CHOICE OF STUDY REGION
We propose as a study site, a portion of the WAP that includes Marguerite Bay and extends to the north and south of this region (indicated by solid lines on Fig. 1). This area overlaps with the four southern sampling transects that have been occupied as part of the Palmer Long-Term Ecological Research program (dotted lines on Fig. 1). The LTER sampling and that from other programs, have shown that this region is accessible during the time proposed for the U.S. SO GLOBEC study.
This choice of study site is based upon the recommendations put forward by the International Southern Ocean GLOBEC Planning Group as reported in International GLOBEC reports Nos. 5, 7, and 7A. This group indicated that the WAP continental shelf in the region of Marguerite Bay was the preferred site for the SO GLOBEC field studies. This choice of study site is firmly based on planning that has taken place over the past decade and that has involved an international team of scientists.
The Marguerite Bay site is appropriate to the SO GLOBEC central hypothesis, which requires:
- a persistent overwinter population of krill,
- the presence of predators in winter,
- persistent winter sea ice cover, and
- physical circulation that will retain planktonic populations.
The rationale for our choice of the Marguerite Bay region as the focal point for a study site is that this area of the WAP shelf is known to:
- contain a dependable population of Antarctic krill,
- be a site for predators during the winter,
- have dependable sea ice in winter, and
- is characterized by a gyre circulation that likely retains planktonic populations.
Further, this region will be studied in the field by the German and United Kingdom SO GLOBEC programs, thereby providing an unparalleled opportunity for coordination that can provide enhanced data coverage.
Having a U.S. SO GLOBEC program that overlaps in location with other national programs is the only means to obtain the seasonal continuity that is regarded as essential by the international planning committee. It is noteworthy that during the 1999 austral winter, only minimal sea ice was present to the north of this region and that Marguerite Bay and the surrounding area was the only WAP site with winter sea ice. The importance of sea ice to the overarching program hypothesis suggests that siting a field study focused on winter sea ice effects in this region is prudent and essential to program coherence at the national and international levels.
An additional point favoring the choice of the Marguerite Bay region is that the focus on a retentive shelf circulation will facilitate comparison among the SO GLOBEC results and those obtained from other GLOBEC programs, such as the Georges Bank and Northeast Pacific studies.
We emphasize that we have chosen a study site that extends to the north and south of Marguerite Bay and that encompasses the closed shelf circulation. Our sampling program consists of broad-scale surveys that include the entire region and selected sites for process studies (discussed below). We propose a fine scale survey around each of the process sites in the first year. This will give information that will allow us to scale these sites to the larger broad scale region. Additionally, they will provide information for these sites that can be used in the second year if access to the region by the RV Gould is limited by ice cover.
BACKGROUND ON STUDY REGION
i. Physical Oceanography.
The continental shelf in the WAP region is 200 to 500 m deep and is intersected by several depressions (500 to 700 m), that provide connections between shelf and offshore oceanic waters [Hofmann et al., 1996]. One of the deeper of these channels connects Marguerite Bay, located between Adelaide and Alexander Islands, to the outer shelf (Fig. 1). Shallow plateaus, deep basins, and an outer shelf that deepens rapidly towards Drake Passage characterize the remainder of the shelf.The hydrography of the WAP continental shelf is influenced by on-shelf intrusions of warm, salty, nutrient-rich UCDW that are related to on-shelf meandering of the southern boundary of the ACC. This across-shelf exchange occurs at specific sites that are associated with bottom topography and at time scales of a weeks to months [Hofmann & Klinck, 1998a]. The region offshore of Marguerite Bay is a preferred site for on-shelf movement of UCDW [Smith et al., 1999]. As this water mass moves across the shelf it is modified by the overlying cold and fresh Antarctic Surface Water and becomes a modified form of UCDW.
A 1994 hydrographic section across the channel in front of the George VI ice shelf (stations 150 to 155 on Fig. 1) shows clearly the influence of modified UCDW in Marguerite Bay (Fig. 2a). Water below 400 m is warmer than C with salinity between 34.64 and 34.7, providing clear evidence for the presence of modified UCDW in Marguerite Bay. A second hydrographic section (Fig. 2b) constructed from stations along the channel into Marguerite Bay (stations 148 to 154 on Fig. 1) provides additional evidence for across-shelf movement of modified UCDW into Marguerite Bay.
Figs. 2a and 2b illustrate the presence of modified UCDW in Marguerite Bay but raise the question of how it arrived there. The maximum water depth in the section shown in Fig. 2a is about 1000 m, apparently deeper than the sills to the west and north. One possible explanation is that the water may cross the shelf through channels that are not apparent in the currently available bathymetric data. The vertical temperature and salinity gradients show that it is cooler and less salty than the UCDW source water that intrudes onto the continental shelf. Modifications of the T-S properties may result from double diffusive fluxes (the deeper water is warmer), modifications of water by contact with the ice shelf, or buoyancy forcing by the ice shelf resulting in upwelling and across-shelf transport [Potter & Paren, 1985]. However, existence of the vertical gradients rather than uniform T-S properties suggests that the modified UCDW is relatively recent and has not equilibrated to T-S properties of the inner shelf water. This argues for continual renewal of this water mass in Marguerite Bay, which suggests connections to the circulation on the adjacent continental shelf. Thus, the processes and pathways that allow modified UCDW to be present in Marguerite Bay form the basis for important research questions.
Dynamic topography (200 m/400 m) for the WAP shelf shows a large cyclonic gyre, with sub-gyres at the northeastern and southwestern ends of the shelf [Stein, 1992; Smith et al., 1999]. A recent simulation of the circulation distribution at 200 m (Fig. 3) show southward flow on the continental shelf adjacent to Marguerite Bay and northward flow along the outer shelf, which forms the cyclonic gyre in this region. A portion of the southward flow on the inner shelf turns and enters Marguerite Bay, coupling the Bay and the shelf gyre circulation. It should be noted that the cyclonic gyre and flow into Marguerite Bay appear in a circulation simulation that is based on poorly documented bathymetry. Better definition of the bathymetry will help refine the modeled circulation patterns. Hence, the available observational and theoretical circulation studies provide evidence for the gyre circulation that serves as a potential retention mechanism for krill and other planktonic organisms and which is so important to planning the Southern Ocean GLOBEC program.
ii. Sea-ice Distributions.
The surface waters of the WAP shelf vary from nearly ice free (<10% coverage) in austral summer to fully ice covered in austral winter. Most of the sea ice in this region is first-year ice with perennial ice occasionally present in coastal inlets and in the south near the base of the Antarctic Peninsula. There have been no systematic investigations of ice properties, small-scale dynamics or biochemical composition in this region. Available information suggests that the ice thickness appears to vary between 0.5 and 2 m and the Marguerite Bay region shows consistently lower sea ice concentrations in winter [see Plate 1 in Naganobu et al., 1999] relative to other regions of the WAP shelf. This ice thickness is well within the stated capabilities of the RVIB Palmer and RV Gould. Further, the ice cover is likely to be heavily fractured because of its open western boundary, and broken ice is easier to navigate than solid pack ice.Ice motions in the region are largely unknown, but early results derived from satellite passive microwave data indicate a mean onshore flow [Emery et al., 1997]. The timing of advance and retreat and the extent of maximum winter sea ice cover in the WAP region shows interannual variability, which may be related to interannual variations in mean winter air temperature [Stammerjohn & Smith, 1996] or to the Antarctic Circumpolar atmospheric wave [White & Peterson, 1996]. The large-scale sea ice coverage in the WAP region is modulated by upwelling of warm UCDW, which may either prevent ice formation or reduce the existing ice cover [Smith, 1999]. This is likely the process underlying the reduced winter ice concentration in this region that is visible in satellite observations [Naganobu et al., 1999]. There is some evidence that coastal polynyas or shore leads occur near Marguerite Bay [Stammerjohn & Smith, 1996]. These could be due to upwelling of warm (<C) UCDW (a sensible heat polynya) or ice removal due to winds (latent heat polynya). These are also potential water-access sites for predators in winter.
iii. Biological Distributions.
The WAP is one of the most biologically-rich areas of the Southern Ocean and is known to support a large standing stock of Antarctic krill [Marr, 1962]. The shelf region around Marguerite Bay is of particular note in that it is an area where reproducing and nonreproducing krill are consistently found [Marr, 1962]. This has been reaffirmed by a recent study that covered all seasons [Lascara et al., 1999]. Also found in this region are large populations of top predators, such as Adelie penguins (Pygoscelis adeliae) and seals [Fraser & Trivelpiece, 1996; Costa & Crocker, 1996], that depend entirely or to a large extent on krill as a food source. This provides a strong justification for targeting the Marguerite Bay area as the study area for SO GLOBEC.The historical data on phytoplankton biomass on the WAP shelf indicate that these shelf waters, when not coincident with the retreating marginal ice zone in the spring, are generally characterized by low phytoplankton biomass [Smith et al., 1996; Bidigare et al., 1996; Smith et al., 1998]. Recent austral summer observations of macronutrients in this area [Prezelin et al., 2000] indicate that the WAP shelf/slope waters are often characterized by a silicate drawdown of significant magnitude that seems to coincide with a shift to increasingly diatom-dominated communities. These diatom blooms are found consistently in WAP shelf regions where UCDW intrudes and is upwelled [Prezelin et al., 2000]. These regions may favor diatom growth at levels exceeding those predicted from biomass and carbon fixation rates alone. Whether or not this process operates in winter is unknown. Moreover, the many other factors potentially controlling water column phytoplankton production are largely undescribed for winter. Although microheterotrophs are relatively more abundant during winter, little is known about the community dynamics or their role as an alternative food source for grazers.
Information derived from less than a dozen ice cores in the winter [Kottmeier & Sullivan, 1987] show both microalgal and bacterial biomass and potential production being 10 to 100-fold higher in the pack ice than in the water column. Such characteristics are often typical for ice-covered waters [e.g., Kottmeier & Sullivan, 1990; Cota et al., 1990; Grossmann, 1994; Garrison & Mathot, 1996; Gleitz et al.,1998]. These highly productive surface habitats are known to occur in Marguerite Bay during autumn [Burkholder & Mandelli, 1965], yet the seasonal progression and distribution of such ice-associated productivity remain speculative and topics of mechanistic simulation models [Fritsen et al., 1998].
Determining and understanding how environmental and biological processes interact to provide a strategy by which Antarctic krill survive over winter is the dominant research question for SO GLOBEC (International GLOBEC Reports No. 7 and 7A). The extent to which individuals mitigate unfavorable winter conditions (i.e., low food supply) will affect not only survival, but also recruitment and reproductive success during the following spring and summer. Spatial distributions and overwintering strategies are expected to vary among life history stages of krill. Previous findings indicate that larvae are the dominant stage associated with sea ice during winter [e.g., Daly, 1990; Frazer et al., 1997], whereas juveniles and adults usually are found in the upper 100 m of the water column and may be concentrated away from the ice edge [Siegel, 1989; Daly & Macaulay, 1991; Zhou et al., 1994]. Overwintering behaviors include: 1.) feeding on sea-ice biota [Daly, 1990], 2.) carnivory [Hopkins et al., 1993; Huntley et al., 1994], 3.) combustion of lipid stores [Hagen et al., 1996] or body tissue shrinkage [Ikeda & Dixon, 1982; Nicol et al., 1992], and 4.) reduced metabolism [Torres et al., 1994a]. However, the relative importance of these strategies under different environmental regimes (e.g., heavy vs. light ice cover years), what factors influence the use of different strategies, how these behaviors impact subsequent recruitment and reproduction, and how spatial distributions of krill and overwintering strategies vary among life history stages of krill, which differ in the need to acquire food and avoid predation, are poorly known.
PROPOSED SAMPLING PROGRAM
Mooring Deployment and Siting
In austral summer 2001, prior to the survey and process cruises, a moored array to measure currents and physical/biological water properties will be deployed on the WAP shelf. The array will consist of six moorings, with three aligned across-shelf at the northern portion of the study area (A1-A3 on Fig. 1) and three sited southwest of Adelaide Island (B1-B3 on Fig. 1). The northern moorings provide estimates of alongshore southwesterly flow as well as across-shelf flow. The southern moorings provide information on the across-shelf flow of modified UCDW plus flow over the deep basin in Marguerite Bay.
The time series of moored current measurements will be combined with shipboard Acoustic Doppler Current Profiler (ADCP) currents and hydrographic measurements to develop a detailed description of currents, temperature and salinity, and their response to surface and boundary forcing. Lagrangian current measurements will be made with satellite-tracked surface drifters and isobaric floats released during the mooring deployments and at other times during the program, such as the German SO GLOBEC cruise that will occur in May 2001 and the U.K. SO GLOBEC cruise proposed for November-December 2001, both in the Marguerite Bay region. A reduced moored array, consisting of the northern moorings only, will be deployed in austral summer 2002 to provide in situ current estimates during the second field season. Two of the moorings will include upward-looking sonar to obtain ice draft records and sea ice motion will be obtained from GPS-equipped buoys deployed on ice floes, assuming funding is made available for these instruments.
Ship-based Surveys
The basic design for the ship-based sampling portion of this program consists of 12-14 across-shelf transects with a 40-km alongshelf spacing between transects and 20-km spacing between stations on individual transects. This station spacing is larger than the internal Rossby radius (5-10 km), but it is consistent with the length scales of hydrographic features observed on the WAP shelf [Hofmann & Klinck, 1998a]. These transects serve as the basis for the 2 survey cruises that will take place in austral fall (April-May 2001) and late winter (July-August 2001). An estimate of time required at each station indicates that the entire grid can be occupied once during the six to seven weeks allocated for the survey cruises.
Process Study Sites
Concurrent with the survey cruises will be process-oriented cruises which will occupy 4 to 5 sites, each for a minimum of 5-6 days. The southern-most process site will be located near the perennial ice edge and at the edge of or just inside the developing pack ice. We propose to occupy this site first during the April-May 2001 period since the conditions at this site will determine the initial conditions for the winter study.
In March 2001, ARGOS-linked Platform Transmitter Terminals (PTTs) will be deployed on Adlie penguins and on Crabeater seals (Lobodon carcinophagus). Penguins will be instrumented near Palmer Station, Anvers Island during March 2001, while Crabeater seals will be instrumented during the process cruise. The PTTs will provide data on the winter movements and distribution of these predators, information that will in turn be used to help plan the location of the Marguerite Bay process study site. The process study in Marguerite Bay will provide late fall/early winter conditions for krill, penguin, and seal populations.
Suggested sites for other process studies are areas that provide observations of on-shelf intrusions of UCDW, the region near Marguerite Bay where krill are consistently observed, and a site, designated as a control site, that is primarily influenced by new ice.
During the survey cruises a more closely spaced, e.g., 10-km interval, station grid will be made in the area of the process sites. These stations will be made by the survey vessel and will consist primarily of hydroacoustic mapping of biological distributions, ADCP current measurements, fast CTDs with minimal water sampling, predator census, and some net tows for calibration of the hydroacoustics. The finer-spaced grid will provide detailed information in the area of the process sites that can be used to provide linkages between the survey and process cruises in the second year when only one research vessel is available. The detailed mapping in the first year will provide measurements that can be compared with the combined survey/process cruises in the second year. This is especially important if the process sites in the second year are not accessible by the RV Gould.
Project Structure
The U.S. SO GLOBEC principal investigators, primary research interest, and proposed measurements and models are summarized in Table 1.
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Fig. 1. Proposed study region for the U.S. SO GLOBEC field study. The solid line outlines a region of the west Antarctic Peninsula continental shelf that extends to the north and south of Marguerite Bay. The dotted lines inside the proposed study region are the four southern-most across-shelf transects occupied by the Palmer Long-term Ecological Research program. The open squares indicate hydrographic stations that were occupied in late March 1994 by the RVIB Palmer. Proposed sites for mooring locations are designated by A1, A2, A3 and B1, B2 and B3. The 500 m bathymetric contour is shown, as well as other isobaths in km. Stippled regions indicate ice shelves, and the George VI Ice Shelf is denoted by GVIIS.
Fig. 2a. Vertical sections of temperature and salinity constructed from stations occupied in late March 1994 inside Marguerite Bay and extending across the channel in front of the George VI ice shelf (see Fig. 1 for station locations). Data courtesy of S. Jacobs, Lamont-Doherty Earth Observatory.
Fig. 2b. Vertical sections of temperature and salinity constructed from stations occupied in late March 1994 along a transect that extends from the WAP continental shelf into Marguerite Bay (see Fig. 1 for station locations). Vertical and horizontal scales are different from those in Fig. 2a. Data courtesy of S. Jacobs, Lamont-Doherty Earth Observatory.
Fig. 3. Velocity distribution at 200 m obtained from a theoretical circulation model (R. Matano, Oregon State University, work in progress). Velocity vectors show a southward flow along the continental Shelf off Marguerite Bay which then turns and flows into the Bay. The northward flow at the shelf break comprises the offshore portion of the cyclonic shelf gyre.