Sea Ice Production in the 2016 and 2017 Maud Rise Polynyas

Sea ice production within polynyas, an outcome of the atmosphere-ice-ocean interaction, is a major source of dense water and hence key to the global overturning circulation, but is poorly quantified over open-ocean polynyas. Using the two recent extensive open-ocean polynyas within the wider Maud Rise region of the Weddell Sea in 2016 and 2017, we here explore the sea ice energy budget and estimate their sea ice production based on satellite retrievals, in-situ hydrographic observations and the Japanese 55-year Reanalysis. We find that the oceanic heat flux amounts to 36.1 and 30.7 W m−2 within the 2016 and 2017 polynyas, respectively. Especially the 2017 open-ocean polynya produced nearly 200 km3 of new sea ice, which is comparable to the production in the largest Antarctic coastal polynyas. Finally, we determine that ice production is highly correlated with and sensitive to skin temperature and wind speed, which affect the turbulent fluxes. It is also strongly sensitive to uncertainties in the sea ice concentration and 1,000 hPa temperature, which all urgently need to be better monitored at high latitudes. Lastly, more process-oriented campaigns are required to further elucidate the role of open-ocean polynya on the local and global ocean circulations

Modeling Iron and Light Controls on the Summer \u3ci\u3ePhaeocystis antarctica\u3c/i\u3e Bloom in the Amundsen Sea Polynya

Of all the Antarctic coastal polynyas, the Amundsen Sea Polynya is the most productive per unit area. Observations from the 2010-2011 Amundsen Sea Polynya International Research Expedition (ASPIRE) revealed that both light and iron can limit the growth of phytoplankton (Phaeocystis antarctica), but how these controls manifest over the bloom season is poorly understood, especially with respect to their climate sensitivity. Using a 1-D biogeochemical model, we examine the influence of light and iron limitation on the phytoplankton bloom and vertical carbon flux at 12 stations representing different bloom stages within the polynya. Model parameters are determined by Bayesian optimization and assimilation of ASPIRE observations. The model-data fit is most sensitive to phytoplankton physiological parameters, which among all model parameters are best constrained by the optimization. We find that the 1-D model captures the basic elements of the bloom observed during ASPIRE, despite some discrepancies between modeled and observed dissolved iron distributions. With this model, we explore the way iron availability, in combination with light availability, controlled the rise, peak, and decline of the bloom at the 12 stations. Modeled light limitation by self-shading is very strong, but iron is drawn down as the bloom rises, becoming limiting in combination with light as the bloom declines. These model results mechanistically confirm the importance of climate-sensitive controls like stratification and meltwater on phytoplankton bloom development and carbon export in this region

Pelagic microbial heterotrophy in response to a highly productive bloom of <i>Phaeocystis antarctica</i> in the Amundsen Sea Polynya, Antarctica

Abstract Heterotrophic bacteria play a key role in marine carbon cycling, and understanding their activities in polar systems is important for considering climate change impacts there. One goal of the ASPIRE project was to examine the relationship between the phytoplankton bloom and bacterial heterotrophy in the Amundsen Sea Polynya (ASP). Bacterial abundance, production (BP), respiration, growth efficiency, and extracellular enzyme activity (EEA) were compared to nutrient and organic matter inventories, chlorophyll a (Chl a), viral and microzooplankton abundance, and net primary production (NPP). Bacterial production and respiration clearly responded (0.04–4.0 and 10–53 µg C L−1 d−1, respectively) to the buildup of a massive Phaeocystis antarctica bloom (Chl a: 0.2–22 µg L−1), with highest rates observed in the central polynya where Chl a and particulate organic carbon (POC) were greatest. The highest BP rates exceeded those reported for the Ross Sea or any other Antarctic coastal system, yet the BP:NPP ratio (2.1–9.4%) was relatively low. Bacterial respiration was also high, and growth efficiency (2–27%; median = 10%) was similar to oligotrophic systems. Thus, the integrated bacterial carbon demand (0.8–2.8 g C m−2 d−1) was a high fraction (25–128%; median = 43%) of NPP during bloom development. During peak bloom, activity was particle-associated: BP and EEA correlated well with POC, and size fractionation experiments showed that the larger size fraction (> 3 µm) accounted for a majority (∼ 75%) of the BP. The community was psychrophilic, with a 5x reduction in BP when warmed to 20°C. In deeper waters, respiration remained relatively high, likely fueled by the significant downward particle flux in the region. A highly active, particle-associated, heterotrophic microbial community clearly responded to the extraordinary phytoplankton bloom in the ASP, likely limiting biological pump efficiency during the early season

Self‐shading and meltwater spreading control the transition from light to iron limitation in an Antarctic coastal polynya

Dotson Ice Shelf (DIS) in West Antarctica is undergoing rapid basal melting driven by intrusions of warm, saline Circumpolar Deep Water (CDW) onto the continental shelf. Meltwater from DIS is thought to influence biology in the adjacent Amundsen Sea Polynya (ASP), which exhibits the highest Net Primary Productivity (NPP) per unit area of any coastal polynya in the Southern Ocean. However, the relative importance of iron and light in colimiting the spring phytoplankton bloom in the ASP remains poorly understood. In this modelling study we first investigate the mechanisms by which ice shelves impact NPP, then map spatio‐temporal patterns in iron‐light colimitation, and finally examine the environmental drivers of iron and light supply. We find that ice shelf melting leads to greater upper ocean iron concentrations, both directly due to release of iron from sediments entrained at the glacier bed, and indirectly via a buoyancy‐driven overturning circulation which pulls iron from CDW to the surface. Both of these mechanisms increase NPP compared to experiments where ice shelf melt is suppressed. We then show that the phytoplankton self‐shading feedback delays the bloom and reduces peak NPP by 80% compared to experiments where light penetration is independent of chlorophyll. Compared to light limitation, iron limitation due to phytoplankton uptake is more important a) later in the season, b) higher in the water column and c) further from the ice shelf. Finally, sensitivity experiments show that variability in CDW intrusion influences NPP by controlling the horizontal spreading of iron‐rich meltwater

A carbon budget for the Amundsen Sea Polynya, Antarctica: Estimating net community production and export in a highly productive polar ecosystem

Polynyas, or recurring areas of seasonally open water surrounded by sea ice, are foci for energy and material transfer between the atmosphere and the polar ocean. They are also climate sensitive, with both sea ice extent and glacial melt influencing their productivity. The Amundsen Sea Polynya (ASP) is the greenest polynya in the Southern Ocean, with summertime chlorophyll a concentrations exceeding 20 μg L−1. During the Amundsen Sea Polynya International Research Expedition (ASPIRE) in austral summer 2010–11, we aimed to determine the fate of this high algal productivity. We collected water column profiles for total dissolved inorganic carbon (DIC) and nutrients, particulate and dissolved organic matter, chlorophyll a, mesozooplankton, and microbial biomass to make a carbon budget for this ecosystem. We also measured primary and secondary production, community respiration rates, vertical particle flux and fecal pellet production and grazing. With observations arranged along a gradient of increasing integrated dissolved inorganic nitrogen drawdown (ΔDIN; 0.027–0.74 mol N m−2), changes in DIC in the upper water column (ranging from 0.2 to 4.7 mol C m−2) and gas exchange (0–1.7 mol C m−2) were combined to estimate early season net community production (sNCP; 0.2–5.9 mol C m−2) and then compared to organic matter inventories to estimate export. From a phytoplankton bloom dominated by Phaeocystis antarctica, a high fraction (up to ∼60%) of sNCP was exported to sub-euphotic depths. Microbial respiration remineralized much of this export in the mid waters. Comparisons to short-term (2–3 days) drifting traps and a year-long moored sediment trap capturing the downward flux confirmed that a relatively high fraction (3–6%) of the export from ∼100 m made it through the mid waters to depth. We discuss the climate-sensitive nature of these carbon fluxes, in light of the changing sea ice cover and melting ice sheets in the region

Characterizing cobalamin cycling by Antarctic marine microbes across multiple scales

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Biological Oceanography at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 2020.Highly productive marine microbial communities in the coastal Southern Ocean sustain the broader Antarctic ecosystem and play a key role in Earth’s climate via the biological pump. Regional phytoplankton growth is primarily limited by iron and co-limited by cobalamin (vitamin B12), a trace cobalt-containing organometallic compound only synthesized by some bacteria and archaea. These micronutrients impact primary production and the microbial ecology of the two keystone phytoplankton types: diatoms and Phaeocystis antarctica. This thesis investigates microbe-driven cobalamin cycling in Antarctic seas across multiple spatiotemporal scales. I conducted laboratory culture experiments with complementary proteomics and transcriptomics to investigate the B12-ecophysiology of P. antarctica strain CCMP 1871 morphotypes under iron-B12 co-limitation. We observed colony formation under higher iron treatments, and a facultative use of B12-dependent (MetH) and B12-independent (MetE) methionine synthase isoforms in response to vitamin availability, demonstrating that this strain is not B12-auxotrophic. Through comparative ’omics, we identified a putative MetE protein in P. antarctica abundant under low B12, which is also found in other marine microbes. Across Antarctic seas, community-scale cobalt and B12 uptake rates were measured by 57Co radiotracer incubation experiments and integrated with hydrographic and phytoplankton pigment data. I observed significant correlations between uptake fluxes and environmental variables, providing evidence for predominantly diatom-driven uptake of these micronutrients in warmer, fresher surface waters with notable regional differences. To date, this work is the most comprehensive attempt to elucidate the processes governing the co-cycling of cobalt and B12 in any marine system. At the ecosystem-scale, I developed and tested a hypothesis of micronutrient-driven community dynamics through a trait-based model with cross-feeding interactions. The model demonstrates how the observed seasonal succession of springtime P. antarctica from solitary to colonial cells, bacterioplankton, and summertime diatoms may be explained by the microbial cycling of iron, dissolved organic carbon, and B12. Overall, this dissertation provides new information about the micronutrient-driven ecology of Antarctic marine microbes and adds to our understanding of the interconnections between organismal life cycle, trace metals, and trace organics in marine environments.My training as a scientist during my time in the MIT–WHOI Joint Program (2014-2020) and the work presented in this dissertation were financially supported by the Academic Programs Office (APO) at the Woods Hole Oceanographic Institution (WHOI) and various funding agencies. My first semester was supported by the WHOI Von Damm Fellowship (2014). Subsequent years and endeavors were supported by awards from the Gordon and Betty Moore Foundation to Professor Michael Follows (Award 3778, M.J.F.) and Simons Collaboration on Computational Biogeochemical Modeling of Marine Ecosystems (Award 549931, M.J.F.); National Science Foundation (NSF) grant to Dr. Stephanie Dutkiewicz (Grant number 1434007, S.D.), and NSF Office of Polar Program (OPP) grant to Dr. Makoto Saito (M.S.) for the CICLOPS research expedition (OPP-1643684, OPP-1643845, and OPP-1644073)

Analyses spatiotemporelles des conditions de glace de mer et des tendances de formation des polynies de l’Archipel Arctique Canadien

L’augmentation des températures des régions nordiques est 1.9 fois plus marquée que sur le reste du globe. Ceci est causé par une série de rétroactions positives contribuant au phénomène connu sous le nom d’amplification arctique. Cette amplification mène à une diminution de 13.5% de la superficie de glace de mer dans cette région par décennie. Plus spécifiquement, sur ce territoire glacé se trouvent des secteurs d’eau libre connus sous le nom de polynies. Ces polynies sont formées suite à l’action de processus dynamiques ou thermodynamiques et ont une grande importante écologique et environnementale. Avec la réduction soutenue de la superficie de glace de mer, les impacts sur les tendances de formation de polynies demeurent inconnus, plus particulièrement dans le secteur de l’Archipel Arctique canadien (AAC). Le but du projet est donc d’évaluer ces tendances de formation de polynies pour cette région. Pour ce faire, les conditions de glace pour toute la région furent évaluées en y calculant les anomalies de concentration de glace de mer pour une période critique de formation de polynies. Aucune tendance significative ne fut observée à ce point de vue. Ensuite, les périodes d’ouvertures de dix grandes polynies de l’AAC ont été évaluées. Une fin plus hâtive de certains évènements de polynies récurrentes de l’AAC à partir de l’an 2000 fut constatée suite à ces analyses. Enfin, le nombre d’ouvertures dans chaque grande région couvrant l’AAC par les cartes de glace de mer fut calculé. Une augmentation du nombre d’ouvertures pour les mois de juin et juillet fut observée depuis 2005. Cette même tendance est constatée pour la région ouest de l’AAC, mais à un plus faible degré. Ainsi, bien qu’aucune tendance ne soit observée à l’échelle de l’AAC, des changements plutôt significatifs furent observés à l’échelle des sous-régions de l’AAC et des polynies récurrentes s’y trouvant

​​Observing Antarctic Bottom Water in the Southern Ocean​

Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system