Southwestern Ross Sea Research Articles

Quantifying Seasonal Particulate Organic Carbon Concentrations and Export Potential in the Southwestern Ross Sea Using Autonomous Gliders

AbstractTo assess the temporal biological and hydrographic features of the southwestern Ross Sea, we deployed a glider in a spatially restricted, ice‐free area during the austral summer (1 December–6 February), and quantified from sensor measurements the particulate organic carbon (POC; via particulate backscatter) concentrations, their changes through time, and net community production (NCP; via dissolved O2 concentrations). The POC levels could be divided into three distinct phases (I, II, and III, respectively) characterized by changes in NCP, surface‐layer POC concentrations, remineralization, and export. Surface POC concentrations increased from 215 mg C m−3 in early December to a peak of >400 mg C m−3 by mid‐December, before decreasing to 227 mg C m−3 in late January–early February. NCP was highly variable throughout the summer, becoming maximal in mid‐December. By constructing a carbon budget, we estimated rates of change of POC and export potential to the mesopelagic in each phase. Changes in euphotic zone POC concentrations and NCP suggested that the system is slightly net autotrophic during the observational period (average NCP is 0.05 g C m−2 d−1), and POC removal from the top 240 m of the water column averaged 0.22 g C m−2 d−1. Our data confirm that the southern Ross Sea during the ice‐free season is a high productivity, low export system while providing high‐resolution POC dynamics that had not been previously observed. Although the Ross Sea is a site of substantial carbon fixation, there remains an incomplete understanding both of the processes involved in export and the rates and controls of remineralization.

Ocean‐Driven and Topography‐Controlled Nonlinear Glacier Retreat During the Holocene: Southwestern Ross Sea, Antarctica

AbstractRecent ice sheet mass loss in Antarctica has been attributed to an influx of warm ocean waters, which drove grounding‐line retreat and ice thinning. Episodic retreat and rapid thinning also occurred in the southwestern Ross Sea during the Holocene, which today accommodates cold ocean waters. We applied finite element ice‐flow modeling to investigate the roles of ocean temperature and bed topography in the deglaciation of this region. First, our experiments demonstrate that bed topography controlled the spatial pattern of grounding‐line retreat. Topographic pinning points limited the rate of ice loss until retreat progressed beyond a bathymetric threshold. Second, ocean thermal forcing determined the timing of this ice loss. Enhanced ocean‐driven melt is required during the Early‐to‐Mid Holocene to replicate geological records of deglaciation, possibly indicating that warm ocean waters were once present in this region. On multi‐centennial timescales, ocean temperature drove, while bed topography controlled, nonlinear rates of ice mass loss.

A clustering-based approach to ocean model–data comparison around Antarctica

Abstract. The Antarctic Continental Shelf seas (ACSS) are a critical, rapidly changing element of the Earth system. Analyses of global-scale general circulation model (GCM) simulations, including those available through the Coupled Model Intercomparison Project, Phase 6 (CMIP6), can help reveal the origins of observed changes and predict the future evolution of the ACSS. However, an evaluation of ACSS hydrography in GCMs is vital: previous CMIP ensembles exhibit substantial mean-state biases (reflecting, for example, misplaced water masses) with a wide inter-model spread. Because the ACSS are also a sparely sampled region, grid-point-based model assessments are of limited value. Our goal is to demonstrate the utility of clustering tools for identifying hydrographic regimes that are common to different source fields (model or data), while allowing for biases in other metrics (e.g., water mass core properties) and shifts in region boundaries. We apply K-means clustering to hydrographic metrics based on the stratification from one GCM (Community Earth System Model version 2; CESM2) and one observation-based product (World Ocean Atlas 2018; WOA), focusing on the Amundsen, Bellingshausen and Ross seas. When applied to WOA temperature and salinity profiles, clustering identifies “primary” and “mixed” regimes that have physically interpretable bases. For example, meltwater-freshened coastal currents in the Amundsen Sea and a region of high-salinity shelf water formation in the southwestern Ross Sea emerge naturally from the algorithm. Both regions also exhibit clearly differentiated inner- and outer-shelf regimes. The same analysis applied to CESM2 demonstrates that, although mean-state model biases in water mass T–S characteristics can be substantial, using a clustering approach highlights that the relative differences between regimes and the locations where each regime dominates are well represented in the model. CESM2 is generally fresher and warmer than WOA and has a limited fresh-water-enriched coastal regimes. Given the sparsity of observations of the ACSS, this technique is a promising tool for the evaluation of a larger model ensemble (e.g., CMIP6) on a circum-Antarctic basis.

The buoyancy-based biotope axis of the evolutionary radiation of Antarctic cryonotothenioid fishes

In the absence of any prior comprehensive analysis, I evaluate divergence along the biotope axis in the habitat stage of the evolutionary radiation of Antarctic cryonotothenioids. I utilize the available percentage buoyancy (%B) measurements as habitat proxies for recognition of the pelagic, semipelagic, demersal, and benthic biotopes that include, respectively, 5%, 10%, 73%, and 12% of the 59 species and 1749 specimens in the study. The majority of species retain the ancestral demersal biotope of Eleginops maclovinus, and this probably enhances ecological plasticity. Divergence into the pelagic biotope is the most distinctive organismal feature of the radiation and, although only 5% of species are pelagic, this biotope is not depauperate in global comparisons. Pelagic or potentially pelagic species are Dissostichus mawsoni, D. eleginoides, Pleuragramma antarctica, Aethotaxis mitopteryx, and Gvozdarus svetovidovi. Small ontogenetic changes in %B with growth are typical; however, this is extensive in D. mawsoni, a species with the potential to transition through benthic to pelagic biotopes over ontogeny. Occupation of the pelagic biotope by large D. mawsoni may be impermanent as it is lipid-dependent, a contingency reliant on the availability of P. antarctica as prey. In unusual conditions, the specialized sacs of P. antarctica can also yield their lipid for metabolism with possible loss of buoyancy. Pelagic species are inordinately important in the food web. In the southwestern Ross Sea a guild of large mammalian and avian predators, which includes D. mawsoni, is reliant on lipid-rich, energy-dense cryonotothenioid prey. This includes asymmetrical intraguild predation on D. mawsoni, with P. antarctica as a basal resource for the guild.

Interannual variability of vertical particle fluxes in the Ross Sea (Antarctica)

This study presents new data on biogenic and terrigenous particle fluxes collected by an oceanographic mooring (Mooring A) deployed in the south-western Ross Sea (Antarctica) in the frame of the Italian Long-Term Ecological Research network (LTER-Italy). Results from the years 2005 and 2008 document high mass fluxes during the summer and early autumn seasons, not coincident with the algal bloom. Downward particle fluxes exhibit a high inter-annual variability of both particulate fluxes and composition that seem related to the different factors as the phytoplankton increases, occurring between the beginning of February and the end of March, to the variations in the sea ice extent and to the resuspension and/or lateral advection processes. The flux variability may have been influenced by Iceberg B-15 that resided in the investigated area between 2000 and 2005. The decoupling of biogenic silica and organic carbon cycles is documented by differences in the rates of their respective key processes: biogenic silica dissolution and organic carbon degradation.

A method for correcting seal-borne oceanographic data and application to the estimation of regional sea ice thickness

Abstract The high-latitude oceans surrounding Antarctica are substantially undersampled compared to lower latitudes. Mammal based instruments such as Conductivity-Temperature-Depth Satellite Relay Data Loggers (CTD-SRDLs) present one possible solution. Unfortunately, these are subject to instrument-dependent offsets in absolute salinity. This study investigates a set of satellite-transmitted data collected by CTD-SRDLs mounted on Weddell seals (Leptonychotes weddellii) in the South-western Ross Sea in 2011. The uncorrected salinity offset between devices was found to be up to 1.4 g kg−1, making the data unsuitable for some oceanographic studies without correction. Here, a correction method was developed that uses profiles from pairs of CTD-SRDLs that are considered to be co-located and to sample the same body of water if they occur within defined time and space windows. Using least squares, a best-fit solution to the matrix of offsets in co-located pairs was found that reduces salinity offsets between the CTD-SRDLs. These offsets are smaller than the original offsets by a factor of 10. A calibrated reference instrument, that was co-located with some of the devices, provided further improvement in the absolute accuracy of all the CTD-SRDLs. Using the corrected CTD-SRDL data we estimate the rejection of salt into the water column by sea ice formation, and derived the time evolution of sea ice thickness in the South-western Ross Sea. Our estimates of regional sea ice thickness are in agreement with direct sea ice thickness measurements taken over a limited area in November 2011, providing further affirmation of our method.

Formation of a spatter-rich pyroclastic density current deposit in a Neogene sequence of trachytic-mafic igneous rocks at Mason Spur, Erebus volcanic province, Antarctica

Erosion has revealed a remarkable section through the heart of a volcanic island, Mason Spur, in the southwestern Ross Sea, Antarctica, including an unusually well-exposed section of caldera fill. The near-continuous exposure, 10 km laterally and > 1 km vertically, cuts through Cenozoic alkalic volcanic rocks of the Erebus volcanic province (McMurdo Volcanic Group) and permits the study of an ancient volcanic succession that is rarely available due to subsequent burial or erosion. The caldera filling sequence includes an unusual trachytic spatter-rich lapilli tuff (ignimbrite) facies that is particularly striking because of the presence of abundant black fluidal, dense juvenile spatter clasts of trachytic obsidian up to 2 m long supported in a pale cream-coloured pumiceous lapilli tuff matrix. Field mapping indicates that the deposit is an ignimbrite and, together with petrological considerations, it is suggested that mixing of dense spatter and pumiceous lapilli tuff in the investigated deposit occurred during emplacement, not necessarily in the same vent, with the mixed fragmental material emplaced as a pyroclastic density current. Liquid water was not initially present but a steam phase was probably generated during transport and may represent water ingested during passage of the current as it passed over either wet ground, stream, shallow lake or (possibly) snow. Well-exposed caldera interiors are uncommon and that at Mason Spur is helping understand eruption dynamics associated with a complex large island volcano. The results of our study should help to elucidate interpretations of other, less well exposed, pyroclastic density current deposits elsewhere in Antarctica and globally.

Widespread persistence of expanded East Antarctic glaciers in the southwest Ross Sea during the last deglaciation

It has been suggested that the grounding line of the Last Glacial Maximum (LGM) ice sheet in the Ross Sea, Antarctica, receded in an approximately north-to-south pattern during the Holocene. An implication of this hypothesis is that geological evidence from the southwestern Ross Sea has been used widely to interpret retreat histories of the West Antarctic Ice Sheet (WAIS) across the wider Ross Sea embayment. Accurately constraining the timing and pattern of marine-based ice sheet retreat in this embayment is critical to understanding the drivers that may have triggered this event, and its contribution to rapid sea-level rise events. Here, we present new multibeam swath bathymetry data that identifies well-preserved glacial features indicating that thick (>700 m) marine-based ice derived from the East Antarctic Ice Sheet coastal outlet glaciers dominated the ice sheet input into the southwestern Ross Sea during the last phases of glaciation. Subglacial geomorphic features indicate that ice derived from present outlet glacier valleys in South Victoria Land flowed southeastward. This is more consistent with flowlines from model-based interpretations of an earlier retreat of the WAIS in the central Ross Sea than with previous land-based geological reconstructions. This implies that coastal records of deglaciation along the Transantarctic Mountains front record only the final phases of glacial retreat in the Ross Sea. Therefore, chronological data from the central embayment are required to accurately constrain the timing of large-scale glacial retreat in the Ross Sea and to identify the mechanisms that drove it.

Spatio-temporal occurrence patterns of cetaceans near Ross Island, Antarctica, 2002–2015: implications for food web dynamics

The Ross Sea pelagic food web is closely coupled, with the foraging among abundant upper level species affecting the foraging of one another. To investigate the roles cetaceans may have in such interspecific interactions in this system, we studied within-season and interannual occurrence patterns of Antarctic minke whales (Balaenoptera bonaerensis) and type-B and type-C killer whales (Orcinus orca) within the southwestern Ross Sea, 2002–2015. Time series analysis summarized daily observations made from 3 shore localities: Capes Crozier, Bird, and Royds distributed around the ~120 km periphery of Ross Island. In early mid-November, both species arrived at Crozier, the easternmost point and westward edge of the Ross Sea Polynya marginal ice zone. Subsequently, coinciding with decreased sea ice cover and numbers off Crozier, both species appeared off Bird, then Royds, 80 and 117 km to the west/southwest, respectively. Arrival in either area coincided with SIC decreasing to <80%, consistent with observations elsewhere. Within-season off Crozier, both species occurred in cycles of multi-day presence followed by absence, perhaps reflecting the spatio-temporal patchiness of prey indicated also by penguin foraging patterns. Within-season off Royds, especially for killer whales, occurrence was even more episodic; type-Bs arrived before type-Cs, and results support previously described resident and transient portions of the type-C population. Combined with results showing that whale arrival leads to food stress among penguins, we suggest that relatively few cetaceans can alter food availability to alter the foraging behavior of other mesopredators, despite primary productivity being the richest in the Southern Ocean.

The influence of short-term events on the hydrographic and biological structure of the southwestern Ross Sea

The Ross Sea continental shelf supports very high productivity and phytoplankton biomass. Conventional methods, including ship-based sampling, instrumented moorings, satellite imagery, and modelling, have illustrated the typical patterns of seasonal progression of the phytoplankton blooms. While variability in the Ross Sea is substantial over relatively large scales, our understanding of smaller scales of variability (on the order of a few hours or several kilometers) is limited. Utilizing data from an autonomous glider, we examined the mechanisms driving both the transitions between stages of the phytoplankton bloom and the short-term perturbations in chlorophyll concentrations. Three phases within the bloom were defined based on chlorophyll changes: an accumulation phase, a dissipation period, and a post-dissipation phase. Short-term perturbations in chlorophyll were repeatedly observed and correlated with wind speed. These chlorophyll perturbations were strongly influenced by the degree of temporal coupling between wind events and the depth of mixing, which varied among phases. Delays of 12–24h between wind events and chlorophyll changes were observed during the accumulation phase, but shortened to 2–12h in the dissipation phase. Furthermore, while physical factors contributed to the observed short-term reductions in biomass and the appearance of chlorophyll at depth throughout the study, we hypothesize that turbulence during the period of maximum biomass induced aggregate formation and led to rapid vertical flux. These results suggest that the small-scale, short-term physical perturbations may induce substantial vertical redistribution of biogenic material, which in turn can have significant biogeochemical impacts.

Increases in Adélie penguins in the Ross Sea: Could the fishery for Antarctic toothfish be responsible?

The Ross Sea is home to about a third of the world population of Adélie penguins (Pygoscelis adeliae). Between 2001 and 2013 the number of breeding pairs of Adélie penguins at colonies in the southwestern Ross Sea more than doubled. It has been suggested that this increase was caused by the fishery for Antarctic toothfish (Dissostichus mawsoni) leading to mesopredator release of Antarctic silverfish (Pleuragramma antarctica), a shared prey of toothfish and Adélie penguins. Cases of mesopredator release in marine systems have been widely reported, but limited information on marine predators often hinders quantitative investigation of these changes. The present study has brought together information from multiple models to estimate the biomass of silverfish that could be released from predation through the effects of the toothfish fishery. New diet data for toothfish are presented which show that Antarctic toothfish and Adélie penguins do not have significantly overlapping diets in the southwestern Ross Sea. The mass of silverfish released from predation due to the effects of fishing was estimated to be 128tonnes wet-weight per year (tWW/y; 5th–95th estimation interval of 46–358tWW/y) in 2013, equivalent to less than 2% of the biomass of silverfish estimated to be consumed annually by Adélie penguins. Even if toothfish consumed only silverfish, the predicted predation release effect would still not be sufficient to explain the observed increase in the number of Adélie penguins in the southern Ross Sea. The results of the modelling are hence inconsistent with predation release of silverfish due to the toothfish fishery being responsible for recent increases in the number of Adélie penguins breeding in the southwestern Ross Sea.

The origin of lithogenic sediment in the south-western Ross Sea and implications for iron fertilization

AbstractSummer iron (Fe) fertilization in the Ross Sea has previously been observed in association with diatom productivity, lithogenic particles and excess Fe in the water column. This productivity event occurred during an early breakout of sea ice via katabatic winds, suggesting that aeolian dust could be an important source of lithogenic Fe required for diatom growth in the Ross Sea. Here we investigate the provenance of size-selected dust deposited on sea ice in McMurdo Sound, south-western (SW) Ross Sea. The isotopic signature of McMurdo Sound dust (0.70533&lt;87Sr/86Sr&lt;0.70915 and -1.1&lt;εNd(0)&lt;3.45) confirms that dust is locally sourced from the McMurdo Sound debris bands and comprises a two-component mixture of McMurdo Volcanic Group and southern Victoria Land lithologies. In addition, the provenance of lithogenic sediment trapped in the water column was investigated, and the isotopic signature (εNd(0)=3.9, 87Sr/86Sr=0.70434) is differentiated from long-range transported dust originating from South America and Australia. Elevated lithogenic accumulation rates in deeper sediment traps in the Ross Sea suggest that sinking particles in the water column cannot simply result from dust input at the surface. This discrepancy can be best explained by significant upwelling and remobilization of lithogenic Fe from the sea floor.

The sub-ice platelet layer and its influence on freeboard to thickness conversion of Antarctic sea ice

Abstract. This is an investigation to quantify the influence of the sub-ice platelet layer on satellite measurements of total freeboard and their conversion to thickness of Antarctic sea ice. The sub-ice platelet layer forms as a result of the seaward advection of supercooled ice shelf water from beneath ice shelves. This ice shelf water provides an oceanic heat sink promoting the formation of platelet crystals which accumulate at the sea ice–ocean interface. The build-up of this porous layer increases sea ice freeboard, and if not accounted for, leads to overestimates of sea ice thickness from surface elevation measurements. In order to quantify this buoyant effect, the solid fraction of the sub-ice platelet layer must be estimated. An extensive in situ data set measured in 2011 in McMurdo Sound in the southwestern Ross Sea is used to achieve this. We use drill-hole measurements and the hydrostatic equilibrium assumption to estimate a mean value for the solid fraction of this sub-ice platelet layer of 0.16. This is highly dependent upon the uncertainty in sea ice density. We test this value with independent Global Navigation Satellite System (GNSS) surface elevation data to estimate sea ice thickness. We find that sea ice thickness can be overestimated by up to 19%, with a mean deviation of 12% as a result of the influence of the sub-ice platelet layer. It is concluded that within 100 km of an ice shelf this influence might need to be considered when undertaking sea ice thickness investigations using remote sensing surface elevation measurements.

The contribution of aeolian sand and dust to iron fertilization of phytoplankton blooms in southwestern Ross Sea, Antarctica

AbstractIron is a limiting micronutrient for primary production in the Ross Sea, Antarctica. Recent observations reveal low dissolved Fe (dFe) concentrations in the Ross Sea polynya following high initial rates of primary production in summer, after the dFe winter reserve has been consumed. Significant new sources of dFe are therefore required to further sustain phytoplankton blooms. Iron from aeolian sand and dust (ASD) released from melting sea ice is one potential source. To constrain aeolian Fe inputs, we determined ASD mass accumulation rates and the total and soluble Fe content of ASD on sea ice in McMurdo Sound, southwestern (SW) Ross Sea. The mean mass accumulation rate was ~1.5 g m−2 yr−1, total Fe content of this ASD was 4 ± 1 wt %, and the percentage of soluble Fe was 11 ± 1%. Our mean estimate of the bulk aeolian dFe flux of 122.1 µmol m−2 yr−1 for the McMurdo Sound region suggests that aeolian Fe can support between 9.0 × 109 and 4.1 × 1011 mol C yr−1 (0.1–4.9 Tg C yr−1) of new primary production. This equates to only ~15% of new primary production in the SW Ross Sea, suggesting that aeolian dFe is a minor component of seasonal Fe supply. The very high ASD accumulation on sea ice in McMurdo Sound compared to other regions of Antarctica suggests that our results represent the upper limit of dFe supply to the ocean from this source in the Ross Sea.

Does large-scale ocean circulation structure life history connectivity in Antarctic toothfish (Dissostichus mawsoni)?

A multidisciplinary approach incorporating otolith chemistry, age data, and numerical Lagrangian particle simulations indicated a single, self-recruiting population of Antarctic toothfish (Dissostichus mawsoni) in the Southeast Pacific Basin (SPB) and Ross Sea, with a life history structured by the large-scale circulation. Chemistry deposited prior to capture along otolith edges demonstrated strong environmental heterogeneity, yet the chemistry in otolith nuclei, deposited during early life, showed no differences. Age data showed only adult fish in catches on the Pacific–Antarctic Ridge in the SPB and structuring of life stages consistent with transport pathways from the northern Ross Sea. Lagrangian particle simulations predicted that early life stages following the flow in the SPB would be transported to areas in the Ross Sea where juveniles are caught, whereas the circulation would facilitate adult movement along the shelf slope and back into the SPB where spawning adults are caught. These results suggest that successfully spawning fish spend only a part of their adult life history in the Ross Sea, areas in the eastern Ross Sea contribute disproportionately to the spawning population, and areas in the southwestern Ross Sea may supply fisheries in the southern Indian Ocean.

Altered Sea Ice Thickness and Permanence Affects Benthic Ecosystem Functioning in Coastal Antarctica

Antarctic sea ice and the cold waters surrounding the continent are key elements of the global climate system, influencing heat redistribution, oceanic circulation and the absorption of carbon dioxide from the atmosphere. However, the Southern Ocean is predicted to warm by 1–6°C over the next century, altering sea ice extent, thickness and permanence. To better understand the connections between coastal sea ice conditions and the functioning of Antarctica’s unique marine benthic ecosystems, we performed manipulative experiments on the seafloor at two southwestern Ross Sea sites with contrasting sea ice conditions. Benthic systems at both study sites were net heterotrophic during the study period (early November), with primary production most likely limited by light availability rather than nutrients. There was five times more fresh algal detrital material in benthic sediments at the site with the thinner, snow-free, annually formed sea ice, relative to the site with thicker, multiyear sea ice. This elevated quantity and quality of algal detrital matter corresponded with a significantly greater rate of sediment oxygen utilization by the benthos and an altered pathway of nitrogen regeneration (tighter coupling between nitrification and denitrification). Large benthic animals (brittle stars, Ophionotus victoriae) enhanced the efflux of dissolved inorganic nutrients from the sediment to the water column and played a greater role in nutrient regeneration at the site with more food. Although changes in sea ice characteristics in the Western Ross Sea are difficult to predict at present, large benthic organisms can be expected to have an expanded role in mediating the effects of elevated coastal productivity and detritus supply on ecosystem dynamics in this part of Antarctica.

Diatom evidence for the onset of Pliocene cooling from AND-1B, McMurdo Sound, Antarctica

The late Pliocene, ~3.3–3.0Ma, is the most recent interval of sustained global warmth in the geologic past. This window is the focus of climate reconstruction efforts by the U.S. Geological Survey's Pliocene Research, Interpretation, and Synoptic Mapping (PRISM) Data/Model Cooperative, and may provide a useful climate analog for the coming century. Reconstructions of past surface ocean conditions proximal to the Antarctic continent are essential to understanding the sensitivity of the cryosphere to this key interval in Earth's climate evolution. An exceptional marine sediment core collected from the southwestern Ross Sea (78° S), Antarctica, during ANDRILL's McMurdo Ice Shelf Project preserves evidence of dramatic fluctuations between grounded ice and productive, open ocean conditions during the late Pliocene, reflecting orbitally-paced glacial/interglacial cycling. In this near-shore record, diatom-rich sediments are recovered from interglacial intervals; two of these diatomites, from ~3.2Ma and 3.03Ma, are within the PRISM chronologic window. The diatom assemblages identified in PRISM-age late Pliocene diatom-rich sediments are distinct from those in mid-Pliocene and later Pliocene/Pleistocene intervals recovered from AND-1B, and comprise both extant taxa with well-constrained ecological preferences and a diverse extinct flora, some members of which are previously undescribed from Antarctic sediments. Both units are dominated by Chaetoceros resting spores, an indicator of high productivity and stratification that is present at much lower abundance in materials both older and younger than the PRISM-age sediments. Newly described species of the genus Fragilariopsis, which first appear in the AND-1B record at 3.2Ma, are the most abundant extinct members of the PRISM-age assemblages. Other extant species with established environmental affinities, such as Fragilariopsis sublinearis, F. curta, Stellarima microtrias, and Thalassiothrix antarctica, are present at lower abundances. Environmental inferences drawn from extant diatom assemblages are in good agreement with those from Chaetoceros resting spores and the Fragilariopsis radiation. All three lines of evidence indicate the onset of late Pliocene cooling in the Ross Sea near-shore environment at 3.2Ma, with intensification and possible regional persistence of summer sea ice by 3.03Ma. An important implication of this research is the indication that the Ross Ice Shelf fluctuated dramatically on orbital timescales at a time when nearshore Antarctic conditions were only modestly warmer than present.

Iceberg-induced changes to polynya operation and regional oceanography in the southern Ross Sea, Antarctica, from in situ observations

AbstractTwo massive tabular icebergs calved from the Ross Ice Shelf in 2000 (B-15) and 2002 (C-19) and perturbed regional ocean processes for several years. Here we document the ocean's response in McMurdo Sound to the icebergs using in situ data collected before, during and after the icebergs’ residence in the Ross Sea. Departures from typical McMurdo Sound seasonal oceanography included the non-appearance of Antarctic Surface Water in summer, a cooler and more homogeneous water column during winter and ‘super-fresh’ High Salinity Shelf Water that gradually recovered its salinity. We found that each iceberg triggered a distinct response to regional ocean processes. B-15a, the largest piece of iceberg B-15, restricted surface circulation, cooled and freshened the upper water column and reduced melting near the ice shelf front for four years. Iceberg C-19 interrupted the operation of the Ross Sea polynya, from which McMurdo Sound took three to four years to recover, and was responsible for a geographic shift in the dense water formation region for the south-western Ross Sea. These results differ from earlier modelling studies and highlight the challenges of modelling the polar ocean. We also show that one pathway previously thought to supply dense water to the Ross Ice Shelf cavity was not operating at that time.

Control of phytoplankton bloom inception in the Ross Sea, Antarctica, by Ekman restratification

Observations from November 2006 in the southwestern Ross Sea indicate that stratification developed in a localized fashion, proximal to upper ocean fronts. These regions were hotspots for biological productivity, exhibiting greater drawdown of CO2and accumulation of oxygen, indicative of enhanced photosynthesis and air‐sea gas exchange. While the effect of stratification is clear, the reasons for its development was not; air temperatures were unseasonably cold, sea‐ice melt and sea surface warming were not significant. By comparing a one‐dimensional mixed layer model with two‐dimensional numerical simulations that include horizontal density gradients characteristic of the region, it is shown that Ekman advection is critical to structuring early season stratification. Where fronts are forced by winds that oppose the surface frontal current, Ekman advection displaces lighter water over dense. As biological productivity is light limited in the Ross Sea, and thus sensitive to the depth of the mixed layer, Ekman restratification plays an important role in determining the spatial distribution and development of the annual phytoplankton bloom in the region. The presence of fronts is therefore of first‐order importance to the restratification and bloom dynamics of the Ross Sea in the early spring.

Influence of hydrography on phytoplankton distribution in the Amundsen and Ross Seas, Antarctica

The phytoplankton of the Ross Sea have been intensively studied, in contrast to that of the Amundsen Sea. This study focused on understanding the environmental variables that influence the spatial patterns of assemblages during late summer, 2007, and late spring–early summer, 2008 in the Amundsen and Ross Seas. Blooms of the prymnesiophyte Phaeocystis antarctica, and the silicoflagellate Dictyocha speculum occurred in the southwestern to eastern parts of the Ross Sea, respectively, whereas diatoms dominated in southeastern Ross and the Amundsen Sea. Shallow mixed layers supported the growth of diatoms, but were not the only factor required for diatom bloom development. Modified Circumpolar Deep Water intruded into the subsurface waters (< 200 m) in the southwestern Ross Sea during February 2007, and possibly favored the formation of P. antarctica blooms. Photosynthetic quantum yield data suggest that blooms from the southwestern Ross Sea were approaching stress during January 2008, likely due to iron limitation, in contrast to blooms close to the ice edge in the Amundsen Sea, where iron may be more available to the phytoplankton. A detailed comparison between the Amundsen and Ross Seas may allow a greater understanding of the environmental-induced impacts on phytoplankton distribution and regional biogeochemical cycles.