Modeling polar marine ecosystem functions guided by bacterial physiological and taxonomic traits

Heterotrophic marine bacteria utilize organic carbon for growth and biomass synthesis. Thus, their physiological variability is key to the balance between the production and consumption of organic matter and ultimately particle export in the ocean. Here we investigate a potential link between bacterial traits and ecosystem functions in the rapidly warming West Antarctic Peninsula (WAP) region based on a bacteria-oriented ecosystem model. Using a data assimilation scheme, we utilize the observations of bacterial groups with different physiological traits to constrain the group-specific bacterial ecosystem functions in the model. We then examine the association of the modeled bacterial and other key ecosystem functions with eight recurrent modes representative of different bacterial taxonomic traits. Both taxonomic and physiological traits reflect the variability in bacterial carbon demand, net primary production, and particle sinking flux. Numerical experiments under perturbed climate conditions demonstrate a potential shift from low nucleic acid bacteria to high nucleic acid bacteria-dominated communities in the coastal WAP. Our study suggests that bacterial diversity via different taxonomic and physiological traits can guide the modeling of the polar marine ecosystem functions under climate change.

Seasonal Dietary Shifts of the Gammarid Amphipod Gondogeneia antarctica in a Rapidly Warming Fjord of the West Antarctic Peninsula

The amphipod Gondogeneia antarctica is among the most abundant benthic organisms, and a key food web species along the rapidly warming West Antarctic Peninsula (WAP). However, little is known about its trophic strategy for dealing with the extreme seasonality of Antarctic marine primary production. This study, using trophic markers, for the first time investigated seasonal dietary shifts of G. antarctica in a WAP fjord. We analyzed δ13C and δ15N in G. antarctica and its potential food sources. The isotopic signatures revealed a substantial contribution of red algae to the amphipod diet and also indicated a significant contribution of benthic diatoms. The isotope results were further supported by fatty acid (FA) analysis, which showed high similarities in FA composition (64% spring–summer, 58% fall–winter) between G. antarctica and the red algal species. G. antarctica δ13C showed a small shift seasonally (−18.9 to −21.4‰), suggesting that the main diets do not change much year-round. However, the relatively high δ15N values as for primary consumers indicated additional dietary sources such as animal parts. Interestingly, G. antarctica and its potential food sources were significantly enriched with δ15N during the fall–winter season, presumably through a degradation process, suggesting that G. antarctica consumes a substantial portion of its diets in the form of detritus. Overall, the results revealed that G. antarctica relies primarily on food sources derived from benthic primary producers throughout much of the year. Thus, G. antarctica is unlikely very affected by seasonal Antarctic primary production, and this strategy seems to have allowed them to adapt to shallow Antarctic nearshore waters.

Spatiotemporal Variations in Antarctic Protistan Communities Highlight Phytoplankton Diversity and Seasonal Dominance by a Novel Cryptophyte Lineage

The climate-sensitive waters of the West Antarctic Peninsula (WAP), including its many fjords, are hot spots of productivity that support multiple marine mammal species. Here, we profiled protistan molecular diversity in a WAP fjord known for high productivity and found distinct spatiotemporal patterns across protistan groups.

Subglacial Conditions of the Kamb Ice Stream and its Response to Environmental Change

<p>The Siple Coast ice streams, which drain the West Antarctic Ice Sheet into the Ross Ice Shelf, are susceptible to temporal changes in flow dynamics. The Kamb Ice Stream on the Siple Coast, stagnated approximately 160 years ago, thought to partially be the result of basal water diversion. The character of its subglacial environment can exert an important control on long- and short-term ice sheet and ice stream fluctuations. Were the Kamb Ice Stream to reactivate in response to subglacial or future climate change, it would have the potential to contribute more substantially to ice discharge into the Ross Ice Shelf. Therefore, it is important to characterise the present-day subglacial environment and climatic conditions that may reactivate this flow. This study investigates the present-day subglacial conditions of the Kamb Ice Stream and how these conditions may be affected by environmental perturbations. Due to the difficult nature of making direct observations of ice sheet basal conditions, other methods are employed to investigate the response of the Kamb Ice Stream to environmental change. Active source seismic surveying data obtained during the 2015/16 and 2018/19 austral summer seasons provides an instantaneous snapshot of the present-day basal conditions. Flowline and whole-continent numerical ice sheet modelling is used to investigate the longer-term response of the Kamb Ice Stream and the West Antarctic Ice Sheet. Amplitude analysis of seismic lines indicate saturated till beneath the Ross Ice Shelf in the vicinity of the grounding zone, which is supported by retreat rates of the Kamb Ice Stream grounding zone post-stagnation. Seismic reflection imaging suggests potential dewatered till conditions beneath the grounded Kamb Ice Stream. Flowline modelling of the Kamb Ice Stream indicates that changes to the water content of the subglacial sediments appear to be self regulating, with high reversibility over centennial timescales. Oceanic temperature forcings are the key driver of change of the Kamb Ice Stream, and the ice stream is susceptible to topographic pinning points in 2D and lateral drag. Future glaciological change is more likely to occur in response to oceanic than to atmospheric temperature perturbations. Results from 3D continent-wide modelling experiments also find that precipitation increases offset the effect of air temperature perturbations and influence subglacial conditions, indicating more dynamic ice stream behaviour on the Siple Coast. This study has worked to re-enforce and strengthen our existing understanding of the Kamb Ice Stream and its sensitivity to environmental change. Future work using higher-resolution simulations and a higher density of observational data may help refine these results.</p>

Subglacial Conditions of the Kamb Ice Stream and its Response to Environmental Change

<p>The Siple Coast ice streams, which drain the West Antarctic Ice Sheet into the Ross Ice Shelf, are susceptible to temporal changes in flow dynamics. The Kamb Ice Stream on the Siple Coast, stagnated approximately 160 years ago, thought to partially be the result of basal water diversion. The character of its subglacial environment can exert an important control on long- and short-term ice sheet and ice stream fluctuations. Were the Kamb Ice Stream to reactivate in response to subglacial or future climate change, it would have the potential to contribute more substantially to ice discharge into the Ross Ice Shelf. Therefore, it is important to characterise the present-day subglacial environment and climatic conditions that may reactivate this flow. This study investigates the present-day subglacial conditions of the Kamb Ice Stream and how these conditions may be affected by environmental perturbations. Due to the difficult nature of making direct observations of ice sheet basal conditions, other methods are employed to investigate the response of the Kamb Ice Stream to environmental change. Active source seismic surveying data obtained during the 2015/16 and 2018/19 austral summer seasons provides an instantaneous snapshot of the present-day basal conditions. Flowline and whole-continent numerical ice sheet modelling is used to investigate the longer-term response of the Kamb Ice Stream and the West Antarctic Ice Sheet. Amplitude analysis of seismic lines indicate saturated till beneath the Ross Ice Shelf in the vicinity of the grounding zone, which is supported by retreat rates of the Kamb Ice Stream grounding zone post-stagnation. Seismic reflection imaging suggests potential dewatered till conditions beneath the grounded Kamb Ice Stream. Flowline modelling of the Kamb Ice Stream indicates that changes to the water content of the subglacial sediments appear to be self regulating, with high reversibility over centennial timescales. Oceanic temperature forcings are the key driver of change of the Kamb Ice Stream, and the ice stream is susceptible to topographic pinning points in 2D and lateral drag. Future glaciological change is more likely to occur in response to oceanic than to atmospheric temperature perturbations. Results from 3D continent-wide modelling experiments also find that precipitation increases offset the effect of air temperature perturbations and influence subglacial conditions, indicating more dynamic ice stream behaviour on the Siple Coast. This study has worked to re-enforce and strengthen our existing understanding of the Kamb Ice Stream and its sensitivity to environmental change. Future work using higher-resolution simulations and a higher density of observational data may help refine these results.</p>

The Southern Ocean Exchange: porous boundaries between humpback whale breeding populations in southern polar waters

Humpback whales (Megaptera novaeangliae) are a cosmopolitan species and perform long annual migrations between low-latitude breeding areas and high-latitude feeding areas. Their breeding populations appear to be spatially and genetically segregated due to long-term, maternally inherited fidelity to natal breeding areas. In the Southern Hemisphere, some humpback whale breeding populations mix in Southern Ocean waters in summer, but very little movement between Pacific and Atlantic waters has been identified to date, suggesting these waters constituted an oceanic boundary between genetically distinct populations. Here, we present new evidence of summer co-occurrence in the West Antarctic Peninsula feeding area of two recovering humpback whale breeding populations from the Atlantic (Brazil) and Pacific (Central and South America). As humpback whale populations recover, observations like this point to the need to revise our perceptions of boundaries between stocks, particularly on high latitude feeding grounds. We suggest that this “Southern Ocean Exchange” may become more frequent as populations recover from commercial whaling and climate change modifies environmental dynamics and humpback whale prey availability.

Marine Palynomorphs from the Plio-Pleistocene interval of the AND-1B Drill-Core McMurdo Sound, Antarctica

<p>The ANDRILL project recovered over 600 m of Plio-Pleistocene sediments within the Ross embayment, Antarctica. These sediments contain a record of local and regional paleoenvironmental conditions and glacial dynamism. They also provide a proxy for ice dynamics of the West Antarctic Ice Sheet (WAIS) during a period when global temperatures were ~3OC higher than modern. This unique record provides an analogue for future global climate change, which is expected to rise by 3OC by the end of the 21st century. Sixty-one samples from the upper 600 m of the AND-1B core were analysed for their palynomorph content yielding 4 to 5380 grains per sample (with an average frequency of 34 grains per gram). Marine palynomorphs including fossil dinoflagellate cysts, acritarchs, and prasinophyte algae were the focus of this study and fluctuations in their abundance and diversity reflect changes in paleoenvironment and glacial dynamics. The upper 600 m can be divided into 6 discrete units based on the palynomorph assemblage: The early-Pliocene (~5.0 – 4.6 Ma. Unit 1) is characterised by relatively high abundances of in situ round brown dinoflagellate cysts, microforaminiferal linings, and Leiosphaeridia, suggesting warmer than modern paleoenvironmental conditions and seasonal ice within the Ross embayment. The WAIS was likely small and highly dynamic during Unit 1. The mid-Pliocene (~4.6 – 3.4 Ma. Unit 2) exhibits relatively high abundances of round brown dinoflagellate cysts, microforaminiferal linings, and scolecodonts. The relatively low abundance of Leiosphaeridia (understood to indicate proximal/seasonal ice) suggests that ice free conditions at the drill site may have existed for up to ~1.2 Ma and that this may be the warmest period recorded in the core. During the warm, mid-Pliocene interval a sudden increase in scolecodonts (fossilized polychaete remains) may give indications into the water depth at the drill site because of their dependence on physical disturbance (decreasing with depth) for population growth. Further study of the scolecodonts is required before confident estimates of water depth can be made. The mid- to late-Pliocene (~3.4 – 2.6 Ma. Units 3, 4 & 5) is characterised by a variable palynomorph assemblage indicating variability in paleoenvironmental conditions, ice cover at the drill site, and ultimately a variable WAIS. A spike in the prasinophyte alga Cymatiosphaera (understood to indicate reduced salinity) at the base of a diatomite unit in the late-Pliocene may be a record of algae thriving in meltwater from the collapse of the WAIS. Further highresolution analysis is needed to help resolve this event. The Quaternary interval (~2.6 Ma and younger. Unit 6) is significantly different from previous units and is dominated by reworked Eocene dinoflagellate cysts and acritarchs (the “Transantarctic Flora”). This interval records a period of significant cooling and glacial expansion and the WAIS likely grew to its modern “polar” state. The WAIS may have undergone several collapses during super-interglacial periods in the Pleistocene but if it did it did not persist in its collapsed state for significant periods of time.</p>