Western Antarctic Peninsula Shelf Research Articles

Remote Sensing of Particulate Organic Carbon on the Western Antarctic Peninsula Shelf Using a Color Index-Based Algorithm

A color index (CI)–based algorithm (CI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">POC</sub> ) for estimating particulate organic carbon (POC) concentrations from satellite observations is evaluated for the Western Antarctic Peninsula (WAP) shelf. The CI utilizes three spectral bands centered approximately at 490, 550, and 670 nm of the ocean color satellite sensors. The CI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">POC</sub> algorithm is calibrated and validated using field–measured POC concentration data and match–up satellite–derived remote–sensing reflectance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_rs</i> ) data. For comparison, the performance of the blue–green (BG) band ratio algorithm is also evaluated using the same match–up dataset. Results show that the CI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">POC</sub> algorithm outperforms the BG band ratio algorithm in determining the POC concentration for the WAP shelf with good statistical parameters of algorithm performance for the CI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">POC</sub> algorithm. Given that the CI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">POC</sub> algorithm is built upon a three–band–difference concept and is less sensitive to artificial errors in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_rs</i> than the BG algorithm, CI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">POC</sub> can correct POC concentration by retrieving uncertainties caused by sea ice and cloud contaminations. Further evaluation showed that the CI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">POC</sub> algorithm can derive POC from next–generation ocean color satellite sensors with robust performance and noise tolerance.

History of Anvers-Hugo Trough, western Antarctic Peninsula shelf, since the Last Glacial Maximum. Part I: Deglacial history based on new sedimentological and chronological data

Reconstructing the advance and retreat of past ice sheets provides important long-term context for recent change(s) and enables us to better understand ice sheet responses to forcing mechanisms and external boundary conditions that regulate grounding line retreat. This study applies various radiocarbon dating techniques, guided by a detailed sedimentological analyses, to reconstruct the glacial history of Anvers-Hugo Trough (AHT), one of the largest bathymetric troughs on the western Antarctic Peninsula (WAP) shelf. Existing records from AHT indicate that the expanded Antarctic Peninsula Ice Sheet (APIS) advanced to, or close to, the continental shelf edge during the Last Glacial Maximum (LGM; 23-19 cal kyr BP [ = calibrated kiloyears before present]), with deglaciation of the outer shelf after ∼16.3 cal kyr BP. Our new chronological data show that the APIS had retreated to the middle shelf by ∼15.7 cal kyr BP. Over this 600-year interval, two large grounding-zone wedges (GZW) were deposited across the middle (GZW2) and inner shelf (GZW3), suggesting that their formation occurred on centennial rather than millennial timescales. Expanded sequences of sub-ice shelf sediments occur seaward of the inner GZW3, which suggests that the grounding line remained stationary for a prolonged period over the middle shelf. Grounding-line retreat rates indicate faster retreat across the outer to middle shelf compared to retreat across the middle to inner shelf. We suggest that variable retreat rates relate to the broad-scale morphology of the trough, which is characterised by a relatively smooth, retrograde seabed on the outer to middle shelf and rugged morphology with a locally landward shallowing bed and deep basin on the inner shelf. A slowdown in retreat rate could also have been promoted by convergent ice flow over the inner shelf and the availability of pinning points associated with bathymetric highs around Anvers Island and Hugo Island.

History of Anvers-Hugo Trough, western Antarctic Peninsula shelf, since the Last Glacial Maximum. Part II: Palaeo-productivity and palaeoceanographic changes during the Last Glacial Transition

Following the Last Glacial Maximum (LGM; ca. 23-19 calibrated [cal.] kyr before present [BP]), atmospheric and oceanic warming, together with global sea-level rise, drove widespread deglaciation of the Antarctic Ice Sheet, increasing the flux of freshwater to the ocean and leading to substantial changes in marine biological productivity. On the Antarctic continental shelf, periods of elevated biological productivity, often preserved in the sediment record as laminated (and sometimes varved) diatomaceous oozes (LDO), have been reported from several locations and are typically associated with the formation of calving bay re-entrants during ice sheet retreat. Understanding what drives the formation and deposition of LDOs, and the impact of deglacial processes on biogenic productivity more generally, can help inform how Antarctic coastal environments will respond to current and future ice sheet melting. In this study we utilise a suite of sediment cores recovered from Anvers-Hugo Trough (AHT), western Antarctic Peninsula shelf, which documents the transition from subglacial to glacimarine conditions following retreat of an expanded ice stream after the LGM. We present quantitative absolute diatom abundance (ADA) and species assemblage data, to investigate changes in biological productivity during the Last Glacial Transition (19-11 cal kyr BP). In combination with radiocarbon dating, we show that seasonally open marine conditions were established on the mid-shelf by 13.6 cal kyr BP, but LDOs did not start to accumulate until ∼11.5 cal kyr BP. The ∼1.4 kyr delay between the onset of seasonally open marine conditions and LDO deposition indicates that physiographic changes, and specifically the establishment of a calving bay in AHT, is insufficient to explain LDO deposition alone. LDO deposition in AHT coincides with the early Holocene climatic optimum (∼11.5 – 9.0 kyr) and is therefore explained in terms of increased atmospheric/ocean temperatures, high rates of sea and glacial ice melt and the formation of a well-stratified water column in the austral spring. An implication of our study is that extensive bathymetric mapping in conjunction with detailed core analyses is required to reliably infer environmental controls on LDO deposition.

Microplanktonic diatom assemblages dominated the primary production but not the biomass in an Antarctic fjord

The western Antarctic Peninsula (WAP) fjords, currently threatened by climate change, are known as high krill congregation areas, however, the primary productivity in relation to phytoplankton groups in these coastal areas remains uncertain. To test this, we studied the specific composition, diversity and cellular carbon content of phytoplankton communities in relation to primary production (PP) and growth rates in Andvord Bay, its connection to Gerlache Strait and a near station on the continental shelf during spring (2015) and autumn (2016). Based on the microscopic determination of the relative biomass of the different microalgal taxa we determined three main phytoplankton assemblages that differ in their composition, PP, growth, and spatial and temporal distribution: (1) an assemblage dominated by cryptophytes, with high biomass (59.2 ± 28.2 μgC L−1), intermediate PP (63.1 ± 32 mgC m−3 d−1), and low growth (0.2 ± 1.1 d−1) present in the inner part of the fjord during spring, and related to higher surface water temperatures; (2) an assemblage dominated by microplanktonic diatoms (mainly Odontella weissflogii) with intermediate biomass (40.1 ± 17.2 μgC L−1), high PP (151.2 ± 62.5 mgC m−3 d−1), and high growth (0.9 ± 1 d−1) present from the fjord mouth to the Gerlache Strait waters during spring and in shelf waters during autumn, related to high irradiance and microzooplankton biomass; and (3) an assemblage dominated by dinoflagellates, mainly small gymnodinioids (< 15 μm long), with low biomass (3.5 ± 3.5 μgC L−1), low PP (6.9 ± 5.1 mgC m−3 d−1), and intermediate growth (0.5 ± 0.6 d−1) present in the whole fjord and the Gerlache Strait during autumn and infrequently during spring, related to high meltwater fraction. During both seasons, the highest phytoplankton biomass, richness and productivity in Andvord Bay was located in a frontal area at the fjord's mouth. Overall, the results of this study support the notion that WAP fjords are highly productive, highlighting the role of microplanktonic diatom as the main organic carbon producers, even when they occur in low abundances, due to their photosynthetic potential and higher Carbon-to-Chlorophyll a ratio. Our results, together with previous phytoplankton growth rates measured in the WAP nearshore waters from King George Island to Marguerite Bay, are higher than those measured on the WAP shelf and in the Ross Sea suggesting micro- and macronutrient enrichment from glacial melt maintains high productivity throughout the growth season in fjords. Our results support the hypothesis that climate change could highly affect the coastal WAP ecosystems by changing the balance between more productive phytoplankton assemblages (diatoms) to considerably less productive phytoplankton assemblages (cryptophytes).

Wind‐Driven Barotropic Velocity Dynamics on an Antarctic Shelf

AbstractReanalysis surface stress around the Antarctic continent is used to obtain the cross‐shelf sea surface height (SSH) gradient from the shallow water equations in the long‐wave limit, and the result is compared to the observed barotropic current sampled by current meters on the western Antarctic Peninsula shelf. Similar to the dynamics of SSH, intraseasonal velocity fluctuations (periods 3–100 days) largely consist of a circumpolar‐coherent wavenumber zero mode and of barotropic shelf waves; however, an important distinction from previous studies is the importance of second mode barotropic shelf waves in the velocity signal. Fluctuations with period 40–60 days are particularly energetic. This is partly due to strong fluctuations in the wavenumber zero mode at this period, as previously demonstrated in Drake Passage transport, but also due to excitement of the second mode barotropic shelf wave. After diagnosing the wind‐driven dynamics, some of their implications for shelf‐slope exchange are discussed. Firstly, it is shown that wintertime upwelling of warm water at a coastal canyon head is associated with a coastal SSH drop. Secondly, it is argued that the waves' subtle baroclinicity over the steep continental slope modifies isopycnal depth and affects shelf access to warm water.

Spatially Coherent Intraseasonal Velocity Fluctuations on the Western Antarctic Peninsula Shelf

AbstractWe analyze several year‐long moored current meter records collected on the midshelf and shelf break of the western Antarctic Peninsula in order to describe the major features of the intraseasonal circulation. An empirical orthogonal function (EOF) analysis of all current meter records in any given year reveals that over 49% of the circulation variance follows a spatially coherent, nearly depth‐invariant anticyclonic cell in the downstream vicinity of Marguerite Trough, qualitatively consistent with flow‐topography interaction with the canyon. An analysis of kinetic energy spectra supports the interpretation that the shelf break current is diverted shoreward at the location of the canyon and generates substantial counterclockwise energy at periods shorter than ~10 days as the flow is compressed by a downstream bank. The circulation is well correlated with the long‐shore wind stress both local and remote. Composite velocity profiles under upwelling and downwelling winds show that the change in the barotropic current under different wind states is comparable to or larger than the shear of the mean profile in all years. The barotropic velocity fluctuations are generally not coherent with subpycnocline heat content.

Origin and Attenuation of Mesoscale Structure in Circumpolar Deep Water Intrusions to an Antarctic Shelf

AbstractCross-isobath transport of Upper Circumpolar Deep Water (UCDW) provides a major source of heat to the Antarctic continental shelves. Adaptive sampling with a Slocum glider revealed that the UCDW regularly intrudes onto the western Antarctic Peninsula shelf within mesoscale eddies, and a linear stability analysis of the shelf-break current upstream confirmed eddy length scales and vertical structure are consistent with the baroclinic instability of the current. The properties of the most unstable mode are insensitive to current orientation but sensitive to bottom slope in accordance with modified Eady theory. Once on the shelf, the eddies’ core properties mix with ambient shelf water to form modified CDW (mCDW). Concurrent shipboard CTD and ADCP data are used to diagnose the responsible mixing processes and highlight the importance of thermohaline intrusions. The genesis mechanism of the interleaving layers cannot be confirmed, however a simple analytic model suggests the upper limit contribution of advection by internal waves cannot account for the observed temperature variance unless the cross-eddy temperature gradient is large. Data-adaptive sampling of an eddy with the glider revealed it lost heat across two isopycnals and a fixed radius at a rate of 7 × 109 J s−1 over 3.9 days. This rate is corroborated by a diffusion model initialized with the eddy’s initial hydrographic properties and informed by the heat fluxes parameterized from the shipboard data. The results suggest eddies predominately lose heat laterally and downward, which preserves subsurface heat for melting of marine-terminating glaciers.

Biogeographical patterns among deep sea megabenthic communities across the Drake Passage

AbstractBiogeographical patterns among deep sea benthic communities in the Drake Passage remain poorly understood as a consequence of poor sampling resolution and the spatial remoteness of many sea floor features. Hard-bottom features, including at least 20 seamounts, remain uncharacterized with respect to their benthic megafaunal community assemblages. Here, we present community assemblage patterns from several locations across the Drake Passage to better understand the faunal relationships between deep sea floor communities in the region. Towed camera surveys were conducted on nine topographical features ranging from shelf environments on the southern Chilean Margin, the western Antarctic Peninsula shelf and seamounts in the central Drake Passage. These are the first quantitative measurements of megafaunal abundance at two seamount complexes in the central Drake Passage and multivariate analyses are used to examine the factors influencing species distributions. Three biogeographical groupings were identified based on species assemblages and environmental variables specific to major water mass boundaries in the region: sub-Antarctic Mode Water (318–523 m), Antarctic Intermediate Water (504–1128 m) and Circumpolar Deep Water (1837–3034 m). Further examination of megafaunal associations between sea floor structures may provide clues as to how sub-Antarctic communities are connected throughout the greater Southern Ocean.

Controls on dissolved and particulate iron distributions in surface waters of the Western Antarctic Peninsula shelf

The Western Antarctic Peninsula (WAP) displays high but variable productivity and is also undergoing rapid change. Long-term studies of phytoplankton communities and primary production have suggested transient limitation by the micronutrient iron (Fe), but to date no data have been available to test this hypothesis. Here, we present the first spatially extensive, multi-year measurements of dissolved and particulate trace metals in surface waters to investigate the key sources and sinks of Fe in the central WAP shelf. Surface samples of dissolved and particulate metals were collected throughout the 700 × 200 km grid of the Palmer Long-Term Ecological Research program in three consecutive austral summers (2010 − 2012). Iron concentrations varied widely. Both dissolved and particulate Fe were high in coastal waters (up to 8 nmol kg− 1 and 42 nmol kg− 1, respectively). In contrast, very low Fe concentrations (< 0.1 nmol kg− 1) were widespread in mid- to outer-shelf surface waters, especially in the northern half of the sampling grid, suggesting possible Fe limitation of primary production on the shelf. Sea ice and dust inputs of Fe were minor, although their relative importance increased with distance from shore due to the larger near-shore sources. Sedimentary inputs were inferred from manganese distributions; these were more significant in the northern portion of the grid, and showed interannual variation in intensity. Overall, the interannual distribution of Fe was most closely correlated to that of meteoric water (glacial melt and precipitation). Although the Fe concentrations and relative contributions of dissolved and particulate Fe attributed to meltwater were variable throughout the sampling region, increasing glacial meltwater flux can be expected to increase the delivery of Fe to surface waters of the coastal WAP in the future.

A Decadal (2002–2014) Analysis for Dynamics of Heterotrophic Bacteria in an Antarctic Coastal Ecosystem: Variability and Physical and Biogeochemical Forcings

We investigated the dynamics of heterotrophic bacteria in the coastal western Antarctic Peninsula (WAP), using decadal (2002-2014) time series of two bacterial variables, bacterial production (BP) via 3H-leucine incorporation rates and bacterial biomass (BB) via bacterial abundance, collected at Palmer Antarctica Long Term Ecological Research (LTER) Station B (64.8°S, 64.1°W) over a full austral growing season (October-March). Strong seasonal and interannual variability in the degree of bacterial coupling with phytoplankton processes were observed with varying lags. On average, BP was only 4% of primary production (PP), consistent with low BP:PP ratios observed in polar waters. BP was more strongly correlated with chlorophyll (Chl), than with PP, implying that bacteria feed on DOC produced from a variety of trophic levels (e.g. zooplankton sloppy feeding and excretion) as well as directly on phytoplankton-derived DOC. The degree of bottom-up control on bacterial abundance was moderate and relatively consistent across entire growing seasons, suggesting that bacteria in the coastal WAP are under consistent DOC limitation. Temperature also influenced BP rates, though its effect was weaker than DOC. We established generalized linear models (GLMs) for monthly composites of BP and BB via stepwise regression to explore a set of physical and biogeochemical predictors. Physically, high BP and large BB were shaped by a stratified water-column, similar to forcing mechanisms favoring phytoplankton blooms, but high sea surface temperature (SST) also significantly promoted bacterial processes. High BP and large BB were influenced by high PP and bulk DOC concentrations. Based on these findings, we suggest an increasingly important role of marine heterotrophic bacteria in the coastal WAP food-web as climate change introduces a more favorable environmental setting for promoting BP, with increased DOC from retreating glaciers, a more stabilized upper water-column from ice-melt, and a baseline shift of water temperature due to more frequent delivery of warming Upper Circumpolar Deep Water (UCDW) onto the WAP shelf.

Metabolism of gymnosomatous pteropods in waters of the western Antarctic Peninsula shelf during austral fall

Two species of Southern Ocean gymnosomatous pteropods with dissimilar distribu- tional ranges were collected from western Antarctic Peninsula (WAP) shelf waters in the vicinity of Anvers, Lavoisier, Adelaide and Charcot Islands from March to April 2010 and between 0 and 500 m. The sub-Antarctic gymnosome species, Spongiobranchaea australis, typically occupies regions north of the Polar Front, whereas the true Antarctic gymnosome species, Clione antarc- tica, inhabits colder waters and higher latitudes. Oxygen consumption rates, ammonia excretion rates, proximate body compositions and the activities of 3 metabolic enzymes — lactate dehydro- genase, malate dehydrogenase, and citrate synthase (CS) — were determined in both gymnosome species. Oxygen consumption rates of S. australis and C. antarctica were found to be similar; how- ever, the mean ratio of oxygen consumed to ammonia excreted (O:N, 61.26 ± 18.68:1) indicated that S. australis was oxidizing primarily lipids while C. antarctica was oxidizing a mixture of proteins and lipids (26.41 ± 14.82:1). Proximate body compositions based on percent protein, per- cent lipid, and carbon to nitrogen ratios, suggested larger lipid storage in C. antarctica (~5%) than in S. australis (~3%). CS activities among gymnosomes were dissimilar, and comparisons of enzyme activities were made to other Antarctic organisms. Observed differences in S. australis' physiological indicators may be related to prolonged starvation, whereas C. antarctica appears ready to survive overwintering in Antarctica. Water mass advection from the Antarctic Circum - polar Current is thought to be transporting S. australis onto the WAP shelf, and away from its typical sub-Antarctic habitat.

Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum

This paper compiles and reviews marine and terrestrial data constraining the dimensions and configuration of the Antarctic Peninsula Ice Sheet (APIS) from the Last Glacial Maximum (LGM) through deglaciation to the present day. These data are used to reconstruct grounding-line retreat in 5 ka time-steps from 25 ka BP to present. Glacial landforms and subglacial tills on the eastern and western Antarctic Peninsula (AP) shelf indicate that the APIS was grounded to the outer shelf/shelf edge at the LGM and contained a series of fast-flowing ice streams that drained along cross-shelf bathymetric troughs. The ice sheet was grounded at the shelf edge until ∼20 cal ka BP. Chronological control on retreat is provided by radiocarbon dates on glacimarine sediments from the shelf troughs and on lacustrine and terrestrial organic remains, as well as cosmogenic nuclide dates on erratics and ice moulded bedrock. Retreat in the east was underway by about 18 cal ka BP. The earliest dates on recession in the west are from Bransfield Basin where recession was underway by 17.5 cal ka BP. Ice streams were active during deglaciation at least until the ice sheet had pulled back to the mid-shelf. The timing of initial retreat decreased progressively southwards along the western AP shelf; the large ice stream in Marguerite Trough may have remained grounded at the shelf edge until about 14 cal ka BP, although terrestrial cosmogenic nuclide ages indicate that thinning had commenced by 18 ka BP. Between 15 and 10 cal ka BP the APIS underwent significant recession along the western AP margin, although retreat between individual troughs was asynchronous. Ice in Marguerite Trough may have still been grounded on the mid-shelf at 10 cal ka BP. In the Larsen-A region the transition from grounded to floating ice was established by 10.7–10.6 cal ka BP. The APIS had retreated towards its present configuration in the western AP by the mid-Holocene but on the eastern peninsula may have approached its present configuration several thousand years earlier, by the start of the Holocene. Mid to late-Holocene retreat was diachronous with stillstands, re-advances and changes in ice-shelf configuration being recorded in most places. Subglacial topography exerted a major control on grounding-line retreat with grounding-zone wedges, and thus by inference slow-downs or stillstands in the retreat of the grounding line, occurring in some cases on reverse bed slopes.

Environmental influences on the at-sea behaviour of a major consumer,Mirounga leonina, in a rapidly changing environment

Understanding the distribution and foraging ecology of major consumers within pelagic systems, specifically in relation to physical parameters, can be important for the management of bentho-pelagic systems undergoing rapid change associated with global climate change and other anthropogenic disturbances such as fishing (i.e., the Antarctic Peninsula and Scotia Sea). We tracked 11 adult male southern elephant seals (Mirounga leonina), during their five-month post-moult foraging migrations from King George Island (Isla 25 de Mayo), northern Antarctic Peninsula, using tags capable of recording and transmitting behavioural data and in situ temperature and salinity data. Seals foraged mostly within the Weddell–Scotia Confluence, while a few foraged along the western Antarctic Peninsula shelf of the Bellingshausen Sea. Mixed model outputs suggest that the at-sea behaviour of seals was associated with a number of environmental parameters, especially seafloor depth, sea-ice concentrations and the temperature structure of the water column. Seals increased dive bottom times and travelled at slower speeds in shallower areas and areas with increased sea-ice concentrations. Changes in dive depth and durations, as well as relative amount of time spent during the bottom phases of dives, were observed in relation to differences in overall temperature gradient, likely as a response to vertical changes in prey distribution associated with temperature stratification in the water column. Our results illustrate the likely complex influences of bathymetry, hydrography and sea ice on the behaviour of male southern elephant seals in a changing environment and highlight the need for region-specific approaches to studying environmental influences on behaviour.Keywords: Southern elephant seals; foraging ecology; satellite-relay data loggers; King George Island; Isla 25 de Mayo; at-sea behaviourTo access the supplementary material for this article, please see the supplementary files in the column to the right (under Article Tools).(Published: 26 November 2014)Citation: Polar Research 2014, 33, 23808, http://dx.doi.org/10.3402/polar.v33.23808

Distribution of gymnosomatous pteropods in western Antarctic Peninsula shelf waters: influences of Southern Ocean water masses

ABSTRACTDistributions of gymnosomatous pteropods,Spongiobranchaea australisandClione antarctica, were determined at six sites along a latitudinal gradient in western Antarctica Peninsula shelf waters using vertically stratified trawls. Hydrographic data were collected at the same sites with conductivity-temperature-depth casts, and correlations of explanatory variables to gymnosome distributions were determined using statistical analyses performed in Matlab, a high level programming software to conduct numerical computation and visualisation. Explanatory variables included sampling site, latitude, longitude and depth, seawater temperature, salinity and density, Southern Ocean Antarctic Surface Water, Winter Water, Upper Circumpolar Deep Water and warm transitional waters, as well as oceanographic remote sensing data for coloured dissolved organic matter, chlorophyllaconcentration, normalised fluorescence line height, nighttime sea surface temperature, photosynthetically active radiation, particulate inorganic carbon, particulate organic carbon, daytime sea surface temperature and daily sea ice concentration. Hydrographic data revealed that warmer water masses were prevalent along the western Antarctic Peninsula, and the distributions of both gymnosome species were primarily influenced by water masses, temperature, sampling site and latitude. As a consequence, distributional shifts of gymnosomes are predicted in response to the current warming trends.

Antifreeze proteins in pelagic fishes from Marguerite Bay (Western Antarctica)

The Southern Ocean is home to two major types of fishes: the largely endemic suborder Notothenioidei and representatives of oceanic fish families that are widely distributed in the midwater and benthic environments elsewhere (e.g. bathylagids, myctophids, liparids, and zoarcids). In most regions of the coastal Antarctic, e.g. the Ross Sea, there is a distinct separation in the pelagic communities at the shelf break between the oceanics (off-shelf) and the endemics (on-shelf). Coincidentally, in much of the coastal Antarctic, the shelf break also marks the boundary between a water column entirely composed of the very cold (−2 °C) Ice Shelf Water and an oceanic profile that includes warmer circumpolar deep water (2 °C at 200 m) at intermediate depths. The distinct separation in pelagic communities observed in most coastal regions of the Antarctic is not seen on the western Antarctic Peninsula (WAP), where circumpolar deep water intrudes to form a warmer midwater and oceanic species are strongly represented. It was hypothesized that the cold ice-shelf water, lethal to fishes without antifreeze glycoproteins (AFGPs) in their blood, was excluding the oceanic species from most of the Antarctic continental shelf waters. To test the hypothesis, nine species of fish captured in WAP shelf waters were tested for the presence of AFGPs. The oceanic fish families analyzed: Myctophidae ( Electrona and Gymnoscopelus), Zoarcidae ( Melanostigma), Gempylidae ( Paradiplospinus), Paralepididae ( Notolepis), and Bathylagidae ( Bathylagus) showed no antifreeze activity. Two endemic species captured in the same sampling program did show antifreeze activity: the important pelagic species Pleuragramma antarcticum (Nototheniidae) and the Bathydraconid species Vomeridens infuscipinnus. The absence of AFGPs in the blood of Antarctic oceanic species makes a strong case for temperature exclusion of oceanic fishes in the coastal Antarctic.

Spatial variation in seabed temperatures in the Southern Ocean: Implications for benthic ecology and biogeography

The Antarctic seabed has traditionally been regarded as cold and thermally stable, with little spatial or seasonal variation in temperature. Here we demonstrate marked spatial variations in continental shelf seabed temperature around Antarctica, with the western Antarctic Peninsula shelf significantly warmer than shelves around continental Antarctica as a result of flooding of the shelf by Circumpolar Deep Water from the Antarctic Circumpolar Current. The coldest shelf seabed temperatures are in the Weddell Sea, Ross Sea, and Prydz Bay as a consequence of seasonal convection associated with strong air‐sea heat fluxes and sea‐ice formation. These waters constitute the dense precursors of Antarctic Bottom Water, and can descend down the adjacent slope to inject cold water into the Southern Ocean deep sea. Deep sea seabed temperatures are coldest in the Weddell Sea and are progressively warmer to the east. There is a distinct latitudinal gradient in the difference between seabed temperatures on the shelf and in the deep sea, with the deep sea warmer by up to ∼2 K at high latitudes and colder by ∼2 K around sub‐Antarctic islands. These differences have important consequences for benthic ecology and biogeography, understanding the evolutionary history of the Antarctic marine biota, and the impact of regional climate change.

Comparison of contemporary and fossil diatom assemblages from the western Antarctic Peninsula shelf

During the past ten years, the Antarctic Peninsula has been identified as the most rapidly warming region of the Southern Hemisphere and it is important to place this warming in the context of the natural climate and oceanographic variability of the recent geological past. Many biological proxies, such as marine diatom assemblages, have been used to determine Southern Ocean palaeoceanographic conditions during the Late Quaternary, however, few investigations have attempted to link observations of modern floras with the fossil record. In this study we examine a suite of modern austral spring (December 2003) and summer (February 2002) surface water samples from along the western Antarctic Peninsula (WAP) continental shelf and compare these to core-top, surface sediment samples. Using detrended correspondence analysis (DCA) and principal component analysis (PCA) of diatom abundance data we investigate the relationship of contemporary diatom floras with the fossil record. This multivariate analysis reveals that our modern assemblages can be divided into three groups: summer southern WAP sites, summer northern WAP sites, and spring WAP sites. Sea surface temperature (SST) is an important environmental variable for explaining seasonal differences in diatom assemblages between spring and summer, but sea surface salinity (SSS) is more important for understanding temporally-equivalent regional variations in assemblage. Our summer diatom samples are more reminiscent of early season assemblages, reflecting the unusually late sea ice retreat from the region that year. When the modern assemblages are compared to the fossil record, it is clear that most of the important diatoms from the summer assemblage are not preserved into the sediments, and that the fossil record more closely reflects spring assemblages. This observation is important for any future attempts to quantitatively reconstruct palaeoceanographic conditions along the WAP during the Late Quaternary and highlights the need for many more such studies in order to address longer timescales, such as interannual variability, in the context of the fossil record.

Pelagic fishes in the Marguerite Bay region of the West Antarctic Peninsula continental shelf

Abstract Pelagic fishes in the Marguerite Bay region of the western Antarctic Peninsula (WAP) continental margin were sampled using a 10-m 2 MOCNESS as part of the Southern Ocean Global Ocean Ecosystems Dynamics (SO GLOBEC) program. Sixty-two tows were completed during the course of four cruises conducted during the austral fall and winter, 22 each during the austral fall, and 9 each during the austral winter. Six thousand and sixty individuals of 34 species representing 13 families were collected in the fall, while 672 individuals of 22 species from 10 families were collected in the winter. Nearly all of the notothenioid specimens collected (families Artedraconidae, Bathydraconidae, Channichthyidae, and Nototheniidae) were either larvae or young juveniles (0–2 years). Conversely, except for the paralepidid Notolepis coatsi and the occasional juveniles of the bathylagid Bathylagus antarcticus , the gonostomatid Cyclothone kobayashii , or the myctophid Electrona antarctica , the non-notothenioid specimens collected were predominantly adults. In the fall, the nototheniids Pleuragramma antarcticum and Trematomus scotti , and the myctophid E. antarctica numerically dominated the overall assemblage, collectively accounting for 89.7% of the total catch. In the winter, E. antarctica , Cyclothone microdon , and B. antarcticus were the numerical dominants, each contributing 14–20% of the total. The pelagic fish community within the Marguerite Bay region of the WAP continental shelf is a variable mixture of mesopelagic and neritic fauna. At one extreme is an oceanic assemblage exhibiting high-diversity indices and characterized by the genera Electrona , Gymnoscopelus , Protomyctophum , Bathylagus , Cyclothone , and Notolepis . Minor components of this group include numerous less common mesopelagic genera (e.g., Paradiplospinus , Lampanyctus , Benthalbella , Borostomias ) and the occasional larval/juvenile notothenioid. At the other extreme is a coastal assemblage with low-diversity indices dominated by larval and juvenile notothenioids, particularly Pleuragramma antarcticum This assemblage is also characterized by a numerically low but consistent liparid and zoarcid component, with the latter group often contributing disproportionately to the total biomass. The degree of overlap between the two assemblages and the relative dominance of representative species is directly related to local hydrographic conditions, in particular the presence of Circumpolar Deep Water (CDW). The unique hydrographic conditions of the WAP shelf and the accompanying spatial heterogeneity in pelagic ichthyofauna provide a striking contrast to the continental margin areas of the Ross, Weddell, Davis, and Dumont d’Urville Seas where sharp temperature gradients near the shelf break result in a clear separation of oceanic and coastal assemblages.

Estimating sea-ice coverage, draft, and velocity in Marguerite Bay (Antarctica) using a subsurface moored upward-looking acoustic Doppler current profiler (ADCP)

A technique for the analysis of data from a subsurface moored upward-looking acoustic Doppler current profiler (ADCP) to determine ice coverage, draft and velocity is presented and applied to data collected in Marguerite Bay on the western Antarctic Peninsula shelf. This method provides sea-ice information when no dedicated upward-looking sonar (ULS) data are available. Ice detection is accomplished using windowed variances of ADCP vertical velocity, vertical error velocity, and surface horizontal speed. ADCP signal correlation and backscatter intensity were poor indicators of the presence of ice at this site. Ice draft is estimated using a combination of ADCP backscatter data, atmospheric and oceanic pressure data, and information about the thermal stratification. This estimate requires corrections to the ADCP-derived range for instrument tilt and sound speed profile. Uncertainties of ±0.20 m during midwinter and ±0.40 m when the base of the surface mixed layer is above the ADCP for ice draft are estimated based on: (a) a Monte Carlo simulation, (b) uncertainty in the sound speed correction, and (c) performance of the zero-draft estimate during times of known open water. Ice velocity is taken as the ADCP horizontal velocity in the depth bin specified by the range estimate.

Oceanic and atmospheric forcing of early Holocene ice shelf retreat, George VI Ice Shelf, Antarctica Peninsula

We use lake sediment records from an epishelf lake on Alexander Island to provide a detailed picture of the Holocene history of George VI Ice Shelf (GVI-IS). Core analyses included; micropaleontology (diatoms/foraminifera), stable isotope (δ18O, δ13C), geochemistry (total organic carbon (TOC), total nitrogen (TN), C/N ratios) and grain-size analyses. These data provide robust evidence for one period of past ice shelf absence during the early Holocene. The timing of this period has been constrained by 10 AMS 14C dates performed on mono-specific foraminifera samples. These dates suggest that GVI-IS was absent between c 9600calyrBP and c 7730calyrBP. This early Holocene collapse immediately followed a period of maximum Holocene warmth that is recorded in some Antarctic ice cores and coincides with an influx of warmer ocean water onto the western Antarctic Peninsula (AP) shelf at c 9000calyrBP. The absence of a currently extant ice shelf during this time interval suggests that early Holocene ocean-atmosphere variability in the AP was greater than that measured in recent decades.