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PART 1: ENVIRONMENTAL SETTING
INTRODUCTION
Abiotic factors that have been assessed to have biological significance for the
distribution of benthic and pelagic organisms on the Antarctic margin and the
Southern Ocean have been collated for inclusion in this biogeographic atlas. In total,
42 abiotic factors were included as described in Appendix I. Detailed descriptions of
these datasets and their relevance to the biota are discussed in the following sections.
ABIOTIC FACTORS
Geomorphology
Geomorphic features delineate distinct sedimentary and oceanographic environments
that can be related to major habitat characteristics. Such characteristics include sea
floor type (hard versus soft substrate), ice keel scouring, sediment deposition or
erosion and current regimes. On the Antarctic shelf, geomorphic features have been
shown to provide an effective guide to the broad-scale distribution of benthic
communities (Barry et al. 2003, Beaman & Harris 2005, Koubbi et al. 2010, Post et
al. 2010a, Post et al. 2010b).
Geomorphic units were mapped using GIS for the Southern Ocean based on
bathymetry data. The key datasets used were the GEBCO08 bathymetry contours,
which are derived from ship track data, and the ETOPO2 satellite bathymetry (Smith
& Sandwell 1997). The ETOPO2 data comprises a bathymetric grid derived by
inverting satellite gravity measurements controlled by available ship based
bathymetry. This grid has the advantage of providing a depth grid in areas where shipbased data are not available, an important feature for the Southern Ocean region
where ship tracks can be sparse. The limitations of ETOPO2 that affect this study are
that the method has problems resolving features less than 12.5 km across and there are
uncertainties in the absolute accuracy of depth values, particularly for areas of
continental shelf (Marks & Smith 2006). To overcome some of these uncertainties,
particularly for areas of shelf and slope, GEBCO contours were overlain on ETOPO2
to provide a cross check on the location and depth of features.
Based on interpretation of the seafloor bathymetry, 28 geomorphic units were
identified at a scale of about 1: 1-2 million (Figure 1). The geomorphic features were
digitised in ArcGIS by hand as polygons based on the criteria shown in Table 1 {see
\O’Brien, 2009 #383 for more details}. In this classification, the International
Hydrographic Organisation classification of undersea features (International
Hydrographic Organisation 2001) was used as a starting point which was expanded to
accommodate the geomorphic features of the region and to recognise those that are
likely to have differing substrates and influence on oceanography. This approach was
used in order to improve the technique as a predictor of physical conditions that may
influence benthic communities. We have tried to avoid the traditional classification
continental slope and rise because distinguishing between slope and rise around much
of Antarctica is very difficult because of the very gradual slope changes associated
with the large sediment wedges supplied to the margin by glaciation. The geomorphic
mapping around the New Zealand and South American margins is not intended to
replace detailed schemes produced for these regions.
To provide a calibration of the approach, seismic lines from the SCAR Seismic Data
Library System were used to give a profile view of the sea floor and to understand the
geology of the substrate. Seamounts and other rocky features protruding through the
sediment cover were compared with the mapped extent of features from the ETOPO2
data. Highly detailed multibeam bathymetry (50 m grid cell) obtained by the OGS
Explora in 2006 from the continental slope off George V Land {De Santis, 2007
#351} were also overlain on the broader grids as a guide to the geometry of canyons
and slope ridges in that area as a test of the global compilations. This comparison
showed that there was good consistency in the position of these features between the
multibeam and global bathymetric datasets, though the global data lacks the detailed
dimensions provided by the high resolution multibeam data.
Figure 1: Geomorphic features of the Southern Ocean south of 45°S.
Table 1: Description of geomorphic features mapped in this study
Feature
Description
Shelf bank
Banks on the shelf at depths <500 m and therefore subject to
iceberg scouring.
Shelf deep
Areas on the shelf delineated by closed contours deeper than
500 m.
Cross shelf valley
Shelf depressions, commonly shallower than 500 m, that are
connected to the shelf edge by valleys.
Coastal terrane
Inshore areas at depths delimited roughly by the 200 m
contour, and therefore within the photic zone.
Island coastal terrane
Mapped as for coastal terrane around large, rugged islands.
Iceshelf cavity
Areas beneath floating ice tongues.
Upper slope
Upper limit mapped as position at which the rate of change
in gradient is at a maximum, to a lower limit ~2500 m where
the gradient reduces.
Lower slope
Mapped from ~2500 m, or where there is a reduction in
slope gradient, to a lower limit at the point where canyons
are no longer obvious (~3500 m).
Trough mouth fan
Broad aprons of sediment on the upper slope, extending
from the shelf break to 2500-3000 m water depth.
Structural slope
Low relief topographic features formed from underlying
structures, such as basement protrusions, that extend beyond
the lower slope.
Marginal ridges
Large protrusions extending hundreds of meters above the
abyssal plain formed from igneous or basement intrusions.
Marginal plateau
Areas of relatively flat seafloor that extend from the
continental margin, but are separated from the shelf by a
saddle.
Abyssal plain
Smooth, sediment covered area of seafloor.
Contourite drift
Mounds of sediment that rise gently above the surrounding
sea floor, constructed by strong bottom currents.
Rough seafloor
Rugged seafloor consisting of a mixture of hard and soft
substrates reflecting the protrusion of small basement hills
and ridges beneath the sediment surface.
Seamount
Roughly circular areas which rise above the surrounding sea
floor by at least 1000 m.
Seamount ridges
Elongate ridges which are hundreds to thousands of meters
high relative to the surrounding seafloor.
Mid-Ocean Ridge
Valley
Elongate troughs several hundreds of meters deeper than the
rift shoulders, with a pronounced central rift valley.
Feature
Description
Fracture zone
Steep cliffs developed on major crustal fracture zones,
formed during rifting and seafloor spreading.
Trough
Closed elongate depressions more than 4500 m deep and
hundreds of kilometres long. Mostly straight.
Trench
Arcuate areas of very deep ocean floor, more than 5000 m
deep. Formed by subduction of oceanic crust at convergent
plate margins.
Island arc
Arcuate ridges capped with volcanic islands formed adjacent
to subduction zones.
Volcano
Active volcanoes that impinge directly on the marine
environment.
Plateau
Relatively flat regions elevated by at least a few hundred
meters above the surrounding seafloor. The edge is defined
as the line of maximum change in slope above the region
that slopes to the ocean floor.
Plateau slope
Broad sloping regions around the margins of larger plateaus.
Ridge
Elongate ridges that extend from large plateaus and other
features.
Wave-affected Bank
Areas of banks shallower than 200 m which are likely
impacted by large, long period swells and storm waves.
No data
Features could not be mapped due to lack of data. Usually in
areas of heavy sea ice accumulation.
The geomorphic features are potentially powerful predictors for the distribution of the
benthic biota. Depth alone does not describe the seafloor environment experienced by
the organisms. For instance, deep basins on the shelf, which can extend to depths
>1000 m, are a very different environment to regions of similar depth on the
continental slope. Deep shelf basins often contain thick accumulations of muddy
biogenic material produced in the shelf surface waters (Domack 1982, Beaman &
Harris 2005), while similar depths on the slope are often energetic current-swept
environments deeply incised by canyons. The distinction between deep shelf basins
and shelf banks can also delineate between communities affected by iceberg scouring,
with the basins typically below the depth affected by iceberg disturbance (Barnes &
Lien 1988, Dowdeswell & Bamber 2007). Testing of abiotic factors against benthic
communities on the George V shelf indicates that distinct communities occur between
geomorphic units, with sediment type also shown to be a significant factor (Beaman
& Harris 2005, Koubbi et al. 2010, Post et al. 2010a, Post et al. 2010b). The
distribution of Gutt’s (2007) core shelf communities can also be broadly
approximated by shelf geomorphology and oceanography, with communities such as
the Mobile Deposit Feeder Community (MDC) and Infauna Community (INC)
confined to areas where modern fine sediment can accumulate, such as is found
within shelf depressions. Seamounts are another feature that have been shown to be
biologically significant, supporting rich benthic communities with a high number of
endemic species (Richer de Forges et al. 2000). Other features which potentially
provide unusual substrates and modify local ocean currents, such as marginal ridges
and plateaus, may similarly support distinct and diverse benthic communities.
Bottom sediments
Numerous studies have demonstrated the significance of substrate properties to the
distribution of benthic biota. The distinction between hard and soft substrates has
been associated with distinct species assemblages (Williams & Bax 2001, Beaman &
Harris 2007), while the grainsize composition of the sediments, and particularly the
mud content, has been shown to have a strong correlation to the benthic communities
in a range of settings. These regions include the Antarctic margin (Beaman & Harris
2005, Koubbi et al. 2010, Post et al. 2010a), the NW Atlantic (Thouzeau et al. 1991,
Kostylev et al. 2001), the Great Barrier Reef (Beaman & Harris 2007, Pitcher et al.
2007) and the Gulf of Carpentaria, NE Australia (Long et al. 1995, Post et al. 2006).
Unfortunately sediment composition was not explicitly included in the modelling for
the Biogeographic Atlas as there are no suitable datasets compiled at the scale of the
region. However, layers such as water depth and geomorphic features distinguish
sedimentary environments at a broad scale.
Last glacial maximum grounding line
The Antarctic shelf fauna have been strongly influenced by the expansion and retreat
of the Antarctic ice sheets on glacial–interglacial time scales (Clarke & Crame 1992,
Thatje et al. 2005). The expansion of the ice sheets across the continental shelf during
glacial periods largely eradicated the available shelf habitats and evidence suggests
that these shelf fauna may have migrated to either the Antarctic slope or the deep sea
(e.g. Zinmeister & Feldmann 1984, Brandt 1991, Brey et al. 1996). In some regions,
however, shelf fauna may have found refugia during glacial periods beneath floating
ice shelves or small ice-free areas in regions where grounded ice did not advance to
the edge of the continental shelf. Geological evidence suggests that during the last
glaciation the ice sheets did not completely ground to the shelf edge in the western
Ross Sea (Licht et al. 1996, Domack et al. 1998, O'Brien et al. 1999, Shipp et al.
2002), Prydz Bay (Domack et al. 1998, O'Brien et al. 1999) or George Vth Land
(Beaman & Harris 2003).
By including the last glacial grounding line in the model we aim to test whether this
parameter captures variability in the shelf fauna, particularly in areas where shelf
refugia were available. The glacial history on the shelf can also indicate the
sedimentary environments, with deep basins gouged out by the advancing glaciers
forming depocentres for the accumulation of thick sedimentary deposits (e.g. Domack
1982, Beaman & Harris 2005). These deep basins are potential habitats for mobile
deposit feeders and infauna, as observed on the George V Shelf (Beaman & Harris
2005, Post et al. 2010a). The last glacial grounding line has been mapped based on
Anderson et al. {, 2002 #367}, but we have modified its extent in areas where it
exceeded the position of the shelf break as mapped on the geomorphology layer to
maintain consistency between these layers.
Bathymetry and slope (More contributions to this section welcome, especially in
terms of how water depth may affect pelagic organisms???)
For this atlas, bathymetry data has been compiled from a combination of satellite
altimetry and ship depth soundings contained in the ETOPO2 grid (Smith & Sandwell
1997). Slope data has been derived from this grid based on the equation of Burrough
and McDonnell (1998). The Southern Ocean is comprised of three ocean basins: the
Pacific, the Indian and Atlantic basins. These deep basins occur mostly at depths that
exceed 3000 m, and are separated by submarine ridges, plateaus and the island chain
of the Scotia Arc. Around the Antarctic margin the continental shelf is unusually
deep, averaging 450 m, and exceeding 1000 m at times (Clarke & Johnston 2003).
The depth of the shelf largely reflects over-deepening of inner-shelf basins by glacial
erosion, as well as depression of the crust by isostatic loading. The location of the
shelf break varies considerably, located at depths ranging from 200 - 1000 m,
depending on the trajectory of glacial basins and the adjacent shallow banks. The
average width of the Antarctic shelf is ~125 km, almost twice that of shelves
elsewhere in the world. This large average width is mostly due to the occurrence of
large glacial embayments, such as in the Ross and Weddell seas, which form shelf
seas spanning up to 1000 km from the continental margin to the shelf edge. These
embayments are largely covered by floating ice shelves. The narrowest parts of the
shelf occur off Dronning Maud Land.
Water depth has been shown to be a strong delimiter of biological communities in
various settings around the Antarctic margin. Water depth shows a strong correlation
to the distribution of demersal fish and benthic communities on the George V shelf
and slope (Beaman & Harris 2005, Koubbi et al. 2010, Post et al. 2010a, Post et al.
2010b), and benthic communities in the Ross Sea (Barry et al. 2003) and the Weddell
and Lazarev Seas (Gutt & Starmans 1998). Water depth can be linked to the strength
of bottom currents, and hence deposition of organic matter (e.g. as shown for the
Ross, Weddell and Lazarev Seas), and on the Antarctic margin also reflects long-term
sedimentation patterns established following previous glaciations. Deep basins
formed by the expansion of glaciers towards the shelf edge during past glacial periods
now act as depocenters for the accumulation of fine grained siliceous mud and ooze
sourced from the productive shelf waters (e.g. Harris et al. 2001), while shelf banks
have been preserved as relatively shallow areas through the bypass of the mobile
icesheet around these broad features.
Depth on the shelf also largely defines the impact of iceberg scouring, with icebergs
commonly grounding and scouring the seafloor over the shelf banks to depths of up to
500 m (Barnes & Lien 1988, Dowdeswell & Bamber 2007). Coastal areas are also
subject to ice disturbance. Within the subtidal zone, changes in the diversity,
abundance and biomass of benthic assemblage are associated with the degree of
disturbance by sea ice, growlers and anchor ice (Barnes 1995). At Signy Island
frequent disturbance by ice at depths <15 m has been shown to significantly restrict
development of the macrofauna and macroflora communities. Within the sublittoral
zone, the mechanical effects of ice disturbance are less significant, but light levels are
affected by sea ice concentration and snow cover (Thrush et al. 2006).
Sea ice (Alex Fraser, Klaus Meiners, Ben Raymond)
Figure 1. This image will need a reference- probably Spreen et al., 2008.
Sea ice data were compiled from AMSR-E satellite-derived estimates of daily sea ice
concentration at 6.25km spatial resolution (Spreen et al. 2008) and a longer data set
from SMMR and SSM/I satellite instruments at 25km resolution (Cavalieri et al.
1996). Two products were derived from these data: (i) the proportion of time the
ocean was covered by sea ice of concentration 85% or higher, (ii) the distance from
the mean maximum extent of sea ice. The former conveys features such as polynyas
and the spatial variation in duration of sea ice cover, while the latter provides a broad
indication of position relative to the sea ice zone. These data are complemented by
others, including depth and summer chlorophyll-a.
Sea ice is a major feature of the Antarctic marine realm, and has profound and diverse
effects on its ecosystems. The extent of sea ice ranges from approximately 3 million
km2 in February to 18 million km2 in September (Cavalieri and Parkinson, 2008),
making it one of the largest seasonal physical changes in surface conditions anywhere
on the planet. Figure $FIGURE WITH MONTHLY CLIMATOLOGY$ shows the
annual cycle of sea ice extent around the Antarctic coast. The sea ice extent seasonal
cycle is characterised by slow and steady growth from March to September, followed
by relatively rapid decay, particularly in December and January. Minimum extent in
most regions is typically achieved in February or early March, with maximum extent
occurring in September (Gloersen et al., 1992).
Sea ice can be broadly classified into pack ice, which moves horizontally under the
influence of ocean currents and winds, and landfast ice, or fast ice, which is
mechanically locked onto coastal features, fixed to grounded icebergs, or grounded
upon shoals (WMO, 1970). In East Antarctica, fast ice typically forms "upstream" (on
the east side) of protrusions into the westward Antarctic Coastal Current (e.g.,
grounded icebergs, coastal promontories), with a coastal polynya often located on the
corresponding "downstream" (western) side (Fraser et al., 2011). Flaw leads (narrow
strips of open water) typically form when divergent ice conditions occur at the shear
zone between fast and pack ice. Since fast ice is often contiguous with the coast, it has
implications for fauna whose habitat includes the near-coastal zone.
Polynyas are regions of low sea ice concentration enclosed within higherconcentration sea ice (Barber and Massom, 2007). Coastal polynyas typically form in
coastal regions where strong, persistent, offshore winds (e.g. katabatic winds) exist.
The strong, offshore wind advects pack ice away from the coast, exposing open water.
Offshore polynyas can also exist within the pack ice zone (e.g., the recurring
Cosmonaut Polynya, at ~40° E). These are sustained either by divergent
atmospheric/oceanic flow, or an input of heat from the ocean, causing the ice to melt
(Barber and Massom, 2007). Polynyas are important as areas of elevated production
(Arrigo & van Dijken 2003) and also because they provide access to the water for
higher predators such as penguins and seals.
Antarctic sea ice affects pelagic primary production and biomass in multiple and
complex ways. Sea ice, especially when snow-covered, reduces the amount of light in
the under-ice pelagic realm by 1–2 orders of magnitude, thus limiting plankton
growth (Eicken 1992, Fritsen et al. 2011). Sea ice also serves as a temporal reservoir
for nutrients, for example by accumulating the micro-nutrient iron (Fe) during sea ice
formation and releasing it into Fe-depleted surface waters during ice melt in spring
and early summer (Lannuzel 2007, 2010, Wright et al. 2011). In addition, iceassociated microalgae released from the ice are considered to serve as inoculum for
spring phytoplankton blooms (Lizotte 2001, Raymond et al. 2009). The release of
nutrients and algae in combination with meltwater-induced stratification of surface
waters can trigger so-called ice-edge blooms which can result in strong increase in
regional Southern Ocean primary production and export (Smith & Nelson 1985,
Buesseler et al. 2003, Arrigo & Thomas 2004, Smith & Comiso 2008). However, the
formation of ice-edge blooms is moderated by other processes including currentgenerated fronts, polynya expansion, sea ice advection and wind stress patterns (Fitch
& Moore 2007, Smith & Comiso 2008). Where ice-edge blooms occur, they generally
progress temporally and spatially across the marginal sea ice zone and provide an
important food-source for pelagic herbivores such as Antarctic krill Euphausia
superba (Nicol et al. 2000). The sea ice edge is also generally a productive foraging
ground for higher predators such as humpback whales {Thiele, 2004 #16920}{Gales,
2009 #16624} and fur seals {Boyd, 2002 #16915}. Diving predators can access
under-ice communities of fish and invertebrate prey, which may be more abundant
than in open waters. Increased krill biomass has been reported under the edge of sea
ice, compared to adjacent open water {Brierley, 2002 #16899}.
Importantly, Antarctic sea ice serves as a substrate for various groups of organisms
including bacteria, algae, fungi, protozoans, and metazoans, forming distinct
biological communities at the bottom, the interior and surface of sea ice floes (Arrigo
et al. 2010, Meiners et al. 2009, 2011). Sea ice algae have been estimated to
contribute significantly to overall primary production of the Antarctic sea ice zone
with estimates ranging up to 25% of overall primary production of that zone (Arrigo
and Thomas 2004). These ice algae provide an important early-season food source for
pelagic herbivores during late winter and early spring, when food in the water-column
is scarce.
The lifecycle of Antarctic krill appears to be particularly closely linked to sea ice. In
the southwest Atlantic sector, for example, it has been shown that there is a
correlation between the extent of winter sea ice and the subsequent recruitment of
krill (e.g. Atkinson et al. 2004). It has been hypothesized that this apparent direct link
between krill recruitment and sea ice extent/duration arises because krill, particularly
krill larvae, rely on the microbial and diatom communities that grow on the underside
of ice for food (Smetacek et al. 1990). Krill have been observed feeding on ice algae,
particularly in late winter (Stretch et al. 1988). Adult female krill that feed well under
ice in winter enter the spring in a well-nourished condition and spawn successfully
(Quetin and Ross 2003). Juvenile krill that are able to spend their first winter under
sea ice have an increased chance of survival. This is because juveniles are able to feed
on ice algae (Frazer et al. 1997), whereas phytoplankton concentrations in the open
ocean are probably insufficient to sustain juvenile krill. Unlike adults, juvenile krill
are not able to tolerate periods of starvation (O’Brien et al. 2011). Thus, multiple
winters of reduced sea ice may eventually lead to a reduction in krill biomass.
For higher predators such as penguins, seals, flying seabirds, sea ice provides habitat
and foraging grounds, and is important for reproduction, and for moulting for birds
and seals (Ainley et al. 2003). However, the effects of sea ice variability on biota are
again complex. For example, emperor penguins Aptenodytes forsteri rely on stable
fast ice conditions for breeding and rearing chicks, but extensive fast ice (which the
birds must cross in order to access open water for foraging) can impact breeding
success (Massom et al. 2009). Adélie penguins Pygoscelis adeliae nest on ice-free
land, but can be similarly affected by coastal fast ice (Emmerson & Southwell 2008).
Polynyas are therefore important in determining the colony locations of these species
(Ainley et al. 2008).
The impact of ice on the benthic fauna can be, locally, very significant because most
organisms can not escape, reinvade or drift back into their usual habitat, unlike the
nekton and plankton. Ice impacts the benthic fauna both directly and indirectly, and
the impacts are most extreme along the coast. The result is that habitats in these
regions tend to have low species diversity and often low abundance, and are
dominated by those few animals and algae that are able to withstand the harsh
conditions. On the Antarctic coast, limpets tend to be one of the best survivors, with
high abundances despite the threat of ice scour (Gutt, 2001). In McMurdo Sound, ice
scouring is associated with the depth zonation of macroalgal growth (Miller and
Pearse, 1991). The algae Iridea cordata occurs from 3-10m, Phyllophora Antarctica
dominates from 6-18 m and this is replaced by the encrusting coralline algae
Phymatolithon foecundum to depths of ~60 m.
The formation of anchor ice directly impacts on benthic fauna, with a 2 m2 piece of
ice able to lift 25 kg of sediment, including any epifauna that were living within its
radius (Dayton et al., 1970). Temporal variations in the formation of anchor ice in
McMurdo Sound reveal the impact of anchor ice on the benthic community
composition. During the 1960s and the early 1980s there were high levels of anchor
ice disturbance to depths of 30 m in the Sound, and at this time the sponge
Homaxinella and its predators were rare along the shoreline (Dayton, 1989).
However, during the early 1970s changes in oceanographic conditions resulted in very
low levels of anchor ice disturbance and massive recruitment of Homaxinella, which
subsequently covered up to 80% of the seafloor.
The indirect impacts of ice cause variations in community composition associated
with the duration of sea ice cover, as observed at Signy Island {Barnes, 1995 #371}
and in the Davis Harbour region {O'Brien, 2012 #516}.In the Davis region, areas
which are ice-free for significant periods of the year have abundant algal growth of
Himantothalus and Iridea on hard substrates, with relatively low abundances of
attached invertebrates. Areas of more persistent sea ice, by contrast, have a
dominance of attached invertebrates on areas with rocky substrates. The frequency of
ice disturbance decreases with depth, resulting in increased diversity. At Signy Island,
diversity significantly increased at depths greater than 25 m (Barnes,1995).
Figure 2. from: Massom, R.A., A. Davidson , S. O'Farrell, N. Adams, L. Armand, K.M. Meiners,
S. Wright, K. Arrigo, M. Curran, P. Heil & B. Pasquer, In prep. Observing & modelling
Antarctic sea ice habitats: Knowledge gaps and future research directions. For submission to
Progress in Oceanography
Sea ice and climate change
In contrast to the recent rapid decline in sea ice extent observed in the Arctic (Stroeve
et al., 2008), a slight (but statically-significant at the 95% confidence level) positive
trend of approximately +1.2 %/decade is observed in overall Antarctic sea ice extent
over the period 1979-2008 (Cavalieri and Parkinson, 2008; Comiso, 2010). However,
differing trends in sea ice extent are observed on a regional basis, depending largely
on the strength and phase of various modes of atmospheric variability (e.g., the
Antarctic Circumpolar Wave, the Pacific South American pattern, the quasi-stationary
Zonal Wave 3, the Southern Annular Mode, and others, see Comiso 2010 for
references). For the purposes of trend and variability analysis, Antarctic sea ice is
generally sub-divided into five roughly equal areas, as shown in Figure $FIGURE
SHOWING REGIONS$ (Gloersen et al., 1992). The only two statistically-significant
regional trends include a decrease in the Bellingshausen/Amundsen Seas sector (60° 130° W) of -7.1 %/decade; and an increase in the adjacent Ross Sea sector (130° W -
160° E) of +4.9 %/decade. These strong trends are likely driven by atmospheric
forcing, but a detailed analysis is outside the scope of this review (see Massom et al.,
2006; Comiso 2010 for more details). The remaining sectors (Weddell Sea, 20° E 60° W; Indian Ocean, 20° - 90° E; and Western Pacific Ocean, 90° - 160° E) all show
slight (but non-significant) positive trends over the 1979-2008 satellite period
(Comiso 2010).
The latitudinal seasonal maximum/minimum extent of fast ice cover varies by sector.
In the Indian Ocean and Western Pacific Ocean sectors, sea ice typically extends to
~60° S in September, and retreats to the coast in many regions in February. In the
Weddell Sea sector, a persistent oceanic gyre leads to upwelling of cold water,
leading to relatively extensive year-round sea ice. A similar gyre exists in the Ross
Sea sector, leading to generally northward advection and divergent conditions in the
western Ross Sea. The Bellingshausen/Amundsen Seas sector is characterised by
relatively poor vertical ocean mixing, leading to persistent ice cover even during
summertime.
Sea ice not only responds rapidly to oceanic (e.g., ocean heat flux, waves, swell,
ocean currents), atmospheric (e.g., air temperature, winds) and radiative
(shortwave/longwave flux) forcing, but also influences all three components via
complex feedback mechanisms (see Thomas and Dieckmann, 2010, and references
therein). It follows that the future state of Antarctic sea ice will depend strongly on the
future state of the climate. Climate models generally project increases in Antarctic
surface air temperature, snowfall, storminess and waviness, leading to a general
decrease in pack ice cover (Turner et al., 2009; Bracegirdle et al., 2008; Bentley et al.,
2007). The response of fast ice is not well understood, though a projected increase in
iceberg numbers may lead to higher fast ice extent in some regions.
Hydrography and oceanography (Eberhard Fahrbach))

relevant layers: SST, mixed layer depth climatology, locations of fronts
Surface and water column temperature and salinity (Philippe)

relevant layers: 0m, 50m, 200m, 500m T and Salinity
Surface and water column nutrients and oxygen (Philippe?)

relevant layers: 0m, 50m, 200m, 500m NOx, Si, O2
Bottom temperature (Huw)

relevant layer: floor temperature
Some Antarctic marine organisms are sensitive to temperature, and so even small
changes in seabed water temperatures may have important consequences for patterns
in benthic biodiversity {Clarke, 2009 #16924}. Regional differences in circulation,
topography, oceanography, and sea ice production generate spatial variability in
seabed temperatures. In particular, seasonal air-sea fluxes and ice production
generates dense, cold water in autumn and winter that reaches to the shelf bottom, and
in some locations down the continental slope. Other locations, such as the western
Antarctic Peninsula, are primarily influenced by the large-scale circulation of the
Antarctic Circumpolar Current (ACC), which brings warmer, nutrient-rich waters
onto the continental shelf. Hence the seabed temperatures to the west of the Antarctic
Peninsula are considerably warmer than the remainder of the continental shelf areas.
Bottom nutrients?? (Huw?)
Near bottom currents??
Pelagic regionalisation
Regionalisation analyses are used to classify the environments across a region into a
number of discrete classes, thereby providing a spatial and environmental subdivision
of the study area. The classification is made on the basis of a number of
environmental datasets, thereby providing an integrated description of the types of
habitats described by those data. These types of analyses are typically undertaken as
part of spatial management and modelling activities. This section presents the
circumpolar primary pelagic regionalisation from Raymond (2011). It provides an
update to earlier pelagic regionalisation work (Grant et al. 2006), and follows the
advice from the Scientific Committee in 2010 that such analyses should consider
depth, water mass characteristics, and dynamic ice behaviour (SC-CAMLR-XXIX
2010).
Methods
The methods for regionalisation follow those of Grant et al. (2006) and the CCAMLR
Bioregionalisation Workshop (SC-CAMLR-XXVI 2007). Briefly, a non-hierarchical
clustering algorithm was used to reduce the full set of grid cells to 250 clusters. These
250 clusters were then further refined using a hierarchical (UPGMA) clustering
algorithm. The first, non-hierarchical, clustering step is an efficient way of reducing
the large number of grid cells, so that the subsequent hierarchical clustering step is
tractable. The hierarchical clustering algorithm produces a dendrogram, which can be
used to guide the clustering process (e.g. choices of data layers and number of
clusters) but is difficult to use with large data sets. Analyses were conducted in
Matlab (Mathworks, Natick MA, 2011) and R (R Foundation for Statistical
Computing, Vienna 2009).
Earlier work (Grant et al. 2006) used depth, sea surface temperature (SST), and
subsurface (200m) nutrient data. The nutrient data were both spatially smoothed
(based on relatively sparse historical CTD data) and missing in near-coastal shallow
areas. Here, three variables were used as input variables: summer climatological SST,
depth, and the proportion of time covered by sea ice (Appendix I). Sea surface
temperature was used as a general indicator of water masses and of Southern Ocean
fronts (Moore et al. 1999, Kostianoy et al. 2004). Sea surface height (SSH) from
satellite altimetry is also commonly used for this purpose (e.g. Sokolov & Rintoul
2009), and may give front positions that better match those from subsurface
hydrography than does SST. However, SSH data has incomplete coverage in some
near-coastal areas (particularly in the Weddell and Ross seas) and so in the interests of
completeness, SST was used here.
During the hierarchical clustering step, singleton clusters (clusters comprised of only
one datum) were merged back into their parent cluster (5 instances, in cluster groups
2, 3, 8, and 13). Additionally, two branches of the dendrogram relating to temperate
shelf areas (around South America, New Zealand, and Tasmania) were merged to
reduce detail in these areas (since such detail is largely irrelevant in the broader
Southern Ocean context).
Results
20 environmental types were apparent in the results (Figures 3–5), and are
summarised in Table 1.
Figure 3. Spatial distributions of the 20 cluster types from the regionalisation analyses.
Figure 4. Dendrogram from the hierarchical clustering step. The dotted red line shows the level
at which the dendrogram was cut to produce the groups. Note that clusters 19 and 20 represent
merged clusters, to reduce detail in temperate shelf areas.
Figure 5. Properties of the 20 cluster types.
Cluster
number
1
Description
Polynya margins on the continental shelf, the South Orkneys plateau, and
areas off Adelaide and Biscoe Island in the West Antarctic Peninsula.
Moderately shallow (to ~1000m) with ice cover ~20–50% and SST <2°C.
Area (x1000
km2)
287
2
3
4
5
6
7
8–11
12
13,14
15
16
17,18
Polynyas on the continental shelf, as well as areas off the Danco Coast of the
Peninsula and the South Orkney Islands, and part of Banzare Bank. Low ice
cover (~0–20%) and cold sea surface temperatures (<2°C).
Shallow shelf areas with ~25–60% ice cover. Restricted distribution,
generally limited to East Antarctica.
Shallow areas with high ice cover (~75–95%). Patchy distribution scattered
around the continental shelf.
Shelf areas with almost perennial ice cover (~75–100%).
Similar to 7, but shallower and with lower ice cover. Widely but sparsely
distributed around the continental shelf.
Moderate depths (~200–1000m) and ice cover (~50–75%). Many areas
correspond to general regions around polynyas (see e.g. Arrigo & van
Dijken, 2003). Also areas of the southern Scotia Arc.
Sea ice zone. Clusters 8–11 form an approximately latitudinal, deep water
continuum of increasing ice cover and decreasing SST. The northernmost
limit (of cluster 10) is generally just south of the mean maximum winter sea
ice extent.
Moderate depth (~1000–2500m) and sea ice cover (~40%). Restricted to
parts of the southern Scotia Arc, and isolated pockets north of the Balleny
Islands and off the West Ice Shelf.
13: Shallow (~200–1000m) parts of the northern Kerguelen, Crozet, and
South Georgia plateau areas, Conrad Rise.
14: Deeper (~500–2000m) parts of the same plateaux, also Bouvetøya and
the northern tip of the southern Kerguelen plateau.
Deep oceanic waters, encompassing approximately the southern Antarctic
Circumpolar Current front and the Polar Front.
Deep oceanic waters, bounded approximately on the north by the
subantarctic front.
Temperate waters
19
Outer areas of the South American, New Zealand, and Tasmanian shelves,
and scattered temperate banks.
20
Broad distribution around the South American, New Zealand, Tasmanian,
and Crozet shelves. Shallow, ice-free, and with warm SST (~10–20°C).
Table 1. Summaries of the 20 cluster types
167
33.1
42.8
1010
165
1030
8: 1670
9: 5140
10: 3430
11: 3570
Total: 13800
48.9
13: 398
14: 345
Total: 743
14500
16800
17: 17900
18: 6560
Total: 24400
1420
1500
Discussion
The results of these analyses are broadly similar to the 2006 primary regionalisation,
with roughly concentric bands in open ocean areas, corresponding to the Southern
Ocean fronts, and increased heterogeneity in shallower and near-coastal areas. The
open ocean banding differs in detail between the two analyses. These differences are
due in part to the different SST data sets used (1985–1997 Pathfinder data for the
2006 analyses, and 2002–2010 MODIS Aqua data here), as well as the fact that the
open ocean regions experience no sea ice cover. Thus, cluster patterns in these areas
in the current analyses are driven solely by differences in depth and SST. Previously,
the sub-surface nutrient data would also have contributed to the open ocean structure.
The Weddell Gyre, which was previously driven strongly by patterns in nutrient data,
is now much less apparent.
The current results show an increased level of detail in shallow and near-coastal areas,
because subsurface nutrient data (missing in many near coastal areas) were replaced
by sea ice data, providing previously-missing spatial structure, particularly over the
continental shelf. Previously, the Antarctic shelves were represented by a single class.
These regions now have considerable additional substructure (i.e. clusters 1–7). The
clusters representing polynyas (1 and 2; see examples in Figure 5) show spatial
distributions closely resembling the polynyas of Arrigo & van Dijken (2003).
The previous Kerguelen, Heard and McDonald Islands cluster is similar to the current
cluster 13, which is now accompanied by a neighbouring class representing deeper
areas of these plateaux (14). The previous Chatham Rise and Inner Shelf classes are
still present, but merged into cluster 20. The Campbell Plateau and South American
shelf class here (19) is largely identical to its 2006 counterpart.
Figure 6. Locations of polynyas in East Antarctica (top; reproduced from Arrigo & van Dijken
2003). Clusters 1 and 2 (brown) show good correspondence with these locations (bottom).
A number of regionalisation analyses at smaller scales have recently been conducted
(e.g. Constable et al. 2010, Sharp et al. 2010, Koubbi et al. 2011). Such regional-scale
analyses are able to address smaller-scale structure and processes than a circumpolar
analysis, and can make use of data with regional coverage that would be extremely
difficult to include at a circumpolar scale. Thus, the general patterns in the current
results should be similar to those derived at regional scales, but finer-scale details will
likely differ.
The pelagic regionalisation of the Ross Sea region conducted by Sharp et al. (2010) is
shown in Figure 7(a), with the matching subset of the current results shown alongside.
The fine-scale regional analyses separated the continental shelf and off-shelf areas
and conducted independent classification analyses for the two areas (Sharp et al.
2010). Analyses were based on water temperature, salinity, depth, and sea ice
information, and identified 18 bioregions. Despite the differences in variables and
spatial scale, the results from the circumpolar analyses are broadly similar, with a
clear distinction between the shelf and offshore areas.
(a)
(b)
Figure 7. (a) Pelagic regionalisation of the Ross Sea region from Sharp et al. (2010). (b) Subset of
the current results, for the same region. Black lines show (from north to south) the Polar Front,
the southern Antarctic Circumpolar Current front, and the southern boundary of the ACC, as
defined by Orsi et al. (1995). Yellow lines in (a) show CCAMLR small scale research units.
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