West Antarctic Peninsula Continental Shelf Research Articles

Annual Mixed Layer Carbon Budget for the West Antarctic Peninsula Continental Shelf: Insights From Year‐Round Mooring Measurements

AbstractFor the first time the annual carbon budget on the West Antarctic Peninsula shelf was studied with continuously measured CO2 system parameters (pH and pCO2) from a subsurface mooring. The temporal evolution of the mixed layer dissolved inorganic carbon (DIC) is investigated via a mass balance. The annual mixed layer DIC inventory change was 1.1 ± 0.4 mol m−2 yr−1, which was mainly regulated by biological drawdown (−2.8 ± 2.4 mol m−2 yr−1), diapycnal eddy diffusion (2.6 ± 1.3 mol m−2 yr−1), entrainment/detrainment (0.9 ± 0.4 mol m−2 yr−1), and air‐water gas exchange (0.4 ± 2.1 mol m−2 yr−1). Significant carbon drawdown was observed in the spring and summer, which was replenished by the physical processes mentioned above. These observations suggest this area is an annual atmosphere CO2 sink with a mixed layer net community production of 2.8 ± 2.4 mol m−2 yr−1. These results highlight the significant seasonality in the DIC mass balance and the necessity of year‐round continuous observations for robust assessments of biogeochemical cycling in this region.

Sedimentary Nutrient Supply in Productive Hot Spots off the West Antarctic Peninsula Revealed by Silicon Isotopes

In this study we evaluate the benthic fluxes of silicic acid along the West Antarctic Peninsula (WAP). Silicic acid (DSi) is one of the macronutrients essential in fuelling biological hot spots of diatom‐dominated primary production along the WAP. Here we measure the concentration and stable silicon isotopic composition of DSi in porewater profiles, biogenic silica content (BSi), and diatom abundance from sediment cores collected along the WAP. We couple these measurements with reaction‐transport modeling, to assess the DSi flux and the processes that release this key nutrient from the sediment into the overlying waters. Our results show that the benthic DSi flux is dominated by the diffusive flux, which is estimated to be equivalent to 26.7 ± 2.7 Gmol yr−1 for the WAP continental shelf. The DSi isotope profiles reveal the important impact of sedimentary processes on porewater DSi and suggest that biogenic silica dissolution is the main source of DSi in porewaters and consequently of the benthic fluxes. Our integrated data‐model assessment highlights the impact of surface productivity on sedimentary processes and the dynamic environment of core‐top sediments where dissolution and reverse weathering reactions control DSi exchanges.

Assessing the distribution of labile organic carbon from diverse depositional environments on the West Antarctic Peninsula shelf

The seabed distributions of labile organic carbon (LOC), i.e., recently produced organic matter from marine plankton, were studied in a variety of depositional environments on the West Antarctic Peninsula continental shelf using the naturally occurring radioisotopes, 14C, 210Pb and 234Th. The effects of bioturbation on the LOC pool were assessed on short-term (seasonal, using 234Thxs within the upper, 2–5 cm) and long-term (decadal, using 210Pbxs from 3-22 cm) timescales to generate LOC degradation coefficients (k) and mean residence times (MRT or τ-LOC) as a function of depth within the sediment column. Based on 234Th distributions, mixing coefficients (Db) ranged from 2 cm2/y to 36 cm2/y, which resulted in LOC MRT values that varied from 0.8y to 9.8y. Based on 210Pb distributions, Db values at depth ranged from 0.2 cm2/y to 1.8 cm2/y, resulting in MRT values that varied from 32y to 4900y. Along the West Antarctic Peninsula, seabed inventories of LOC varied between 17-70 mg/cm2. Bulk LOC becomes less reactive and ages with time during resuspension and transport across the shelf and as LOC is transported deeper within the sediment column by infaunal feeding activities.

Oceanographic variability on the West Antarctic Peninsula during the Holocene and the influence of upper circumpolar deep water

Recent intensification of wind-driven upwelling of warm upper circumpolar deep water (UCDW) has been linked to accelerated melting of West Antarctic ice shelves and glaciers. To better assess the long term relationship between UCDW upwelling and the stability of the West Antarctic Ice Sheet, we present a multi-proxy reconstruction of surface and bottom water conditions in Marguerite Bay, West Antarctic Peninsula (WAP), through the Holocene. A combination of sedimentological, diatom and foraminiferal records are, for the first time, presented together to infer a decline in UCDW influence within Marguerite Bay through the early to mid Holocene and the dominance of cyclic forcing in the late Holocene. Extensive glacial melt, limited sea ice and enhanced primary productivity between 9.7 and 7.0 ka BP is considered to be most consistent with persistent incursions of UCDW through Marguerite Trough. From 7.0 ka BP sea ice seasons increased and productivity decreased, suggesting that UCDW influence within Marguerite Bay waned, coincident with the equatorward migration of the Southern Hemisphere Westerly Winds (SWW). UCDW influence continued through the mid Holocene, and by 4.2 ka BP lengthy sea ice seasons persisted within Marguerite Bay. Intermittent melting and reforming of this sea ice within the late Holocene may be indicative of episodic incursions of UCDW into Marguerite Bay during this period. The cyclical changes in the oceanography within Marguerite Bay during the late Holocene is consistent with enhanced sensitively to ENSO forcing as opposed to the SWW-forcing that appears to have dominated the early to mid Holocene. Current measurements of the oceanography of the WAP continental shelf suggest that the system has now returned to the early Holocene-like oceanographic configuration reported here, which in both cases has been associated with rapid deglaciation.

Productivity and linkages of the food web of the southern region of the western Antarctic Peninsula continental shelf

The productivity and linkages in the food web of the southern region of the west Antarctic Peninsula continental shelf were investigated using a multi-trophic level mass balance model. Data collected during the Southern Ocean Global Ocean Ecosystem Dynamics field program were combined with data from the literature on the abundance and diet composition of zooplankton, fish, seabirds and marine mammals to calculate energy flows in the food web and to infer the overall food web structure at the annual level. Sensitivity analyses investigated the effects of variability in growth and biomass of Antarctic krill (Euphausia superba) and in the biomass of Antarctic krill predators on the structure and energy fluxes in the food web. Scenario simulations provided insights into the potential responses of the food web to a reduced contribution of large phytoplankton (diatom) production to total primary production, and to reduced consumption of primary production by Antarctic krill and mesozooplankton coincident with increased consumption by microzooplankton and salps. Model-derived estimates of primary production were 187–207gCm−2y−1, which are consistent with observed values (47–351gCm−2y−1). Simulations showed that Antarctic krill provide the majority of energy needed to sustain seabird and marine mammal production, thereby exerting a bottom-up control on higher trophic level predators. Energy transfer to top predators via mesozooplanton was a less efficient pathway, and salps were a production loss pathway because little of the primary production they consumed was passed to higher trophic levels. Increased predominance of small phytoplankton (nanoflagellates and cryptophytes) reduced the production of Antarctic krill and of its predators, including seabirds and seals.

Sensitivity of Circumpolar Deep Water Transport and Ice Shelf Basal Melt along the West Antarctic Peninsula to Changes in the Winds

Abstract Circumpolar Deep Water (CDW) can be found near the continental shelf break around most of Antarctica. Advection of this relatively warm water (up to 2°C) across the continental shelf to the base of floating ice shelves is thought to be a critical source of heat for basal melting in some locations. A high-resolution (4 km) regional ocean–sea ice–ice shelf model of the west Antarctic Peninsula (WAP) coastal ocean was used to examine the effects of changes in the winds on across-shelf CDW transport and ice shelf basal melt. Increases and decreases in the strength of the wind fields were simulated by scaling the present-day winds by a constant factor. Additional simulations considered effects of increased Antarctic Circumpolar Current (ACC) transport. Increased wind strength and ACC transport increased the amount of CDW transported onto the WAP continental shelf but did not necessarily increase CDW flux underneath the nearby ice shelves. The basal melt underneath some of the deeper ice shelves actually decreased with increased wind strength. Increased mixing over the WAP shelf due to stronger winds removed more heat from the deeper shelf waters than the additional heat gained from increased CDW volume transport. The simulation results suggest that the effect on the WAP ice shelves of the projected strengthening of the polar westerlies is not a simple matter of increased winds causing increased (or decreased) basal melt. A simple budget calculation indicated that iron associated with increased vertical mixing of CDW could significantly affect biological productivity of this region.

Characterization of sea ice cover, motion and dynamics in Marguerite Bay, Antarctic Peninsula

As part of the U.S. GLOBEC Southern Ocean Program, data from two automatic weather stations (AWSs) on small islets and upward-looking acoustic Doppler current profilers (ADCPs) and an upward-looking sonar (ULS) on sub-surface moorings have been analyzed to produce time series of atmospheric forcing, sea ice thickness, sea ice and ocean velocities, and sea ice momentum balances within Marguerite Bay and at mid-shelf on the central west Antarctic Peninsula continental shelf for the austral winter–spring seasons of 2001 and 2002. Both years had roughly seven months of nearly complete sea ice cover, but ice onset was about two months earlier in 2002 than 2001 mostly due to extremely cold surface air temperatures in Marguerite Bay during May–June 2002. Sea ice draft was quite variable, but generally thickened with time, reaching ∼2–3 m by the end of September before thinning. In October, 2002, a polynya was observed at one site within Marguerite Bay that lasted for 4 days. The sea ice motion and dynamics in Marguerite Bay were analyzed when sea ice draft was greater than 2 m. Wind stress during these periods was predominantly southward and southeastward. Wind stress, sea ice, and near-surface water motion were partially correlated, with sea ice motion greater than water motion. Large wind stress and sea ice and water motions occurred during short energetic events of a few days or shorter; however, the rms wind stress, sea ice, and water velocities were small, about 0.3 N m −2, 8–16 cm s −1, and 5–7 cm s −1, respectively. Mean sea ice motion was southward, but quite small, of order 2 cm s −1, while mean near-surface water motion was not statistically different from zero. The dominant sea ice momentum balance was between wind and internal ice stresses on time scales of several days and longer, with water stress, Coriolis, and tilt terms playing secondary roles. The mean southward wind stress was balanced, within uncertainty, by the mean northward internal ice stress. The strong and variable internal ice stresses were likely due to the onshore winds, the thickness of the sea ice, and the nearby coastline.

On the characteristics of Circumpolar Deep Water intrusions to the west Antarctic Peninsula Continental Shelf

Hydrographic and current velocity observations collected from March 2001 to February 2003 on the west Antarctic Peninsula shelf as part of the Southern Ocean Global Ecosystems Dynamics program are used to characterize intrusions of Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) onto the shelf and Marguerite Bay. UCDW is found on the middle and outer shelf along Marguerite Trough, which connects the shelf break to Marguerite Bay, and at another location farther south. UCDW intrudes in the form of frequent (four per month) and small horizontal scales (≈4 km) warm eddy‐like structures with maximum vertical scales of a few hundred meters. However, no evidence of UCDW intrusions was found in Marguerite Bay. LCDW was found in several deep depressions connected to the shelf break, including Marguerite Trough, forming a tongue of relatively dense water 95 m thick (on average) that reaches into Marguerite Bay through Marguerite Trough. A steady advective‐diffusive balance for the LCDW intrusion is used to make an estimation of the average upwelling rate and diffusivity in the deep layer within Marguerite Trough, which suggest the LCDW layer is renewed approximately every six weeks.

Macrofaunal abundance and composition on the West Antarctic Peninsula continental shelf: Evidence for a sediment ‘food bank’ and similarities to deep-sea habitats

Abstract To assess the impact and fate of the summer phytoplankton bloom on Antarctic benthos, we evaluated temporal and spatial patterns in macrofaunal abundance and taxonomic composition along a transect crossing the West Antarctic Peninsula (WAP) continental shelf. As part of the FOODBANCS project, we sampled three sites at 550–625 m depths during five cruises occurring in November 1999, February–March 2000, June 2000, October 2000 and March 2001. We used a combination of megacore and box-core samplers to take 81 samples, and collected over 30,000 macrofaunal individuals, one of the largest sampling efforts on the Antarctic shelf to date. Comparison of the two sampling methodologies (box core and megacore) indicates similar macrofaunal densities, but with significant differences in taxonomic composition, a reflection of the different spatial scales of sampling. Macrofaunal abundances on the WAP shelf were relatively high compared to other Antarctic shelf settings. At two of the three sampling sites, macrofaunal abundance remained constant throughout the year, which is consistent with the presence of a sediment ‘food bank’. Differences were observed in taxonomic composition at the site closest to the coast (Station A), driven by higher abundances of sub-surface-deposit feeders. A significant temporal response was observed in the ampharetid polychaetes at Station A, with an abundance peak in the late fall post-bloom period; this may have resulted from juvenile recruitment during the summer bloom. Familial composition of macrofaunal polychaetes on the WAP shelf is more closely related to deep-sea abyssal fauna than to other shelf regions, and we hypothesize that this is a result of both local ecological conditions (low temperatures) and a reflection of historical processes such as extinctions on the Antarctic shelf during previous glacial maxima followed by recolonization from the deep sea.

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.

Upper ocean variability in west Antarctic Peninsula continental shelf waters as measured using instrumented seals

Abstract Temperature profile data for the west Antarctic Peninsula (WAP) continental shelf waters, collected from freely ranging instrumented seals (crabeater, Lobodon carcinophagus and leopard, Hydrurga leptonyx), were used to demonstrate that these platforms can be used to supplement traditional oceanographic sampling methods to investigate the physical properties of the upper water column. The seal-derived profiles were combined with temperature profiles obtained from ship-based CTD measurements and from a numerical circulation model developed for the WAP to describe changes in temperature structure, heat content, and heat flux in the upper ocean waters of the WAP continental shelf. The seal-derived data documented the fall-to-winter transition of the surface waters and the shelf-wide presence of modified Circumpolar Deep Water (CDW) below 150–200 m on the WAP continental shelf. The heat content of the upper 200 m calculated from the seal-derived temperature profiles ranged between 1000 and 1500 MJ m−2; similar estimates were obtained from simulated temperature distributions. The seal-derived temperature measurements provided broader space and time resolution than was possible using any other currently available oceanographic sampling method. As such, the seal-derived measurements provided a valuable dataset for evaluation of temperature fields obtained from a numerical circulation model.

Transport of Antarctic krill ( Euphausia superba) across the Scotia Sea. Part I: Circulation and particle tracking simulations

The Harvard Ocean Prediction System (HOPS) is configured to simulate the circulation of the Scotia Sea and environs. This is part of a study designed to test the hypothesis that Antarctic krill ( Euphausia superba) populations at South Georgia in the eastern Scotia Sea are sustained by import of individuals from upstream regions, such as the western Antarctic Peninsula. Comparison of the simulated circulation fields obtained from HOPS with observations showed good agreement. The surface circulation, particularly through the Drake Passage and across the Scotia Sea, matches observations, with its northeastward flow characterized by three high-speed fronts. Also, the Weddell Sea and the Brazil Current, and their associated transports match observations. In addition, mesoscale variability, an important component of the flow in this region, is found in the simulated circulation and the model is overall well suited to model krill transport. Drifter simulations conducted with HOPS showed that krill spawned in areas coinciding with known krill spawning sites along the west Antarctic Peninsula continental shelf can be entrained into the Southern Antarctic Circumpolar Current Front (SACCF). They are transported across the Scotia Sea to South Georgia in 10 months or less. Drifters originating on the continental shelf of the Weddell Sea can reach South Georgia as well; however, transport from this region averages about 20 months. Additional simulations show that such transport is sensitive to changes in wind stress and the location of the SACCF. The results of this study show that krill populations along the Antarctic Peninsula and the Weddell Sea are possible source populations that can provide krill to the South Georgia population. However, successful transport of krill to South Georgia is shown to depend on a multitude of factors, such as the location of the spawning area and timing of spawning, and variations in the location of the SACCF. Therefore, this study provides insight into which environmental factors control the successful transport of krill across the Scotia Sea and with it a better understanding of krill distribution in the region.

Nutrients in the Southern Ocean GLOBEC region: variations, water circulation, and cycling

As part of the US Southern Ocean Global Ecosystem Dynamics (SO GLOBEC) study on the West Antarctic Peninsula (WAP) continental shelf, four surveys were conducted along a grid of close-spaced stations lying within an area composed of Marguerite Bay (MB) and the adjacent continental margin. The surveys occurred in the autumn and winter of both 2001 and 2002. A detailed repetitive plan of the SO GLOBEC surveys permitted the delineation of the important features of nutrient processes. Horizontal intrusions of Upper Circumpolar Deep Water onto the WAP continental shelf do not appear to produce fluxes of nitrate and phosphate onto the shelf but appear to lead to a dilution of the silica inventory on the shelf. High silica concentrations (>115 μM) observed in the near-bottom waters on the shelf were most likely generated locally through dissolution of opal in sediments. Nutrient deficits and net community production (NCP) were calculated by the integration of decreases in nutrient concentrations in the mixed-layer below corresponding values in the underlying remnant Winter Water layer. Waters in MB and other coastal areas were strongly depleted in nutrients—deficits of total inorganic nitrogen (TIN=NO3−+NO2−+NH4+) and silica were >0.6 mol m−2 and >2.5 mol m−2, respectively—and enriched in ammonium stocks (>0.25 mol m−2) relative to other parts of the grid, especially in the autumn of 2001. Autumn NCP values calculated from TIN deficits in MB equaled or exceeded autumnal NCP values from the Joint Global Ocean Flux Study Ross Sea study in 1997. Observed ΔN/ΔP and ΔSi/ΔN removal ratios were 12.4±1.6 and 3.9±1.2, respectively, in 2001; and 11.1±1.3 and 4.1±1.1, respectively, in 2002. These results strongly suggest the dominance of diatoms in the study region. Co-location of maxima in ammonium stocks with the areas of the highest TIN and silica deficits indicated a spatial coupling between primary and heterotrophic production in both years. This co-location highlights the finding that temporal trends in ammonium were usually the opposite of trends in the other nutrients.

Water-mass properties and circulation on the west Antarctic Peninsula Continental Shelf in Austral Fall and Winter 2001

Abstract Hydrographic measurements made during the US Southern Ocean Global Ocean Ecosystem Dynamics cruises, which took place from April to June and July to September 2001, provide a description of changes in water-mass distributions and circulation patterns in the Marguerite Bay region of the west Antarctic Peninsula continental shelf that result from seasonal variability and offshore forcing by the southern boundary of the Antarctic Circumpolar Current (ACC). The primary seasonal change in water-mass properties is the reduction in Antarctic Surface Water and replacement by a thick Winter Water layer. The primary effect of the ACC is to pump warm ( > 1.5 ° C ), salty (34.65–34.7), and nutrient-rich Circumpolar Deep Water (CDW) onto the continental shelf below 200 m at specific sites that correspond to bathymetric features, such as Marguerite Trough. The CDW intruded onto the continental shelf, moved across shelf, and entered Marguerite Bay. This flow pattern was observed during both cruises, suggesting that onshelf intrusions of CDW are a regular occurrence on this shelf. The number of CDW intrusions observed suggests that 4–6 events can occur in a year. Regions where CDW intrusions occur are characterized by surface waters that are above freezing in winter. Diffusive heat fluxes based on estimated diffusivities are insufficient for the observed rate of temperature decay of CDW intrusions. Localized, bathymetrically controlled vertical mixing is suggested as the primary heat transfer mechanism. The hydrographic and Acoustic Doppler Current Profiler measurements show a southwesterly flowing coastal current along Adelaide Island that enters the north side of Marguerite Bay and exits around Alexander Island. This current, which may result from seasonal, coastal buoyancy forcing, was present in fall and winter, but was better developed in fall. This current may be part of a larger cyclonic gyre that overlies the northern part of the area surveyed during the two cruises.

A model study of circulation and cross-shelf exchange on the west Antarctic Peninsula continental shelf

Abstract Exchange of warm, nutrient-rich Circumpolar Deep Water (CDW) onto Antarctic continental shelves and coastal seas has important effects on physical and biological processes in these regions. The present study investigates the locations of this exchange and their dynamics in the west Antarctic Peninsula with a high-resolution three-dimensional numerical model. The model circulation is forced by daily wind stress along with heat and salt fluxes calculated by bulk formulae. All surface fluxes are modified by an imposed climatological ice cover. The model circulation compares favorably to general schematics of the flow based on dynamic topography, water properties from recent hydrography, and ADCP measurements. The sea-surface temperature is similar to satellite estimates except that the model temperatures are slightly higher than observations in the summer and lower in the winter. The seasonal variation of the depth and temperature of the model mixed layer matches observations reasonably well. Sub-pycnocline temperature shows evidence of persistent intrusion of warm CDW onto the shelf, for example, at the shelf break offshore of Adelaide Island. There is a significant correlation between the curvature of the shelf break and the volume transport across the shelf break, indicating that circulation crosses the shelf break in places where the tendency of the flow to maintain a given direction would have it cross a strongly curved bathymetric contour. A momentum term balance shows that momentum advection helps to force flow across the shelf break in specific locations due to the curvature of the bathymetry. For the model to create a strong intrusion of CDW onto the shelf, it appears two mechanisms are necessary. First, CDW is driven onto the shelf at least partially due to momentum advection and the curvature of the shelf break. Then, the general shelf circulation pulls the CDW into the interior.

Water properties on the west Antarctic Peninsula continental shelf: a model study of effects of surface fluxes and sea ice

Abstract A vertical- and time-dependent numerical mixed-layer and sea-ice model is used to analyze processes responsible for sea-ice and surface mixed-layer water properties on the continental shelf on the west side of the Antarctic Peninsula. Atmospheric observations from Faraday and Palmer stations along with satellite sea-ice observations and shipboard water observations (four hydrographic cruises between January 1993 to February 1994) are used for forcing and verification. The focus of this study is the year 1993 during which the best observations exist. However, a 16-year simulation is completed to analyze interannual variations of ice thickness and mixed-layer depth. This model study shows that surface waters of the west Antarctic Peninsula are heated in the summer by solar radiation and cooled in the winter by sensible heat losses. Diffusive-convection is important for upward heat flux across the pycnocline. Ice melt in the spring is due to solar warming of open water, which then melts ice; the direct melting for ice by solar heating is negligible. The near closure of surface heat and salt budgets over 1 year supports the minor importance, or at least the compensation, of near-surface lateral exchanges. Intrusion of Upper Circumpolar Deep water from the Antarctic Circumpolar Current onto the subpycnocline shelf is a critical element of both salt and heat budgets. A 16-year simulation (1978–1993) reproduces most high and low ice years as observed by satellite microwave measurements, thus supporting the major contribution of thermodynamic (local) processes in creating sea-ice and mixed-layer properties.

The linkage between Upper Circumpolar Deep Water (UCDW) and phytoplankton assemblages on the west Antarctic Peninsula continental shelf

Intrusion of Upper Circumpolar Deep Water (UCDW), which was derived from the Antarctic Circumpolar Current (ACC), onto the western Antarctic Peninsula (WAP) shelf region in January 1993 provided a reservoir of nutrient-rich, warmer water below 150 m that subsequently upwelled into the upper water column. Four sites, at which topographically-induced upwelling of UCDW occurred, were identified in a 50 km by 400 km band along the outer WAP continental shelf. One additional site at which wind-driven upwelling occurred was also identified. Diatom-dominated phytoplankton assemblages were always associated with a topographically-induced upwelling site. Such phytoplankton communities were not detected at any other shelf location, although diatoms were present everywhere in the 80,000 km 2 study area and UCDW covered about one-third the area below 150 m. Phytoplankton communities dominated by taxa other than diatoms were restricted to transition waters between the UCDW and shelf waters, the southerly flowing waters out of the Gerlache Strait, and/or the summertime glacial ice melt surface waters very near shore. We suggest that in the absence of episodic intrusion and upwelling of UCDW, the growth requirements for elevated silicate/nitrate ratios and/or other upwelled constituents (e.g. trace metals) are not sufficiently met for diatoms to achieve high abundance or community dominance. One consequence of this is that the ice-free regions of the outer WAP continental shelf will not experience predictable spring diatom blooms. Rather, this region will experience episodic diatom blooms that occur at variable intervals and during different seasonal conditions, if the physical structuring events are occurring. Preferential drawdown of silicate relative to nitrate was observed at each of the offshore upwelling sites and resulted in a reduction in the ambient silicate:nitrate ratio relative to the corresponding value for unmodified UCDW (1.5 versus 3.0 for UCDW). The magnitude of the nutrient drawdown in areas of topographically-induced upwelling suggested that diatom growth had been elevated in response to recent upwelling but that the resulting increased algal biomass was either dispersed by advective processes and/or consumed by the larger krill that were observed to be associated with each offshore upwelling site. Thus, diatom bloom conditions on the outer WAP shelf may not be recognized based on elevated biomass and/or rates of carbon fixation. It was likely that similar physical forcing of significant phytoplankton growth, especially diatoms, may occur but be undetected in regions where the southern boundary of the ACC nears the Antarctic continental shelf edge. Our analyses from the west Antarctic Peninsula demonstrate coupling of the structure of the physical environment with nutrient distributions and phytoplankton assemblages and through to the higher trophic levels, such as Antarctic krill. This environment-trophic coupling may also occur in other regions of the Antarctic, as suggested by correspondences between the distribution of Southern ACC boundary and regions of high concentrations of Antarctic krill. The many mechanisms underlying this coupling remain to be determined, but it was clear that the ecology and biology of the components of the marine food web of the Antarctic continental shelf cannot be studied in isolation from one another or in isolation from the physical environment.

Hydrography and circulation of the West Antarctic Peninsula Continental Shelf

The water mass structure and circulation of the continental shelf waters west of the Antarctic Peninsula are described from hydrographic observations made in March–May 1993. The observations cover an area that extends 900 km alongshore and 200 km offshore and represent the most extensive hydrographic data set currently available for this region. Waters above 100–150 m are composed of Antarctic Surface Water and its end member Winter Water. Below the permanent pycnocline is a modified version of Circumpolar Deep Water, which is a cooled and freshened version of Upper Circumpolar Deep Water. The distinctive signature of cold and salty water from the Bransfield Strait is found at some inshore locations, but there is little indication of significant exchange between Bransfield Strait and the west Antarctic Peninsula shelf. Dynamic topography at 200 m relative to 400 m indicates that the baroclinic circulation on the shelf is composed of a large, weak, cyclonic gyre, with sub-gyres at the northeastern and southwestern ends of the shelf. The total transport of the shelf gyre is 0.15 Sv, with geostrophic currents of order 0.01 m s -1. A simple model that balances across-shelf diffusion of heat and salt from offshore Upper Circumpolar Deep Water with vertical diffusion of heat and salt across the permanent pycnocline into Winter Water is used to explain the formation of the modified Circumpolar Deep Water that is found on the shelf. Model results show that the observed thermohaline distributions across the shelf can be maintained with a coefficient of vertical diffusion of 10 -4 m 2 s -1 and horizontal diffusion coefficients for heat and salt of 200 and 1200 m 2 s -1, respectively. When the effects of double diffusion are included in the model, the required horizontal diffusion coefficients for heat and salt are 200 and 400 m 2 s -1, respectively.