for millions of years. Adapted to thrive in the cold environment, their specialized phenotypes will likely render them particularly susceptible to future ocean warming and acidification as a result of climate change. Moving from a period of stability to one of environmental change, species persistence will depend on maintaining energetic equilibrium, or sustaining the increased energy demand without compromising important biological functions such as growth and reproduction. Metabolic capacity to acclimate, marked by a return to metabolic equilibrium through physiological compensation of resting metabolic rate (RMR), will likely determine which species will be better poised to cope with shifts in environmental conditions. Focusing on the suborder Notothenioidei, a dominant group of Antarctic fishes, and in particular 4 well-studied species, Trematomus bernacchii, Pagothenia borchgrevinki, Notothenia rossii and N. coriiceps, we discuss metabolic acclimation potential to warming and CO2-acidification using an integrative and comparative framework. There are species-specific differences in the physiological compensation of RMR during warming and the duration of acclimation time required to achieve compensation; for some species RMR fully recovered within 3.5 weeks of exposure, such as P. borchgrevinki, while for other species, such as N. coriiceps, RMR remained significantly elevated past 9 weeks of exposure. In all instances, added exposure to increased PCO2, further compromised the ability of species to return RMR to pre-exposure levels. The period of metabolic imbalance, marked by elevated RMR, was underlined by energetic disturbance and elevated energetic costs, which shifted energy away from fitness-related functions, such as growth. In T. bernacchii and N. coriiceps, long duration of elevated RMR impacted condition factor and/or growth rate. Low growth rate can affect development and ultimately the timing of reproduction, severely compromising the species’ survival potential and the biodiversity of the notothenioid lineage. Therefore, the ability to achieve full compensation of RMR, and in a short-time frame, in order to avoid long term consequences of metabolic imbalance, will likely be an important determinant in a species’ capacity to persist in a changing environment. Much work is still required to develop our understanding of the bioenergetics of Antarctic fishes in the face of environmental change, and a targeted approach of nesting a mechanistic focus in an ecological and comparative framework will better aid our predictions on the effect of global climate change on species persistence in the polar regions.
Ocean acidification reduces growth and grazing impact of Antarctic heterotrophic nanoflagellates
Published 21 August 2020 Science ClosedTags: abundance, Antarctic, biological response, laboratory, otherprocess, phytoplankton
High-latitude oceans have been identified as particularly vulnerable to ocean acidification if anthropogenic CO2 emissions continue. Marine microbes are an essential part of the marine food web and are a critical link in biogeochemical processes in the ocean, such as the cycling of nutrients and carbon. Despite this, the response of Antarctic marine microbial communities to ocean acidification is poorly understood. We investigated the effect of increasing fCO2 on the growth of heterotrophic nanoflagellates (HNFs), nano- and picophytoplankton, and prokaryotes (heterotrophic Bacteria and Archaea) in a natural coastal Antarctic marine microbial community from Prydz Bay, East Antarctica. At CO2 levels ≥634 µatm, HNF abundance was reduced, coinciding with increased abundance of picophytoplankton and prokaryotes. This increase in picophytoplankton and prokaryote abundance was likely due to a reduction in top-down control of grazing HNFs. Nanophytoplankton abundance was elevated in the 634 µatm treatment, suggesting that moderate increases in CO2 may stimulate growth. The taxonomic and morphological differences in CO2 tolerance we observed are likely to favour dominance of microbial communities by prokaryotes, nanophytoplankton, and picophytoplankton. Such changes in predator–prey interactions with ocean acidification could have a significant effect on the food web and biogeochemistry in the Southern Ocean, intensifying organic-matter recycling in surface waters; reducing vertical carbon flux; and reducing the quality, quantity, and availability of food for higher trophic levels.
Geochemical reconstructions of Southern Ocean pH and temperature over the last glacial cycle
Published 13 August 2020 Science ClosedTags: Antarctic, biological response, dissolution, field, laboratory, morphology, paleo, protists
The Southern Ocean is widely thought to play an important role in atmospheric CO₂ change over glacial-interglacial cycles. It has been suggested that as the region that ventilates the majority of the world’s carbon-rich deep waters today, reduced exchange between deep waters and the atmosphere in the Southern Ocean acted to draw down CO₂ over glacial timescales. However, direct evidence of the Southern Ocean’s role in glacial CO₂ drawdown has been lacking thus far. Here I apply the boron-isotope pH-proxy to foraminifera from the Antarctic Zone sediment core PS1506 over the last glacial cycle. The low boron concentrations in these polar foraminifera makes these samples particularly sensitive to boron blank and so a close examination of the sources of blank, and an assessment of the precision of blank measurements, has been made. The ratios of trace elements to calcium in foraminiferal shells are widely applied as proxies for palaeoenvironmental parameters such as temperature. As Southern Ocean carbonate sediments are particularly prone to dissolution, which can affect trace element concentrations, an assessment of dissolution has been made. Firstly, dissolution experiments were conducted to constrain the impact of dissolution in a controlled setting, and secondly, shell mass and trace elements were evaluated for the downcore record. Imaging reveals similar etching textures in both experimentally dissolved samples and deglacial intervals, when shell mass is also low and several trace elements exhibit an excursion to lower values. Boron isotope data for PS1506 show that during the penultimate interglacial, surface water pH was low. At the onset of atmospheric CO₂ drawdown, pH increased, indicating low CO₂ surface waters. This is consistent with the signature predicted for a more stratified Southern Ocean, and is evidence that stratification in the Antarctic Zone acted to contribute to CO₂ drawdown early in the transition to a glacial state.
Effects of ocean acidification on Antarctic microbial communities
Published 23 June 2020 Science ClosedTags: abundance, Antarctic, biological response, BRcommunity, communitymodeling, laboratory, modeling, molecular biology, morphology, otherprocess, phytoplankton, prokaryotes, review
Antarctic waters are amongst the most vulnerable in the world to ocean acidification due to their cold temperatures, naturally low levels of calcium carbonate and upwelling that brings deep CO2-rich waters to the surface. A meta-analysis demonstrated groups of Antarctic marine biota in waters south of 60!S have a range of tolerances to ocean acidification. Invertebrates and phytoplankton showed negative effects above 500 μatm and 1000 μatm CO2 respectively, while bacteria appear tolerant to elevated CO2. Phytoplankton studied as part of a natural microbial community were found to be more
sensitive than those studied as a single species in culture. This highlights the importance of community and ecosystem level studies, which incorporate the interaction and competition among species and trophic levels, to accurately assess the effects of ocean acidification on the Antarctic ecosystem.
Antarctic marine microbes (comprising phytoplankton, protozoa and bacteria) drive ocean productivity, nutrient cycling and mediate trophodynamics and the biological pump. While they appear vulnerable to changes in ocean chemistry, little is known about the nature and magnitude of their responses to ocean acidification, especially for natural communities. To address this lack of information, a six level, dose-response ocean acidification experiment was conducted in Prydz Bay, East Antarctica, using 650 L incubation tanks (minicosms). The minicosms were filled with Antarctic nearshore water and adjusted to a gradient of carbon dioxide (CO2) from 343 to 1641 μatm. Microscopy
and phylogenetic marker gene sequence analysis found the microbial community
composition altered at CO2 levels above approximately 1000 μatm. The CO2-
induced responses of microeukaryotes (>20 μm) and nanoeukaryotes (2 to 20 μm) were taxon-specific. For diatoms the response of taxa was related to cell size with micro-sized diatoms (>20 μm) increasing in abundance with moderate CO2 (506 to 634 μatm), while above this level their abundance declined. In contrast, nano-size diatoms (<20 μm) tolerated elevated CO2. Like large diatoms, Phaeocystis antarctica increased in abundance between 343 to 634 μatm CO2 but fell at higher levels. 18S and 16S rDNA sequencing showed that picoeukaryotic and prokaryotic composition was unaffected by CO2, despite having higher abundances at CO2 levels !634 μatm. This was likely due to the lower abundance of heterotrophic nanoflagellates at CO2 levels exceeding 953 μatm, which reduced the top-down control of their pico- and nanoplanktonic prey. As a result of the differences in the tolerance of individual taxa/size categories, CO2 caused a
significant change in the microbial community structure to one dominated by nano-sized diatoms, picoeukaryotes and prokaryotes.
Based on the CO2-induced changes in the microbial community, modelling was performed to investigate the future effects of different levels of elevated CO2 on the structure and function of microbial communities in Antarctic coastal systems. These models indicate CO2 levels predicted toward the end of the century under a “business as usual scenario” elicit changes in microbial composition, significantly altering trophodynamic pathways, reducing energy transfer to higher trophic levels and favouring respiration of carbon within the microbial loop. Such responses would alter elemental cycles, jeopardise the productivity that underpins the wealth and diversity of life for which Antarctica is renowned. In addition, it would reduce carbon sequestration in coastal Antarctic waters thereby having a positive feedback on global climate change.
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Variability and stability of anthropogenic CO2 in Antarctic Bottom Waters observed in the Indian sector of the Southern Ocean, 1978-2018
Published 19 May 2020 Science ClosedTags: Antarctic, biogeochemistry, chemistry, field
Antarctic bottom waters (AABWs) are known as a long term sink for anthropogenic CO2 (Cant) but is hardly quantified because of the scarcity of the observations, specifically at an interannual scale. We present in this manuscript an original dataset combining 40 years of carbonate system observations in the Indian sector of the Southern Ocean (Enderby Basin) to evaluate and interpret the interannual variability of Cant in the AABW. This investigation is based on regular observations collected at the same location (63° E/56.5° S) in the frame of the French observatory OISO from 1998 to 2018 extended by GEOSECS and INDIGO observations (1978, 1985 and 1987).
At this location the main sources of AABW sampled is the fresh and younger Cape Darnley bottom water (CDBW) and the Weddell Sea deep water (WSDW). Our calculations reveal that Cant concentrations increased significantly in AABW, from about +7 µmol.kg-1 in 1978-1987 to +13 µmol.kg-1 18 in 2010-2018. This is comparable to previous estimates in other SO basins, with the exception of bottom waters close to their formation sites where Cant concentrations are about twice as large. Our analysis shows that the CT and Cant increasing rates in AABW are about the same over the period 1978-2018, and we conclude that the long-term change in CT is mainly due to the uptake of anthropogenic CO2 in the different formation regions. This is however modulated by significant interannual to pluriannual variability associated with variations in hydrological (ϴ, S) and biogeochemical (CT, AT, O2) properties. A surprising result is the apparent stability of Cant concentrations in recent years despite the increase in CT and the gradual acceleration of atmospheric CO2.
The Cant sequestration by AABWs is more variable than expected and depends on a complex combination of physical, chemical and biological processes at the formation sites and during the transit of the different AABWs. The interannual variability at play in AABW needs to be carefully considered on the extrapolated estimation of Cant sequestration based on sparse observations over several years.
Effects of ocean acidification on Antarctic marine organisms: a meta‐analysis
Published 24 April 2020 Science ClosedTags: Antarctic, BRcommunity, review
Southern Ocean waters are among the most vulnerable to ocean acidification. The projected increase in the CO2 level will cause changes in carbonate chemistry that are likely to be damaging to organisms inhabiting these waters. A meta‐analysis was undertaken to examine the vulnerability of Antarctic marine biota occupying waters south of 60°S to ocean acidification. This meta‐analysis showed that ocean acidification negatively affects autotrophic organisms, mainly phytoplankton, at CO2 levels above 1,000 μatm and invertebrates above 1,500 μatm, but positively affects bacterial abundance. The sensitivity of phytoplankton to ocean acidification was influenced by the experimental procedure used. Natural, mixed communities were more sensitive than single species in culture and showed a decline in chlorophyll a concentration, productivity, and photosynthetic health, as well as a shift in community composition at CO2 levels above 1,000 μatm. Invertebrates showed reduced fertilization rates and increased occurrence of larval abnormalities, as well as decreased calcification rates and increased shell dissolution with any increase in CO2 level above 1,500 μatm. Assessment of the vulnerability of fish and macroalgae to ocean acidification was limited by the number of studies available. Overall, this analysis indicates that many marine organisms in the Southern Ocean are likely to be susceptible to ocean acidification and thereby likely to change their contribution to ecosystem services in the future. Further studies are required to address the poor spatial coverage, lack of community or ecosystem‐level studies, and the largely unknown potential for organisms to acclimate and/or adapt to the changing conditions.
Continue reading ‘Effects of ocean acidification on Antarctic marine organisms: a meta‐analysis’
Combined stress of ocean acidification and warming influence survival and drives differential gene expression patterns in the Antarctic pteropod, Limacina helicina antarctica
Published 21 April 2020 Science ClosedTags: Antarctic, biological response, laboratory, molecular biology, mollusks, multiple factors, temperature, zooplankton
The ecologically important thecosome pteropods in the Limacina spp. complex have recently been the focus of studies examining the impacts global change factors – e.g., ocean acidification (OA) and ocean warming (OW) – on their performance and physiology. This focus is driven by conservation concerns where the health of pteropod populations is threatened by the high susceptibility of their shells to dissolution in low aragonite saturation states associated with OA and how coupling of these stressors may push pteropods past the limits of physiological plasticity. In this manipulation experiment, we describe changes in the transcriptome of the Antarctic pteropod, Limacina helicina antarctica, to these combined stressors. The conditions used in the laboratory treatments met or exceeded those projected for the Southern Ocean by the year 2100. We made two general observations regarding the outcome of the data: (1) Temperature was more influential than pH in terms of changing patterns of gene expression, and (2) these Antarctic pteropods appeared to have a significant degree of transcriptomic plasticity to respond to acute abiotic stress in the laboratory. In general, differential gene expression was observed amongst the treatments; here, for example, transcripts associated with maintaining protein structure and cell proliferation were up-regulated. To disentangle the effects of OA and OW, we used a weighted gene co-expression network analysis to explore patterns of change in the transcriptome. This approach identified gene networks associated with OW that were enriched for transcripts proposed to be involved in increasing membrane fluidity at warmer temperatures. Together these data provide evidence that L.h.antarctica has a limited capacity to acclimate to the combined conditions of OA and OW used in this study. This reduced scope of acclimation argues for continued study of how adaptation to polar aquatic environments may limit the plasticity of present-day populations in responding to future environmental change.
Antarctic environmental change and biological responses
Published 7 April 2020 Media coverage , Science ClosedTags: Antarctic, review
Antarctica and the surrounding Southern Ocean are facing complex environmental change. Their native biota has adapted to the region’s extreme conditions over many millions of years. This unique biota is now challenged by environmental change and the direct impacts of human activity. The terrestrial biota is characterized by considerable physiological and ecological flexibility and is expected to show increases in productivity, population sizes and ranges of individual species, and community complexity. However, the establishment of non-native organisms in both terrestrial and marine ecosystems may present an even greater threat than climate change itself. In the marine environment, much more limited response flexibility means that even small levels of warming are threatening. Changing sea ice has large impacts on ecosystem processes, while ocean acidification and coastal freshening are expected to have major impacts.
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Changes in genome-wide methylation and gene expression in response to future pCO2 extremes in the Antarctic pteropod Limacina helicina antarctica
Published 7 February 2020 Science ClosedTags: Antarctic, biological response, laboratory, molecular biology, mollusks, zooplankton
Epigenetic processes such as variation in DNA methylation may promote phenotypic plasticity and the rapid acclimatization of species to environmental change. The extent to which an organism can mount an epigenetic response to current and future climate extremes may influence its capacity to acclimatize or adapt to global change on ecological rather than evolutionary time scales. The thecosome pteropod Limacina helicina antarctica is an abundant macrozooplankton endemic to the Southern Ocean and is considered a bellwether of ocean acidification as it is highly sensitive to variation in carbonate chemistry. In this study, we quantified variation in DNA methylation and gene expression over time across different ocean acidification regimes. We exposed L. helicina antarctica to pCO2 levels mimicking present-day norms in the coastal Southern Ocean of 255 μatm pCO2, present-day extremes of 530 μatm pCO2, and projected extremes of 918 μatm pCO2 for up to 7 days before measuring global DNA methylation and sequencing transcriptomes in animals from each treatment across time. L. helicina antarctica significantly reduced DNA methylation by 29–56% after 1 day of exposure to 918 μatm pCO2 before DNA methylation returned to control levels after 6 days. In addition, L. helicina antarctica exposed to 918 μatm pCO2 exhibited drastically more differential expression compared to cultures replicating present-day pCO2 extremes. Differentially expressed transcripts were predominantly downregulated. Furthermore, downregulated genes were enriched with signatures of gene body methylation. These findings support the potential role of DNA methylation in regulating transcriptomic responses by L. helicina antarctica to future ocean acidification and in situ variation in pCO2 experienced seasonally or during vertical migration. More broadly, L. helicina antarctica was capable of mounting a substantial epigenetic response to ocean acidification despite little evidence of metabolic compensation or recovery of the cellular stress response in this species at future pCO2 levels.
Polar opposites; bacterioplankton susceptibility and mycoplankton resistance to ocean acidification
Published 6 February 2020 Science ClosedTags: Antarctic, biological response, field, laboratory, mesocosms, molecular biology, physiology, phytoplankton
Microorganisms form the basis of ocean ecosystems yet the effects of perturbations such as decreasing pH on microbial community structure, interactions and functionality remain compared to multicellular organisms. Using an experimental manipulation of Southern Ocean seawater, we subjected bacterioplankton and mycoplankton to artificial pH decreases, which are predicted to occur in the future. We show that acidification led to substantial increases of bacterioplankton diversity, while in contrast it had no effect on mycoplankton diversity. Our analyses revealed a loss of putative keystone taxa and a decrease in predicted community interactions as a response to lower pH levels. Bacterioplankton shifted from generalist to specialist community members, suggesting a specific stress response to unfavourable conditions. In addition, enzyme activities involved in nitrogen acquisition were lower at reduced pH levels, suggesting altered organic matter cycling in a more acidic ocean. Our findings suggest that bacterioplankton and mycoplankton may respond differentially to future ocean acidification, with potentially negative impacts on community structure and biogeochemical cycling in the Southern Ocean.
Towards an intensified summer CO2 sink behaviour in the Southern Ocean coastal regions
Published 22 January 2020 Science ClosedTags: Antarctic, biogeochemistry, chemistry
Highlights
• We analysed the FCO2 and CO2 system in an important region of the Southern Ocean.
• The Gerlache Strait acts as a stronger CO2 sink than nearby open ocean areas during the austral summer.
• We identified both strong and near-equilibrium sink scenarios for FCO2.
• The pattern of variability of FCO2 has changed since 2012 to a higher frequency of years with a strong CO2 sink.
Abstract
The Southern Ocean is a globally important carbon sink region. However, the austral coastal zones are usually not considered in global estimations due to their general undersampling and large regional dynamics. Thus, estimations of carbon uptake in the Southern Ocean may differ considerably from current values, i.e., without accounting for coastal regions. Here, we conducted a case study in the Gerlache Strait, an ecologically important Antarctic coastal zone. We show that the net sea-air CO2 flux (FCO2) in the strait may reach the same or greater magnitudes than those in large open sea regions around Antarctica during summer, despite having a much smaller area. A large mean FCO2 of –31 ± 19 mmol m–2 d–1 was observed in the strong CO2 sink years (i.e., FCO2 < –12 mmol m–2 d–1), in contrast to –1 ± 7 mmol m–2 d–1 in CO2 near-equilibrium conditions (i.e., CO2 sea–air difference ≈ 0). This variability is mainly modulated by phytoplankton activity and likely upwelling processes. We also identified two cycles of variability with 2-year and 4-year periodicities from 1999 to 2017. The 2-year periodicity becomes stronger after 2012, intensifying the strong CO2 sink scenario in the Gerlache Strait. Our findings reinforce the importance of polar coastal zones as CO2 sinks during the austral summer and the need to broaden our understanding of the role of these regions at other time scales.
A meta-analysis of microcosm experiments shows that dimethyl sulfide (DMS) production in polar waters is insensitive to ocean acidification
Published 17 January 2020 Science ClosedTags: abundance, Antarctic, Arctic, biogeochemistry, biological response, chemistry, field, laboratory, otherprocess, prokaryotes
Emissions of dimethylsulfide (DMS) from the polar oceans play a key role in atmospheric processes and climate. Therefore, it is important to increase our understanding of how DMS production in these regions may respond to climate change. The polar oceans are particularly vulnerable to ocean acidification (OA). However, our understanding of the polar DMS response is limited to two studies conducted in Arctic waters, where in both cases DMS concentrations decreased with increasing acidity. Here, we report on our findings from seven summertime shipboard microcosm experiments undertaken in a variety of locations in the Arctic Ocean and Southern Ocean. These experiments reveal no significant effects of short-term OA on the net production of DMS by planktonic communities. This is in contrast to similar experiments from temperate north-western European shelf waters where surface ocean communities responded to OA with significant increases in dissolved DMS concentrations. A meta-analysis of the findings from both temperate and polar waters (n=18 experiments) reveals clear regional differences in the DMS response to OA. Based on our findings, we hypothesize that the differences in DMS response between temperate and polar waters reflect the natural variability in carbonate chemistry to which the respective communities of each region may already be adapted. If so, future temperate oceans could be more sensitive to OA, resulting in an increase in DMS emissions to the atmosphere, whilst perhaps surprisingly DMS emissions from the polar oceans may remain relatively unchanged. By demonstrating that DMS emissions from geographically distinct regions may vary in their response to OA, our results may facilitate a better understanding of Earth’s future climate. Our study suggests that the way in which processes that generate DMS respond to OA may be regionally distinct, and this should be taken into account in predicting future DMS emissions and their influence on Earth’s climate.
Ocean freshening and acidification differentially influences mortality and behavior of the Antarctic amphipod Gondogeneia antarctica
Published 3 December 2019 Science ClosedTags: Antarctic, biological response, crustaceans, laboratory, mortality, multiple factors, performance, salinity, zooplankton
Highlights
• Glacier retreat induced by global warming can decrease pH and salinity of the Antarctic ocean.
• The Antarctic amphipod Gondogeneia antarctica was exposed to low pH (7.6) and low salinity (27 psμ) conditions.
• Low pH increased mortality, impaired food detection, reduced shelter-use during daytime. .
• Low salinity increased cannibalism and induced abnormal swimming.
• Ocean acidification and freshening act as independent stressors influencing behavior and physiology of Antarctic amphipods.
Abstract
The Western Antarctic Peninsula (WAP) has experienced rapid atmospheric and ocean warming over the past few decades and many marine-terminating glaciers have considerably retreated. Glacial retreat is accompanied by fresh meltwater intrusion, which may result in the freshening and acidification of coastal waters. Marian Cove (MC), on King George Island in the WAP, undergoes one of the highest rates of glacial retreat. Intertidal and shallow subtidal waters are likely more susceptible to these processes, and sensitive biological responses are expected from the organisms inhabiting this area. The gammarid amphipod Gondogeneia antarctica is one of the most abundant species in the shallow, nearshore Antarctic waters, and it occupies an essential ecological niche in the coastal marine WAP ecosystem. In this study, we tested the sensitivity of G. antarctica to lowered salinity and pH by meltwater intrusion following glacial retreat. We exposed G. antarctica to four different treatments combining two salinities (34 and 27 psμ) and pH (8.0 and 7.6) levels for 26 days. Mortality, excluding cannibalized individuals, increased under low pH but decreased under low salinity conditions. Meanwhile, low salinity increased cannibalism, whereas low pH reduced food detection. Shelter use during the daytime decreased under each low salinity and pH condition, indicating that the two stressors act as disruptors of amphipod behavior. Under low salinity conditions, swimming increased during the daytime but decreased at night. Although interactions between low salinity and low pH were not observed during the experiment, the results suggest that each stressor, likely induced by glacial melting, causes altered behaviors in amphipods. These environmental factors may threaten population persistence in Marian Cove and possibly other similar glacial embayments.
Report card: Potential tipping points for life in the Southern Ocean
Published 13 November 2019 Science ClosedTags: Antarctic, review
There is now clear scientific evidence that the increasing magnitude and rate of anthropogenic carbon dioxide (CO2) emissions are causing rapid and unprecedented changes to the global ocean. These will have potentially serious impacts during the 21st century on the sustainability and management of many marine and coastal ecosystems. Research has shown that the Southern Ocean, in particular, is encountering significant changes linked to climate change. The changes in pH, temperature, circulation and sea ice – along with potential for increased fishing pressure – are all likely to have far-reaching consequences for all species that currently inhabit the Southern Ocean.
One of the fundamental questions for marine scientists studying the Southern Ocean is how climate change will alter the growth of key prey species including phytoplankton, zooplankton and krill. Phytoplankton are the base Baleen whale. iStock of the marine food web, and even seemingly small changes in sea-ice, ocean circulation, chemistry and temperature will affect which species live, thrive and die in the ocean. The biological outcomes from these changes will be determined by the environment, timing, rate and magnitude of change in each stressor, the order in which the changes occur, and the potential for consequences to be compounded when multiple stressors change concurrently.
Hence, understanding the impacts of climate change on Southern Ocean life requires us to consider which key species will be more sensitive to change, if change will have benefical or detrimental effects on marine life, and how change will vary from region to region. These new scientific insights will have important implications for management of fish stocks and high conservation value species throughout the region.
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Lipid biochemistry and physiology of Antarctic krill (Euphausia superba) in the present day and under future ocean acidification scenarios
Published 30 October 2019 Science ClosedTags: Antarctic, biological response, crustaceans, laboratory, morphology, mortality, physiology, respiration
Antarctic krill (Euphausia superba, hereafter ‘krill’) are lipid-rich euphausiids with an important role in the Southern Ocean, including as the primary prey of Antarctic megafauna (whales, seals, penguins), fish, squid and seabirds. They contain high levels of nutritious long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA), specifically eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3). The sheer abundance of krill in the Southern Ocean means that the ecosystem is largely driven by energy derived from krill lipids. In addition to their ecological importance, a Scotia Sea krill fishery harvests krill, including for commercial use of their LC-PUFA. The existence of this year-round krill fishery provides a unique opportunity to collect krill samples for research over large spatial and temporal scales, which is unfeasible using scientific research vessels.
In this thesis, fishery caught krill samples were used to investigate the fatty acid content and composition of krill, during all seasons and over consecutive years (2013 – 2016). This research (presented in Chapter 2) aimed to fill knowledge gaps on the seasonal diet of krill (particularly in winter) in the Scotia Sea region, using fatty acids as dietary biomarkers. Krill were primarily herbivorous in summer (higher levels of 20:5n-3 and 22:6n-3, and low 18:1n-9c/18:1n-7c ratios) and became more omnivorous from autumn to spring (increasing ratios of 18:1n-9c/18:1n-7c and percentages of Σ 20:1 + 22:1 isomers). Seasonal proportions of herbivory and omnivory differed between years, and fatty acid composition differed between fishing locations. Selected samples were also used to investigate the composition of fatty acids in the structural (phospholipids) and storage lipids (triacylglycerols) of krill (Chapter 3). Triacylglycerol fatty acids (thought to better represent recent diet), reflected omnivorous feeding with highest percentages of flagellate biomarkers (18:4n-3) occurring in summer, diatom biomarkers (16:1n-7c) from autumn-spring, and greater carnivory (higher Σ 20:1 + 22:1 and 18:1n-9c/18:1n-7c ratios) in autumn. Phospholipid fatty acids were less variable and were higher in the essential membrane fatty acids 20:5n-3 and 22:6n-3. Percentages of the major krill sterol, cholesterol, were significantly higher in winter and spring compared with summer and autumn. Results presented in Chapters 2 and 3 highlighted the dynamic nature of krill lipids, and the flexible diet of krill, which likely contributes to their huge biomass and success as one of the most abundant organisms on Earth.
Because krill are so important in the Southern Ocean food web, any decreases in krill biomass could result in a major ecological regime shift. Very little is known about how climate change will affect krill. Increasing anthropogenic carbon dioxide (CO2) emissions are causing ocean acidification, as absorption of atmospheric CO2 in seawater alters ocean chemistry. Ocean acidification increases mortality and negatively affects physiological functioning in some marine invertebrates, and is predicted to occur most rapidly at high latitudes. Long-term laboratory studies are needed to understand how keystone species such as krill may respond to predicted future pCO2 levels. A long term experiment was conducted to test whether rising ocean pCO2 is likely to impact krill physiology and biochemistry (Chapters 4 and 5). Adult krill were exposed to near-future ocean acidification (1000 – 2000 μatm pCO2) for one year in the laboratory. Krill reared in near-future pCO2 conditions were able to survive, grow, store fat, mature, and maintain normal respiration rates. Haemolymph pH, lipid and fatty acid composition were also maintained at the same levels as krill in ambient pCO2 (400 μatm). Negative effects on physiology and lipid biochemistry were only observed in extreme pCO2 conditions (4000 μatm), which krill will not experience in the wild. These results place adult krill among the most resilient species in ocean acidification studies to date.
In summary, results in this thesis highlight the remarkable adaptability of krill in a changing environment, from short-term seasonal or annual scales, to longer-term decadal scales. Their flexible phenotype may aid their survival in an ocean that is rapidly changing with increasing anthropogenic CO2 emissions. The data obtained in this thesis can be used for fisheries management to guide fishing activities, and in fisheries models to predict how krill biomass may be affected by climate change. Krill lipid energy fuels the Southern Ocean ecosystem and to date, lipid data has not been included in Antarctic ecosystem models. The large scale of lipid data in this study makes it ideal for inclusion in such models, and it has important implications for the health of the wider Southern Ocean ecosystem.
Elevated temperature and decreased salinity both affect the biochemical composition of the Antarctic sea-ice diatom Nitzschia lecointei, but not increased pCO2
Published 7 October 2019 Science ClosedTags: abundance, Antarctic, biological response, growth, laboratory, otherprocess, photosynthesis, physiology, phytoplankton
Areas in western Antarctica are experiencing rapid climate change, where ocean warming results in more sea ice melt simultaneously as oceanic CO2 levels are increasing. In this study, we have tested how increased temperature (from −1.8 to 3 °C) and decreased salinity (from 35 to 20 and 10) synergistically affect the growth, photophysiology and biochemical composition of the Antarctic sea-ice diatom Nitzschia lecointei. In a separate experiment, we also addressed how ocean acidification (from 400 to 1000 µatm partial pressure of CO2) affects these key physiological parameters. Both positive and negative changes in specific growth rate, particulate organic carbon to particulate organic nitrogen ratio, chl a fluorescence kinetics, lipid peroxidation, carbohydrate content, protein content, fatty acid content and composition were observed when cells were exposed to warming and desalination. However, when cells were subjected to increased pCO2, only Fv/Fm, non-photochemical quenching and lipid peroxidation increased (by 3, 16 and 14%, respectively), and no other of the abovementioned biochemical properties were affected. These results suggest that changes in temperature and salinity may have more effects on the biochemical composition of N. lecointei than ocean acidification. Sea-ice algae are important component of polar food webs, and their nutritional quality may be affected as a result of altered environmental conditions due to climate change and sea ice melt.
Hidden biogeochemical anonymities under Antarctic fast ice
Published 4 October 2019 Science ClosedTags: abundance, Antarctic, biological response, chemistry, field, otherprocess, phytoplankton
Climate change is negatively affecting the extent of summer sea ice and the global oceanic oxygen concentrations, it is therefore imperative to decipher the life processes under the Antarctic fast ice. The biogeochemical parameters like dissolved oxygen, inorganic carbon, macronutrients, phytoplankton, and chlorophyll a (Chl a) were studied under the fast ice (by drilling 1.8 m thick ice) around Larsemann Hills, East Antarctica during India’s Scientific Expedition to Antarctica (31-ISEA-2011/12 and 33-ISEA-2013/14). The waters under ice cover were characterized by hyperoxia (up to 10.6 ml/L). Macronutrient concentrations under sea ice were depleted (<0.1 M NO3, PO4 and <2 M SiO4); this could be ascribed to the nutrient demand from under ice algae. The water under the ice cover exhibited higher algal biomass (Chl a up to 6.1 mg/m3) and CO2 under saturation (pCO2 <10 atm). This is reflected through the Spearman rank correlation indicating a negative correlation between Chl a and pCO2 (). The strong negative correlation between dissolved oxygen and pCO2 () suggests that photosynthesis regulates the concentrations of both these climatically important gases. The waters under the sea ice cover had brownish mucilaginous aggregates comprised of tube-forming diatoms Berkeleya adelienses and Nitzschia lecointei. These unusual biogeochemical changes were seen only at the ice water interface, and not at deeper depths. This study suggests that with global warming and sea ice melting, Antarctica might witness phytoplankton community shifts.
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Life in the freezer : the role of dimethylsulfoniopropionate (DMSP) in the physiological and biochemical adaptations of Antarctic microalgae
Published 30 September 2019 Science ClosedTags: Antarctic, biogeochemistry, biological response, BRcommunity, community composition, laboratory, otherprocess, photosynthesis, phytoplankton, prokaryotes
Marine microalgae are the fuel of the Antarctic ecosystem and changes in primary production can impact the entire food web, as well as the nutritional value at the base of the food web which is dependant not only on biomass but also the macromolecular content of the individual species. Primary production by Antarctic microalgae is also of key importance in the biogeochemical cycling of carbon and sulfur. Antarctica has a unique and dynamic environment where microalgae are evolutionarily adapted to live in freezing temperatures under extreme and oscillating environmental gradients exposing them to solar, osmotic, oxidative and nutrient stress. This thesis investigated the physiological and biochemical adaptations of Antarctic microalgae, focusing on the role dimethylsulfoniopropionate (DMSP) plays in surviving in the harsh Antarctic environment. This thesis provides new knowledge into who are the DMSP producers in Antarctica, the spatial dynamics and role of DMSP in natural Antarctic microbial communities. In a screening study, 16 species of Antarctic microalgae were characterised by their growth rates, physiological health, carbon content, DMSP production and DMSP lyase activity. We found that DMSP production and rates of lyase activity were species-specific, varying within taxa, and that diatom species can produce significant levels of DMSP, in the same magnitude as known DMSP producing haptophytes, 𝘗𝘩𝘢𝘦𝘰𝘤𝘺𝘴𝘵𝘪𝘴 𝘴𝘱𝘱.. In a descriptive study, we take a geographical look at the DMSP content and lyase activity, macromolecular profiles and productivity of three different Antarctic microalgal communities from three unique Antarctic environments; the open ocean to the sea ice and a hypersaline lake. We reveal that species diversity is reduced with more challenging environmental conditions and the species with the greatest phenotypic plasticity dominate in harsher settings. This thesis found that macromolecular content of microalgae changes based on environment, whereby sea-ice microalgae were higher in caloric value due to heavy investment in lipids compared to pelagic species. Using manipulative laboratory studies, we delivered new insight into the response of DMSP to environmental stress and future climate change scenarios as well as macromolecular responses at the species and community levels. Exposure to hypersaline conditions did not induce increased DMSP production, potentially due to the salinity shift being too rapid. In addition, there was no significant change in DMSP or macromolecular concentrations in response to ocean acidification at the species level, however there was a difference at the community level due to a shift in community composition.
