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, published 29 January 2007, doi: 10.1098/rstb.2006.1957362 2007 Phil. Trans. R. Soc. B
Cunningham and A.H Fleming
Rodhouse, P Enderlein, A.G Hirst, A.R Martin, S.L Hill, I.J Staniland, D.W Pond, D.R Briggs, N.J
P.GG.A Tarling, M.A Collins, J Forcada, R.S Shreeve, A Atkinson, R Korb, M.J Whitehouse, P Ward,
E.J Murphy, J.L Watkins, P.N Trathan, K Reid, M.P Meredith, S.E Thorpe, N.M Johnston, A Clarke,
review of large-scale links in a krill centred food web
Spatial and temporal operation of the Scotia Sea ecosystem: a
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Spatial and temporal operation of the
Scotia Sea ecosystem: a review of lar ge-scale
links in a krill centred food web
E. J. Murphy
*
, J. L. Watkins, P. N. Trathan, K. Reid, M. P. Meredith,
S. E. Thorpe, N. M. Johnston, A. Clarke, G. A. Tarling, M. A. Collins,
J. Forcada, R. S. Shreeve, A. Atkinson, R. Korb, M. J. Whitehouse, P. Ward,
P. G. Rodhouse, P. Enderlein, A. G. Hirst, A. R. Martin, S. L. Hill,
I. J. Staniland, D. W. Pond, D. R. Briggs, N. J. Cunningham and A. H. Fleming
British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Roa d,
Cambridge CB3 0ET, UK
The Scotia Sea ecosystem is a major component of the circumpolar Southern Ocean system, where
productivity and predator demand for prey are high. The eastward-flowing Antarctic Circumpolar
Current (ACC) and waters from the Weddell–Scotia Confluence dominate the physics of the Scotia
Sea, leading to a strong advective flow, intense eddy activity and mixing. There is also strong
seasonality, manifest by the changing irradiance and sea ice cover, which leads to shorter summers in
the south. Summer phytoplankton blooms, which at times can cover an area of more than 0.5 million
km
2
, probably result from the mixing of micronutrients into surface waters through the flow of the
ACC over the Scotia Arc. This production is consumed by a range of species including Antarctic krill,
which are the major prey item of large seabird and marine mammal populations. The flow of the ACC
is steered north by the Scotia Arc, pushing polar water to lower latitudes, carrying with it krill during
spring and summer, which subsidize food webs around South Georgia and the northern Scotia Arc.
There is also marked interannual variability in winter sea ice distribution and sea surface
temperatures that is linked to southern hemisphere-scale climate processes such as the El Nin
˜
o–
Southern Oscillation. This variation affects regional primary and secondary production and
influences biogeochemical cycles. It also affects krill population dynamics and dispersal, which in
turn impacts higher trophic level predator foraging, breeding performance and population dynamics.
The ecosystem has also been highly perturbed as a result of harvesting over the last two centuries and
significant ecological changes have also occurred in response to rapid regional warming during the
second half of the twentieth century. This combination of historical perturbation and rapid regional
change highlights that the Scotia Sea ecosystem is likely to show significant change over the next two
to three decades, which may result in major ecological shifts.
Keywords: Scotia Sea; ecosystem; advection; Antarctic krill; heterogeneity; interannual variability
1. INTRODUCTION
Analysis of the operation of ocean ecosystems requires
an understanding of how the structure of the ecosystem
is determined by interactions between physical,
chemical and biological processes. Such analysis
needs to consider the interactions across a wide range
of spatial (approx. 10 m–10 000 km) and temporal
(minutes to centuries) scales, and across all trophic
levels (primary producers to top predators; Murphy
et al. 1988; Angel 1994; Werner et al. 2004). There are,
however, few areas of the global ocean where there is
sufficient knowledge to achieve such an integrated
analysis (deYoung et al. 2004). Circulation patterns of
the major ocean gyres, such as the North Atlantic and
Pacific Oceans, involve movement of water masses
through very different climatic regimes which favour
distinctly different groups of organisms (Longhurst
1998). Generating comprehensive views of the
operation of oceanic ecosystems is complicated as a
result of such heterogeneity in species distribution and
ecosystem structure (Murphy et al. 1988; Levin 1990;
Longhurst 1998).
In contrast to other areas of the global ocean, the
Southern Ocean has two major characteristics that
make the development of large-scale integrated
analyses a realistic possibility. The first is a circumpolar
current with relatively constant environmental con-
ditions along the streamlines, and the second is a
simple food-web structure (Everson 1977; Hempel
1985a). A circumpolar eastward circulation that occurs
within a restricted latitudinal belt dominates the flow
(between approx. 508 and 658 S; Orsi et al. 1995). This
current system, the Antarctic Circumpolar Current
Phil. Trans. R. Soc. B (2007) 362, 113–148
doi:10.1098/rstb.2006.1957
Published online 30 November 2006
One contribution of 8 to a Theme Issue ‘Antarctic ecology: from
genes to ecosystems. I.’
* Author for correspondence (e.murphy@bas.ac.uk).
113 This journal is q 2006 The Royal Society
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(ACC), transports around 130–140 Sv (million m
3
s
K1
)
eastward at Drake Passage (Cunningham et al. 2003), but
shows significant atmospherically forced variability on
time-scales from days to years (Hughes et al.2003;
Meredith et al.2004b). This flow around the continent
results in relatively consistent surface summer tempera-
tures south of the Polar Front (PF) of approximately
4–58C in the north and 0 to K18C in areas just south of
the Southern Boundary (SB) of the ACC (Sievers &
Nowlin 1984; Whitworth & Nowlin 1987; Moore et al.
1997, 1999; Brandon et al. 2004).
Across the circumpolar current, there are therefore
relatively consistent environmental conditions within
which the ecosystem operates. Within this flow regime,
the other major factor that determines the structure of
the ecosystem is the marked seasonality of polar
environments (Clarke 1988). Changes in solar irra-
diance and associated fluctuations in sea ice cover
result in strong seasonal variation in upper ocean
temperature and light levels (Okada & Yamanouchi
2002). This seasonal variation dominates the operation
of Southern Ocean ecosystems in a number of ways.
Temperature changes in surface waters as a result of
fluctuations in irradiance have direct impacts on the
physiological processes of many marine species, and
temperature tolerances are a major determinant of the
geographical boundaries of species distributions
(Mackintosh 1936, 1960; Hempel 1985b; Longhurst
1998; Peck et al. 2004; Peck 2005). However, for most
species, it is marked seasonal fluctuations in the
availability of food that drives key biological processes
(Laws 1983; Clarke 1985a; Peck et al. 2005). During
summer, there is a short period of only two to three
months (or less in the highest latitudes) when
conditions are favourable for primary production.
The resulting phytoplankton blooms are often domi-
nated by species of large diatoms (Laws 1983; Clarke
1985a; Hempel 1985a,b; Clarke & Leakey 1996;
Smetacek et al. 2004). As with the rest of the world
ocean, microbial systems are a key feature of Southern
Ocean ecosystems and can dominate the processes of
production in many regions and through the winter
months (Smetacek et al. 2004). The seasonality
propagates through the food web, so consumers must
be able to make full use of the short summer periods to
breed and survive during the low production periods of
winter (Laws 1983; Clarke 1985a). Such physical and
biological conditions favour the two extremes of
smaller species that can develop quickly in response
to favourable conditions and large-bodied predators
that are often highly mobile. The smaller microbial and
meso-planktonic species opportunistically use available
resources and have strategies in place to survive periods
of low production. The large, mid- and higher trophic
level species, such as penguins and seals, have relatively
long lifespans (often greater than 10 years) and are
highly mobile (foraging over hundreds to thousands of
kilometres), and many move away from the area during
the periods of low production (Clarke 1985b; Croxall
1992). The extreme seasonality in production also
means that there is little capacity to build-up long food
chains involving many steps to the highest trophic levels
(Everson 1977; Clarke 1985b). Southern Ocean
ecosystems therefore have an apparently simple
structure, dominated by short food chains that also
make them tractable for analysing large-scale system
operation (Everson 1977; Laws 1983; Hempel 1985a;
Clarke 1985b). The dominant food-web pathway from
diatoms–zooplankton–predators also provides an
important focus for studying end-to-end ecosystem
processes linking primary production and highest
trophic level predators.
Although there is a consistency in the structure and
composition of the Southern Ocean ecosystem, it is not
operationally homogeneous (Hempel 1985a). South of
the ACC exist large subpolar gyres in the Ross and
Weddell Seas, and there are often complex current
systems at the shelf break and on the shelf of
Antarctica, such as the Antarctic coastal current
(Hempel 1985a; Orsi et al. 1995). The flow of the
ACC is strongly topographically constrained in many
places around its circulation, with the greatest restric-
tion occurring at Drake Passage. Here, the ACC flows
through a ‘choke point’ between South America and
the Antarctic Peninsula and emerges into the Scotia
Sea. The Drake Passage and Scotia Sea region is
therefore an important area in the connection of the
global ocean (Cunningham et al. 2003). To the east of
Drake Passage, the ACC encounters one of the biggest
topographic barriers in the Southern Ocean, the Scotia
Arc, which forms the northern, southern and eastern
boundaries of the Scotia Sea (figure 1). The southern
section of the Scotia Sea also receives input of waters
from the shelf of the Antarctic Peninsula and the
Weddell Sea (Whitworth et al. 1994).
The combination of strong flow and mixing in an area
of rugged bathymetry makes the Scotia Sea one of the
most physically energetic regions of the Southern Ocean.
As a result, the Scotia Sea ecosystem has different
operational characteristics to those in other regions of
the ACC. Over much of the oceanic Southern Ocean, the
concentration of chlorophyll is low even though macro-
nutrient concentrations are high (termed high-nutrient,
low-chlorophyll or HNLC regions). In contrast, exten-
sive blooms of large diatoms occur across the Scotia Sea
during spring. The result is a high-nutrient, high-
chlorophyll region, although at times nutrient concen-
trations can become sufficiently depleted to become
locally limiting (Holm-Hansen et al. 2004b; Korb et al.
2005). The enhanced production supports some of the
largest and most diverse concentrations of seabirds and
marine mammals anywhere on Earth (Everson 1977,
1984). Antarctic krill (Euphausia superba Dana; hereafter
krill) are the major link between primary production and
vertebrate predators in the Southern Ocean food webs.
This is particularly marked in the Scotia Sea, where about
half of the overall krill population occurs (Atkinson et al.
2004). Historically, the Scotia Sea is also where the
majority of harvesting of seals, whales and fishes in the
Southern Ocean occurred (Everson 1977, 2001; Laws
1984). Although the krill fishery has declined during the
last decade, a significant fishery still exits, operating
almost exclusively in the Scotia Sea and the Antarctic
Peninsula regions (Everson 1977, 2001; Everson et al.
2000a).
The Scotia Sea ecosystem is therefore a key part of
the Southern Ocean ecosystem and understanding its
operation has become more urgent as evidence has
114 E. J. Murphy et al. Scotia Sea ecosystem
Phil. Trans. R. Soc. B (2007)
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emerged that rapid environmental change is occurring
in the western Scotia Sea and west Antarctic Peninsula
(WAP) region (Vaughan et al. 2003; Meredith & King
2005). Recent analyses have also suggested that krill
abundance has reduced by over 50% in the Scotia Sea
during the last 30 years and there are indications that
some of the krill-dependent predator populations are in
decline (Reid & Croxall 2001; Atkinson et al. 2004).
Suggestions that these ecological changes are linked to
the climate-related variations have been provided
support by evidence that changes in the Scotia Sea
ecosystem are linked to Southern Ocean and southern
hemisphere scale variations (Forcada et al. 2005;
Murphy et al. submitted; Trathan et al. in press).
Predicting how the Scotia Sea ecosystem will
respond to the climate-related changes presents a
major challenge. Traditional views of food webs have
tended to consider the network of biological
interactions in isolation from the environment. Such
an approach is not realistic because it does not take
account of process interactions of different organisms
at different scales, or the ontogenetic and seasonal
changes in trophic interactions. Including all such
complexity is impossible, so a pragmatic scale-based
approach that focuses on key species within the system
is more realistic (Murphy et al. 1988; deYoung et al.
2004). Such an approach is tractable for the Scotia Sea
ecosystem owing to the importance of krill. To analyse
the operation of the Scotia Sea ecosystem, therefore,
requires detailed analyses of the krill population
dynamics as well as knowledge of the trophic
interactions (figure 2). An analysis of the Scotia Sea
ecosystem also requires consideration of the wider links
to surrounding regions owing to the open nature of the
ecosystem. Here, we concentrate on the Southern
Ocean region of the Scotia Sea south of the PF, but
consider the wider links of the ecosystem to surround-
ing areas of the Antarctic Peninsula, the northern
Weddell Sea and the regions east and north of the
Scotia Arc. Earlier Southern Ocean whole system
reviews were produced by Everson (1977) and Miller &
Hampton (1989). Priddle et al. (1998a) also
considered carbon flows through the food web to
highest trophic levels. Lower trophic level dynamics in
Southern Ocean ecosystems have been reviewed
recently by Smetacek et al. (2004) and Smith &
Lancelot (2004). Specific reviews of aspects of the
dynamics of krill populations have been discussed by
Siegel (2005), of predators by Ainley et al. (2005) and
Trathan et al. in press and the response of the wider
ecosystem to change by Smetacek & Nicol (2005).In
this paper, we review the operation of the ecosystem in
the Scotia Sea and surrounding areas, focusing on the
dominant krill centred food web (figure 2 illustrates the
structure of this paper in relation to the food web).
2. OCEANOGRAPHY AND SEA ICE
Flow through Drake Passage commenced when an
initial shallow gateway opened around 50 Myr ago, but
deep throughflow started only around 34–30 Myr ago,
immediately after the onset of spreading of the west
Scotia Ridge (Livermore et al. in press). The western
–48
–50
–52
–54
–56
–58
latitude
–60
–62
–64
–60 –55 –50 –45
lon
g
itude
–40 –35 –30 –25 –20
Weddell Sea
Georgia B.
South
Georgia
SAF
SAF
MEB
NWGR
N. Scotia Ridge
S. Scotia Ridge
Scotia
Sea
WF
SB
PF
Drake
Passage
SACCF
Figure 1. The Scotia Sea and surrounding areas showing the general position of the major frontal systems in relation to bottom
topography. SAF, sub-Antarctic Front; PF, Polar Front; SACCF, Southern Antarctic Circumpolar Front; SB, Southern ACC
Boundary; WF, Weddell Front; MEB, Maurice Ewing Bank; NWGR, North West Georgia Rise (see text for references; depth
contours shown for 1000 and 2000 m).
Scotia Sea ecosystem E. J. Murphy et al. 115
Phil. Trans. R. Soc. B (2007)
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side of the Scotia Sea is bounded by Drake Passage,
while the other sides are formed by the Scotia Arc
(figure 1). It extends over approximately 750 km
north–south and approximately 2000 km to the east
from Drake Passage, encompassing an area of approxi-
mately 1.5!10
6
km
2
. The waters of the ACC enter the
Scotia Sea through Drake Passage, deflect northwards
and then cross the Scotia Arc that rises from depths of
around 3000–5000 m as a chain of islands from the
Antarctic Peninsula to the tip of South America. Along
this arc are a series of island groups and seamounts.
Much of the central abyssal plain of the Scotia Sea is
3000–4000 m deep with a gradual shallowing from
west to east. Across the region, there are submarine
structures and seamounts such as the Shackleton
Fracture Zone, the Pirie, Bruce, Discovery and Herd-
man Banks and the North West Georgia Rise (NWGR)
to the north of South Georgia.
(a) Upper-ocean circulation and characteristics
in the Scotia Sea
The ACC is split into several fronts, which are at
their narrowest meridional constriction within Drake
Passage and which then diverge as the ACC spreads
into the Scotia Sea (Orsi et al. 1995; Brandon et al.
2004). Following Orsi et al. (1995), the fronts are
termed (from north to south) the sub-Antarctic Front
(SAF), the PF, the Southern ACC Front (SACCF) and
the SB (figure 1). The SAF and PF veer northward
upon entering the Scotia Sea and cross the complex
bathymetry of the North Scotia Ridge (Zenk 1981).
North of the North Scotia Ridge, the PF separates into
two branches over the Falkland Plateau, with one
branch topographically tied to the southern flank of the
Maurice Ewing Bank and the other branch continuing
northward over the plateau. The transport is approxi-
mately equally split between these branches, with the
classical signature of the PF being found in the former
(Trathan et al. 1997, 2000; Arhan et al. 2002; Naveira
Garabato et al. 2002). The SACCF has a more
eastward course, but loops around South Georgia
anticyclonically from the south before retroflecting
eastward (Thorpe et al. 2002; Meredith et al. 2003c).
The SB also maintains a mostly eastward course
through the Scotia Sea, but has a northward topo-
graphically induced loop in the vicinity of the South
Sandwich Island arc (figure 1).
South of the ACC in the Scotia Sea lies the waters of
the Weddell–Scotia Confluence (WSC), formed from
waters spilling off the shelf at the tip of the Antarctic
Peninsula that are injected into oceanic waters flowing
eastward (Whitworth et al. 1994). It should be noted
that the flux of shelf waters into the WSC is not the only
route for such waters to enter the deep ocean from close
to the tip of the Peninsula: Meredith et al. (2003a)
showed that downslope convection occurs to the north
of Elephant Island, with waters dense enough to
contribute to the deep waters of the Scotia Sea,
including Antarctic Bottom Water.
This downslope convection is strongly seasonal and
concentrated in the austral winter. It is speculated that
the flux of shelf waters into the WSC will be similarly
time dependent. The WSC is bounded to the north by
the SB and to the south by the Weddell Front (WF)
(figure 1). It has been suggested recently that the WF
originates from a branching of the Antarctic Slope
Front close to the northwestern limit of the Weddell
Sea (Heywood et al. 2004). Historical observations
have often depicted the WSC to be characterized by
abundant eddies and meanders, but it is now thought
that at least some of this complexity is caused by the
fronts being strongly steered by the convoluted
bathymetry of the South Scotia Ridge.
Close to South Georgia, the flow regime is
dominated by the SACCF. The extent of the SACCF
retroflection has been revised since Orsi et al. (1995)
first represented it schematically reaching to 438 W;
Thorpe et al. (2002) compiled historical hydrographic
measurements and found that the retroflection only
extended as far as 368 W. Subsequently, Meredith et al.
(2003c) showed that the SACCF is steered away from
the shelf of South Georgia by the NWGR, which rises
2000 m above the seabed. It has also been shown that
the course of the SACCF in this region is traceable
using sea surface temperature (SST) imagery from
satellite-borne radiometers (Meredith et al. 2003c).
Such imagery revealed a complex eddy field north of
South Georgia, and this probably accounts for the
predators
krill
-distribution
-growth and age
-reproduction and recruitment
-life cycle interactions
-population variability
zooplankton
intermediate
predators
2. oceanographic and sea ice influences
3. nutrient and lower trophic level plankton dynamics
4.
5. food-web operation
6. ecosystem variability and change
Figure 2. Schematic of the Scotia Sea food web as considered
in this review. Developing the approach of deYoung et al.
(2004), the major focus is on krill, their life history and
interactions, with reduced detail on other groups and trophic
levels. Numbered headings refer to the major sections and
organization of this paper.
116 E. J. Murphy et al. Scotia Sea ecosystem
Phil. Trans. R. Soc. B (2007)
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debate on the westward extent of the SACCF retro-
flection. Waters on the shelf of South Georgia can differ
in potential temperature and salinity characteristics
from those off-shelf, due to retention processes coupled
with freshwater inputs from land and warming through
insolation (Brandon et al. 1999, 2000; Meredith et al.
2005). The transition between the shelf and off-shelf
waters can be abrupt or gradual, with implications for
baroclinic advection around the shelf break (Brandon
et al. 1999, 2000; Meredith et al. 2005).
Although the circulation in the Scotia Sea broadly
follows the pathways of the ACC fronts, it is important
to recognize the role of bathymetry. Not only does this
steer the ACC fronts themselves, but it also controls
the circulation in the zones between the fronts. For
example, Meredith et al. (2003a) presented trajectories
of passive drogued drifters in the Georgia Basin and
demonstrated a general anticyclonic circulation around
the island shelf from the south. However, some of the
drifters did not move to the east in the vicinity of the
SACCF retroflection, but continued to circulate anti-
cyclonically around the periphery of the Georgia Basin
before joining the PF to the west and north of the basin.
Clearly, the advective pathways can be more strongly
influenced by direct topographic steering than by the
ACC frontal pathways in such circumstances. Also of
note is the presence of a variable, but often intense,
warm-core anticyclonic circulation above the NWGR,
with velocities as large as 50 cm s
K1
. Meredith et al.
(2003a) presented dimensional analysis which showed
that the features of this circulation were consistent with
those of a stratified Taylor column and demonstrated
the strong impact that it can have on primary
production and biogeochemistry.
(b) Sea ice dynamics
During winter in the Scotia Sea, sea ice extends out over
the southern areas of the ACC (figures 1 and 3). The ice
is generated mainly in the Weddell Sea, drifting north-
wards driven by ocean currents and surface winds
(Murphy et al. 1995; Harms et al. 2001; Parkinson 2002,
2004). The minimum ice extent in summer occurs
across the Weddell Sea between February and March,
with sea ice advancing across the southern Scotia Arc
around May (figure 3). Although the maximum north-
ward extent of sea ice across the Scotia Sea during winter
usually occurs during September or October, it can
occur anytime between July and November. The timing
of both advance and retreat shows significant inter-
annual variation (figure 3) and is related to changes in air
temperatures and wind speed and direction reflecting
regional atmospheric dynamics (Allison 1997). The
mean position of the maximum winter sea ice extent
generally occurs in the area of the mean summer
position of the SB of the ACC (figures 1 and 3).
However, in extreme years, it can occur much further
north in the region of the SACCF or indeed much
further south around the position of the WF. It should be
noted however that there is little information on the
positions of the fronts when sea ice covers the region.
The average concentration of sea ice across the area
during winter is between 50 and 15%, and at this time it
will be approximately 0.3–0.5 m thick (Allison 1997)
and drifts north and eastward at speeds ranging from 1
to 15 cm s
K1
. These characteristics result in an east-
ward drifting marginal ice zone (MIZ) comprising
variable sized ice floes separated by leads and more
extensive areas of open water (Allison 1997). In spring,
there is an asymmetric southward retreat of sea ice, with
sea ice in the east retreating earlier than that in the west
(October and November). The MIZ in the west is also
more limited in north–south extent by the Antarctic
Peninsula than areas in the eastern Scotia Sea. Some
areas in the western Scotia Sea remain ice covered until
late in the spring (November and December). In areas
where the sea ice retreats slowly, the upper water column
can be stabilized by melt water input, generating shallow
surface mixed layers (10–30 m; Bianchi et al. 1992;
Lancelot et al. 1993; Figueiras et al. 1994; Parkinson
1994; Park et al. 1999). However, the retreat of sea ice
70°W60°W50°W
March
October
May
40°W20°W
L
M
H
L
M
H
L
M
H
50°S
60°S
70°S
80°S
(a)
(b)
(c)
50°S
60°S
70°S
80°S
50°S
60°S
70°S
80°S
30°W
70°W60°W50°W40°W20°W
30°W
Figure 3. Seasonal and interannual changes in extent of sea ice
across the Scotia Sea and Weddell Sea. Mean positions of the
15% ice edge are shown for three months in the year along with
the position of the ice edge in a year of extreme north and south
extent. (a) Mean sea ice extent in March (M) and extent in
March 1986 (L) and 2004 (H). (b) Mean sea ice extent in
October (M) and extent in 1989 (L) and 1987 (H). (c)Meansea
ice extent in May (M) and extent in May 1999 (L) and 1992
(H). Sea ice data from 1979 to 2005 from DMSP–SSMI passive
microwave data produced by NOAA/NCEP.
Scotia Sea ecosystem E. J. Murphy et al. 117
Phil. Trans. R. Soc. B (2007)
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across the Scotia Sea during spring is often rapid and
probably mainly wind driven (Sullivan et al. 1988;
Comiso et al. 1993; Parkinson 1994).
(c) Physical variability and long-term change
With the development of satellite-derived data series of
over 25 years duration, we can now consider variability
and change across the Scotia Sea system. On inter-
annual time-scales, connections between remotely
sensed SST close to South Georgia and the El Nin
˜
o–
Southern Oscillation (ENSO) have been demonstrated
(Trathan & Murphy 2002). These studies show a 2–3
year lag between ENSO variability in the equatorial
Pacific and response around South Georgia, implying a
significant component of oceanic advection in the signal
propagation (Trathan & Murphy 2002; Murphy et al.
submitted). More recently, Meredith et al. (2005)
examined 5 years of hydrographic data from close to
South Georgia and noted particularly cold waters in
early 1998. These were shown to be linked directly to the
very strong 1997/1998 El Nin
˜
o event (Meredith et al .
2005). Murphy et al. (submitted) have further shown
that the propagating oceanic signal dominates the
interannual variation from the Central and west Pacific
sector through to the Scotia Sea, but further support the
view that short-term (less than six months) direct
impacts from atmospheric effects did occur during the
major El Nin
˜
o event. In contrast, variation in the WAP
region appears to be dominated by the direct ENSO-
related atmospheric effects rather than the signal that is
propagated in the ACC (Fraser & Hofmann 2003;
Quetin & Ross 2003; Meredith et al. 2004a).
The interannual changes in SST associated with
these large-scale processes are also closely correlated
with sea ice variation across the region (Fedulov et al.
1996). Warm periods coincide with winters of reduced
ice extent and duration, while in the coldest years the
ice extends further north generating longer winters in
the southern Scotia Sea (Trathan et al. 2006; Murphy
et al. submitted). These changes are linked with the
passage of warm and cold anomalies in ocean SST
through the region from the South Pacific sector of the
Southern Ocean (White & Peterson 1996; Murphy
et al. submitted). Further work is needed to fully
determine the climatic forcings of interannual and
longer period variability in the Scotia Sea.
There is also marked decadal and longer term
change occurring in physical environments around
the Scotia Sea. There is clear evidence that the region
around the Antarctic Peninsula is one of the most
rapidly warming on the planet, with increases in air and
SSTs, and decreases in winter sea ice cover (Smith &
Stammerjohn 2001; Stammerjohn et al. 2003; Vaughan
et al. 2003; Meredith & King 2005). The longest record
of sea ice dynamics for anywhere in the Southern
Ocean also comes from the southern Scotia Sea
(Murphy et al. 1995). Records of the duration of fast
ice in the South Orkney Islands have shown a
significant decline in the mean duration of fast ice
between the first and the second half of the twentieth
century (Murphy et al. 1995; de la Mare 1997). There
are also some indications that upper water column
temperatures around South Georgia increased between
the first and the second half of the century and that this
was related to changes in sea ice extent (Whitehouse
et al. 1996a). This warming was followed by a period of
glacier retreat at South Georgia (Gordon & Timmis
1992). Taken together there is evidence that there was
an abrupt and rapid change in the physical environment
of the Scotia Sea in the middle of the last century. There
are also clear indications that further changes have been
occurring over the last three decades. Meredith & King
(2005) recently showed a warming of the upper ocean
on the west of the Peninsula over the second half of the
last century. There has also been a reduction in the
mean duration of winter sea ice around the WAP and
across the Scotia Sea during the last 25 years (Parkinson
2002). It is likely that this more recent regional change
reflects a downstream influence of the regional warming
that is occurring around the Antarctic Peninsula in
areas where sea ice formation occurs.
3. NUTRIENT AND PLANKTON DYNAMICS
In contrast to much of the Southern Ocean, which is
characterized by HNLC conditions, the Scotia Sea is
an area of both high nutrient concentration and high
productivity (Holm-Hansen et al . 2004a,b). However,
the production regimes are highly variable and reflect
the large-scale variation in physical and chemical
conditions across the region. Pre-bloom surface
macro-nutrient concentrations (nitrate, silicate and
phosphate) are generally high (surface values of
nitrate (NO
3
)O30 mol m
K3
; silicic acid (Si(OH)
4
)O
60 mmol m
K3
;phosphate(PO
4
)O2mmolm
K3
;
Whitehouse et al. 1996a, 2000; Atkinson et al. 2001),
with a gradient from south to north of reducing
nutrient concentration. Across the central Scotia Sea,
summer surface chlorophyll a concentrations are
moderate, between 0.4 and 1.0 mg m
K3
, with some
areas of higher concentration (O1.0 mg m
K3
; figure 4;
Holm-Hansen et al.2004a,b; Korb et al. 2005). Most
areas of enhanced mean surface chlorophyll a concen-
trations (O1.0 mg m
K3
) occur around and down-
stream of islands, across shelf areas, within frontal
jets and in areas recently covered by sea ice (figures 3
and 4; Mitchell et al. 1991; Bianchi et al. 1992;
Treguer & Jacques 1992; Comiso et al. 1993; Perez
et al. 1994; de Baar et al. 1995; Clarke & Leakey 1996;
Korb & Whitehouse 2004; Korb et al. 2004, 2005;
Holm-Hansen et al. 2004a,b). Productivity is also
variable and Korb et al. (2005) estimated primary
production rates of approximately 0.31 g C m
K2
d
K1
in central oceanic regions compared to rates between
0.72 and 2.04 g C m
K2
d
K1
across the shelf areas,
around the Scotia Arc and in the region of the
retreating ice edge in the southern Scotia Sea. These
rates are similar to the empirically derived estimates of
Holm-Hansen et al. (2004b) of between 0.60 and
0.99 g C m
K2
d
K1
for the entire Scotia Sea during
January and February. In the northern areas of
enhanced production, where the summer season can
extend over about five months, annual productivity
may be very high and has been estimated to be
approximately 30–40 g C m
K2
around South Georgia
(Whitehouse et al. 1996a). Further west, near to Drake
Passage, where waters of the ACC have recently
emerged from the South Pacific sector, concentrations
118 E. J. Murphy et al. Scotia Sea ecosystem
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of chlorophyll a are much lower (approx. 0.1 mg m
K3
;
figure 4; Holm-Hansen et al. 2004a). These waters are
similar to much of the Pacific and Indian Ocean sectors
of the Southern Ocean with high concentrations of
nutrients (silicate, phosphate and nitrate, i.e. HNLC;
Korb et al. 2005).
The development of blooms in the southern Scotia
Sea is affected by the timing and pattern of sea ice
retreat during spring (Sullivan et al. 1988; Comiso et al.
1993; Korb et al. 2004, 2005). In the areas of the
northern Scotia Arc, blooms are more regular and
predictable. Over much of the summer, blooms
develop in the shelf areas around South Georgia
(figure 4; Atkinson et al. 2001; Korb & Whitehouse
2004; Korb et al. 2004, 2005). These blooms extend to
the north in the retroflective area of the SACCF.
A large spatially extended bloom is often established
downstream of the island (Korb & Whitehouse 2004).
This bloom can extend downstream from the island
more than 2750 km to the east and at times enhanced
chlorophyll a concentrations are observed beyond the
prime meridian (Korb et al. 2004). These megablooms,
which can occur over an area of between approximately
0.07 and 0.5!10
6
km
2
and can last for over five
64 62 60 58 56 5254 50
latitude (°S)
(b)
(a)
chlorophyll a concentration (mg m
–3
)
1.40
1.20
1.00
0.80
0.60
0.40
0.20
SG
PF
SO
SACCF
FI
AP
(mg m
–3
)
–50
–50
–30 –20–60
–60
–40
0.10 1.00 10.00 20.00
Figure 4. (a) Mean concentration of chlorophyll a (mg m
K3
) derived from the summer (December–February) SeaWiFS data for
the period from 1998 to 2005. The position of the PF and SACCF are also shown. (b) Mean and 95% confidence intervals of the
December–February concentration of chlorophyll a (mg m
K3
) calculated in 18 latitude bands across the Scotia Sea from 55 to
308 W. Data are from the SeaWiFS Project and the NASA Giovanni Ocean Color Project.
Scotia Sea ecosystem E. J. Murphy et al. 119
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months, are potentially globally important in export of
carbon from the surface to the seabed (Schlitzer 2002).
In areas where intense phytoplankton blooms form,
such as north of South Georgia, macronutrients can be
reduced to or near to limiting concentrations (approx.
10 mmol m
K3
NO
3
, approx. 1 mmol m
K3
Si(OH)
4
and
approx. 0.3 mmol m
K3
PO
4
; Whitehouse et al. 1996a,b;
Korb & Whitehouse 2004; Korb et al. 2005).
These views of large-scale chlorophyll a distribution
across the Scotia Sea are based on satellite data
(Comiso et al. 1993; Korb et al. 2004; Holm-Hansen
et al. 2004a,b), which are known to underestimate high
chlorophyll a concentrations in large blooms
(O5mgm
K3
; Korb et al. 2004). A further problem is
that satellites cannot detect sub-surface chlorophyll
maxima which are known to occur in the Scotia
Sea (Korb et al. 2004; Holm-Hansen et al. 2005;
Whitehouse et al. submitted). Such sub-surface pro-
duction is likely to be regionally and temporally
important, but presently remains an uncertain aspect
of the operation of the food web.
It is likely that the waters of the Scotia Sea are
naturally iron enriched and this promotes high
productivity of large diatoms throughout the region.
There is now good evidence, from artificial iron
enrichment experiments, that a lack of iron in surface
waters is a major factor limiting phytoplankton growth
(de Baar et al. 1995; Boyd 2002c). The natural iron
enrichment in the Scotia Sea is likely to come from a
range of sources, including shelf water inputs from the
Antarctic Peninsula region associated with the WSC,
upwelling and interaction of the ACC with the shelf
sediments of the Scotia Arc introducing dissolved iron
into surface waters (de Baar et al. 1995; Korb et al.
2004, 2005; Holm-Hansen et al.2004b). This
enhanced concentration of iron, which is a crucial
micronutrient in the growth process of large diatoms, is
considered to be the major factor that allows phyto-
plankton to bloom across the Scotia Sea (Hart 1942;
Korb & Whitehouse 2004; Korb et al. 2004, 2005;
Holm-Hansen et al. 2004a). A recent study of
phytoplankton growth across the northern Scotia Arc
region gave further support to this view. Holeton et al.
(2005) obtained direct iron measurements which
showed enhanced iron concentrations around South
Georgia that arose from a benthic source. A range of
indirect evidence gives further support for the view that
iron concentrations are high and a major factor
generating the large phytoplankton blooms across the
Scotia Sea; these include the dominance of large
diatoms, large depletions of NO
3
concentration,
observed nutrient deficit ratios and high phytosynthetic
efficiency (Korb & Whitehouse 2004; Korb et al. 2004,
2005; Holm-Hansen et al. 2004a; Holeton et al. 2005;
BAS 2006, unpublished data).
In the Scotia Sea, we therefore see a region of
transition with waters of low iron concentration in the
west that emerge from Drake Passage (Korb et al . 2004;
Holm-Hansen et al. 2004a) and then mix with waters of
high iron concentration that have recently flowed
around and across the Antarctic Peninsula shelf and
southern Scotia Arc. The iron levels of these waters are
likely to be further enhanced as the currents flow over
the northern Scotia Arc, allowing blooms to develop
around the shelf areas. Over time, these blooms
develop downstream away from the shelf areas; there-
fore, they are a function of both the flow and the iron
enhancement (Korb et al. 2005). The strong gradient
north–south in irradiance and ice cover and duration
will affect the timing and development of the plank-
tonic system.
Although iron is important in phytoplankton
growth, a range of studies have shown that realized
population growth rates are the result of multiple
controls (Lancelot et al. 2000; Holm-Hansen et al .
2004b; Korb et al. 2005). The interactive effects of
light, nutrients (micro and macro), temperature and
grazing will all be important in determining the
concentrations of phytoplankton (Smith & Lancelot
2004; Holm-Hansen et al. 2004b). Silicic acid levels
decrease further north across the Scotia Sea and, as
previously noted, are more likely to become limiting
late in the season in northern areas where summer lasts
longer. The long-lasting blooms observed in these areas
are also therefore likely to show shifts in species
composition from diatoms to non-siliceous species.
It is also likely that the dynamics and fate of iron from
the Scotia Sea will also be important in determining
the food-web structure downstream. Indeed, studies
of the food-web operation along the region north of the
Scotia Arc may reveal the time-scales for iron recycling
and its fate in the food web (Smetacek et al. 2004). Of
the grazing controls on production, the impacts of
meso- and macro-zooplankton on phytoplankton
production can often be low, particularly during
summer when blooms have already developed
(Atkinson et al. 2001). However, krill and copepods
also exploit microbial and heterotrophic production, so
grazing impacts on new production will be determined
by food-web structure and interactions (Atkinson et al.
1996; Atkinson & Snyder 1997; Pakhomov et al.
1997a,b; Giesenhagen et al. 1999; Lancelot et al. 2000).
Like much of the global ocean, microbial popu-
lations are undoubtedly an important component of
Scotia Sea nutrient and production systems, but
relatively little is known about connections between
the microbial components and higher trophic levels
and much of the relevant work comes from adjacent
areas of the Weddell Sea and Antarctic Peninsula and
elsewhere in the Southern Ocean (Lancelot et al. 1991;
Mordy et al. 1995; Wright & van den Enden 2000;
Walsh et al. 2001). During summer, the dominant
pathway for energy flow in the Scotia Sea will be
through new production by the larger diatoms, but
ammonium is likely to be an important nitrogen source
over the Scotia Sea (Priddle et al. 2003). Outside of the
summer period, the recycling pathways are much more
important. The seasonal changes in relative importance
of new versus recycled production however is unknown
(Cota et al. 1992; Mordy et al. 1995). In winter, the
microbial communities associated with the sea ice are
important in the food web (Becquevort et al. 1992;
Garrison & Close 1993; Mordy et al. 1995). Bacteria
have an important role in transferring energy through
the consumption of dissolved organic matter and are in
turn consumed by protozoa which are fed upon by
smaller zooplankton (Bak et al. 1992; Kuparinen &
Bjornsen 1992; Grossmann 1994; Tupas et al. 1994;
120 E. J. Murphy et al. Scotia Sea ecosystem
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Mordy et al. 1995; Moran et al. 2001). These microbial
systems introduce important temporal delays into the
food web, making key compounds, organic substrates
and energy available at times during the season when
little new production is available. This will be
particularly important in maintaining energy flows in
the food web during autumn and winter in the Scotia
Sea, where extensive meso- and macro-zooplankton
populations require food (Walsh et al. 2001; Smith &
Lancelot 2004). The recycling pathways are likely to be
the major components of coastal food webs around the
Scotia Sea. In more pelagic waters during winter, there
will be significant sea ice-associated microbial pro-
duction in the drifting ice habitat of the MIZ of the
southern Scotia Sea that will maintain higher trophic
level production (Garrison & Close 1993; Ackley &
Sullivan 1994; Murphy et al. 1998a). Temporal delays
in the food web introduced by recycling will also
result in a spatial disconnect between regions of
production and consumption as the material is
advected in the ocean (Garrison & Buck 1991;
Becquevort et al. 1992; Garrison & Close 1993;
Grossmann 1994; Grossmann & Dieckmann 1994).
The high productivity of the Scotia Sea ecosystem
makes it an important region for examining the effects
of natural iron fertilization on the development of
planktonic systems. The impacts on the wider
operation and structure of the ecosystem provide a
valuable natural contrast with much of the rest of the
oceanic Southern Ocean.
4. KRILL IN THE SCOTIA SEA FOOD WEB
(a) Krill distribution in the Scotia Sea
The physical environment sets the context within
which any species must operate. For Antarctic krill,
the biggest influence may have been the opening of
Drake Passage and the development of the ACC to
generate the relatively isolated circumpolar Southern
Ocean (Patarnello et al. 1996; Jarman et al. 2000; Zane &
Patarnello 2000; Livermore et al. 2005, in press). This
has generated an oceanic environment in the Scotia Sea
which is the most advective in the world (Cunningham
et al. 2003). The life cycle of Antarctic krill, which
appears to have originated at about the time the ACC
became established (Patarnello et al. 1996; Jarman et al.
2000), will have developed in this dispersive system,
which had characteristics similar to the general pattern
of oceanic circulation and seasonality that is observed
today (Spiridonov 1996).
Understanding the factors controlling the large-scale
distribution of krill has become a major focus of
research during the last 5–10 years. These studies have
been advanced by the development of complementary
large-scale modelling, field studies and data syntheses
(Murphy et al. 1998b; Murphy & Reid 2001; Atkinson
et al. 2004; Hofmann & Murphy 2004; Murphy et al.
2004a,b; Siegel 2005; Fach & Klinck 2006; Fach et al.
2006; Nicol et al. 2000; Nicol 2006). The large-scale
distribution of krill is a function of production
(recruitment and growth), mortality, retention and
dispersal. The resultant circumpolar distribution is
highly asymmetric (Marr 1962; Mackintosh 1973),
with at least half of the entire krill population occurring
in the southwest Atlantic sector of the Southern Ocean
(Atkinson et al. 2004). The distribution of krill in the
Scotia Sea also extends further north than in any other
region of the Southern Ocean, with high densities
occurring north of 538 S(figure 5a). Elsewhere in the
Southern Ocean, krill tend to occur mainly near the
continent (between approx. 75 and 658 S). Marr
(1962) suggested that the large-scale distribution was
dominated by the surface currents generally, and that
the ACC, WSC and outflows from the Weddell Sea
were the major determinants of the horizontal distri-
bution in the Scotia Sea. A recent modelling study has
further indicated that the mixing of surface waters in
the Scotia Sea is a key determinant of the large-scale
distribution of krill and brings together plankton from
around the Southern Ocean (Thorpe et al. submitted).
Within the Scotia Sea, high and relatively predictable
concentrations of krill occur in waters less than 1000 m
deep (Miller & Hampton 1989; Murphy et al. 1997).
Detailed analyses of fishery and acoustic survey data
from around South Georgia in the northern Scotia Sea
have shown that maximal values occur in the shelf-break
region (Murphy et al. 1997; Trathan et al. 1998a;
Trathan et al. 2003). However, significant amounts of
krill also occur in oceanic waters across the Scotia Sea
(Siegel 2005). The Discovery Investigations (1925–1951)
found large concentrations of krill in off-shelf regions
(Marr 1962). The most recent comprehensive acoustic
survey to date also showed a large biomass of krill in the
central southern Scotia Sea during summer 2000
(figure 5a; Hewitt et al. 2004). Further support to this
view is given in an analysis of historical net data, which
shows large amounts of krill in off-shelf areas during
summer (Atkinson et al. 2004). These central Scotia Sea
regions were also areas of commercial whaling and
are strongly influenced by oceanic frontal systems
(Hofmann et al.1998; Tyn an 1998 ; Hofmann &
Murphy 2004; Murphy et al. 2004a,b). During autumn
and winter, a combination of dispersal and mortality
leads to a decline in the abundance of krill across
northern oceanic regions. Higher abundances are
maintained across shelf and off-shelf areas further
south through retention, recruitment and seasonal
dispersal (see discussions in Marr 1962).
(b) Krill growth and age in the Scotia Sea
Growth of krill is highly variable, and a function of
animal size and maturity, food availability and
temperature (Ross et al. 2000; Fach et al. 2002; Reid
et al. 2002; Daly 2004; Atkinson et al. 2006; Candy &
Kawaguchi 2006; Kawaguchi et al . 2006). The Scotia
Sea during summer appears to be a generally
favourable habitat for krill growth and development.
Atkinson et al. (2006) and Tarling et al. (2006) recently
measured post-larval growth rates based on samples
from across the entire Scotia Sea in mid-summer. They
derived empirical relationships between krill growth
rates, size and development state with local tempera-
ture and chlorophyll a concentration. Here, we use
these relationships to estimate growth rates across the
whole Scotia Sea during summer (figure 5b). Highest
growth rates, predicted for the 2000 season, are across
the southeast Scotia Arc, down to the eastern Weddell
Sea, across the southern Scotia Sea and in the east
Scotia Sea ecosystem E. J. Murphy et al. 121
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Antarctic Peninsula region. Predicted growth rates are
consistently above zero over most of the area, except in
the more northern and warmer regions nearer the PF.
Calculations of carbon flux indicate that rates of growth
of krill of approximately 3–6% of animal body mass per
day would have occurred in the high growth regions
where the animals were 20–30 mm long. This was
mainly in the central and the eastern Scotia Sea.
Further west and north 40–60 mm animals would have
shown lower rates of 0.5 to 2%. The relationships
indicate that higher temperature regions, to the north
of the Scotia Sea, are poor areas for krill growth and
especially for larger animals. However, it should be
noted that significant growth rates can be maintained
by larger animals in relatively warmer more northern
regions if the chlorophyll a concentrations are suf-
ficiently high. Areas to the north and northeast of
South Georgia show consistent blooms during summer
in areas where temperatures can be more than 38C, and
these could be areas where positive growth rates could
be maintained.
As there are currently no reliable methods to age
individual krill, the variability in growth rate makes
it difficult to examine the development of individual
year classes (Miller & Hampton 1989). There is a
fragmented picture of year class development at South
Georgia compared to that from the WAP. Analyses of
predator diet data have suggested that growth rates
across the Scotia Sea over extended summer periods
are sufficient for animals to reach a size of between 35
and 40 mm in 1 year having overwintered only once
(1C age class; Reid et al. 2002). This view of rapid
growth in the north compared to southern regions of
the Scotia Sea is also supported by analyses of year class
fluctuation across the Scotia Sea (Brierley et al. 1999;
Reid 2002; Reid et al. 2002). However, analyses of
length frequency distributions from net samples have
suggested that the same year classes dominate around
South Georgia and the Elephant Island and WAP
regions (Quetin & Ross 2003; Siegel et al. 2003). In this
interpretation, which is based on a view that the size of
age classes is the same across the Scotia Sea, the
animals would have overwintered twice (2C age class)
before they appear in the population at South Georgia
(Siegel 2005).
This lack of agreement arises from the absence of a
definitive ageing method, the capacity of krill to shrink
in conditions of low food availability, and the short
duration of the available time-series of recruitment
strength data in which the mean size of cohorts are
highly variable and consecutive year classes tend to
occur together. Uncertainty in identifying exactly
which year the animals were spawned affects our ability
to interpret interannual changes in abundance. The
predator and net series also relate to different parts of
the krill population (Reid et al. 1996b; Murphy et al .
1998b; Watkins et al. 1999; Murphy & Reid 2001). The
net sampling has occurred around the whole island on
and off the shelf, which combined with the small mesh
size of the nets can sample size groups in the year before
they dominate the predator diet (Watkins et al. 1999).
Most of the sampling of the length frequency of krill in
the diet of predators is based on animals that forage in
the west of the region, often mainly over the shelf (Reid
et al. 1996b). Comparison of net and predator data has
shown that the sampling needs to be local and
contemporaneous to be comparable (Reid et al.
1996b). Thus, discrepancies can arise through mis-
matches in the scale of sampling, thereby generating
0.30
0.20
0.10
0.00
–0.10
–0.20
50
(a)
(b)
(c)
55
60
65
70 60 50 40 30 20
5
10
20
40
80
160
SACCF
SB
WF
(g m
–2
)
(mm d
–1
)
(°W)
70 60 50 40 30 20(°W)
70 60 50 40 30 20(°W)
(°S)
50
55
60
65
(°S)
50
55
60
65
(°S)
Figure 5. (a) Krill biomass across the Scotia Sea based on
CCAMLR Synoptic Survey during January and February
2000. The positions of the major fronts as determined during
the survey are also shown (Murphy et al.2004a). (b)
Estimated growth rates (mm d
K1
) of krill across the Scotia
Sea during January and February 2000. Values based on
empirical relationships derived by Atkinson et al.(2006;
Calculations use Model 3, Table 5, for all krill sampled) and
Tarling et al. (2006) using satellite-derived mean SST field
and chlorophyll a (SeaWiFS) concentrations for January and
February and assuming a mean length of 40 mm. Blank cells
are where no data were available or where the SST was less
than K1 or greater than 58C. (c) Lagrangian particle tracks
passing through major biomass regions (a) based on tracks
from previous three months using output from the OCCAM
circulation model (Murphy et al. 2004a). (a,c) Reproduced
from Murphy et al. 2004a with permission from Elsevier.
122 E. J. Murphy et al. Scotia Sea ecosystem
Phil. Trans. R. Soc. B (2007)
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difficulties in interpretation of population processes
across the region.
(c) Krill reproduction and recruitment in the
Scotia Sea
Spawning followed by successful recruitment probably
occurs to some extent right across the southern Scotia
Arc and the Scotia Sea between about November and
February (Marr 1962; Hofmann & Husrevoglu 2003;
Tarling et al . in press). Depending on food availability,
krill can probably spawn several times in a year (Ross &
Quetin 1986; Siegel 2005). Mature krill have been
found throughout the Scotia Sea in both on- and
off-shelf areas, and a recent study off South Georgia has
shown that krill complete spawning and produce viable
eggs in the region (Tarling et al. 2006b). Around the
Antarctic Peninsula region mature krill appear to
migrate to the shelf-break regions to spawn (Siegel
2005). Eggs sink rapidly to depths of greater than
500 m, so spawning in shallow shelf areas is unlikely to
be viable as it would result in physical damage when the
eggs come into contact with the substrate and predation
from the benthos (Marr 1962; Miller & Hampton 1989;
Hofmann & Husrevoglu 2003). Larvae develop as they
return to the surface where they begin to feed, in a
process that takes about two to three weeks to complete.
Model simulations of egg development and larval
hatching have shown that there are restricted regions
of the shelf–slope, where the sinking eggs come into
contact with upwelling, relatively warm Upper Circum-
polar Deep Water, where egg development and larval
hatching and ascent can be successfully completed
(Hofmann & Husrevoglu 2003). In the Scotia Sea
sector, these areas are restricted to the WAP and a few
places around the east Antarctic Peninsula. Although
shelf–slope regions around the Scotia Arc do not favour
egg development and larval retention, the model
simulations indicate that oceanic waters right across
the Scotia Sea are suitable for spawning and larval
development (Hofmann & Husrevoglu 2003; Tarling
et al. 2006b). This suggestion is supported by large-scale
surveys of larval distribution that have shown that the
Scotia Sea is an area where high densities of larval krill
occur during the summer months (Marr 1962; Brinton
1985; Ward et al. 2004). These larvae are generally
considered to have come mainly from the major
spawning regions further south along the southern
Scotia Arc and around the Antarctic Peninsula (Marr
1962; Fach et al. 2006). Through a combination of
further spawning and drift, the distribution of larvae
then develops across the southern regions of the Scotia
Sea and north towards South Georgia in the east.
However, recent findings have focused on the fate of
larvae spawned and released over oceanic waters across
the Scotia Sea (Murphy et al. 2004a; Tarling et al. 2006).
Analyses of spawning status have shown that krill at
South Georgia probably complete their maturation
process and spawn over slope and off-shelf areas, where
eggs and larvae will be rapidly transported away from the
island (Murphy et al. 2004a; Tarling et al. 2006b).
Winter survival and growth of the larval krill
produced during summer require access to alternative
food sources. Sea ice is considered to be a key
overwintering habitat for krill generally (Quetin &
Ross 1991; Spiridonov 1995). Ice algae, which develop
on the undersurface and within sea ice, are an
important source of energy that help sustain krill
during the periods of low water column productivity
(Daly & Macaulay 1991; Quetin & Ross 1991, 2001;
Melnikov & Spiridonov 1996; Quetin et al. 1996;
Ross & Quetin 1999; Fraser & Hofmann 2003; Meyer
et al. 2002; Pakhomov et al. 2004). The sea ice also acts
as a potential refuge from predators, reducing mortality
rates (Daly & Macaulay 1991). A relationship has been
found between sea ice conditions during winter and
krill recruitment around the Antarctic Peninsula
(Siegel & Loeb 1995; Loeb et al. 1997; Quetin &
Ross 2003; Siegel et al. 2003; Siegel 2005). It also
appears that consecutive years of extensive sea ice are
required to generate large year classes around the WAP
(Loeb et al. 1997; Fraser & Hofmann 2003; Quetin &
Ross 2003). However, the extension of this concept
that greater winter sea ice extents lead to better food
and refuge conditions as a linear function in every
region is likely to be too simplistic to explain changes
across the whole region. Sea ice conditions vary across
the region, with an area in the west around the
Peninsula where the MIZ is small compared to areas
further east, where low-concentration sea ice cover can
extend over much of the Scotia Sea. Sea ice conditions
around the WAP are dependent on factors to the west,
with much of the ice brought into the region on ocean
currents and driven by wind. Areas around the tip of
the Antarctic Peninsula into the Scotia Sea will be
affected by conditions in the Weddell Sea as well as
further west. It is therefore surprising that a simple
relationship of krill recruitment with ice extent appears
to dominate, given the complexity of the processes
generating the distribution of sea ice. Recent studies
have suggested that the relationships between krill
recruitment and sea ice are more complex. In the
Palmer and Marguerite Bay region, years of enhanced
recruitment were found to be associated with winters of
average ice conditions (Quetin & Ross 2001, 2003).
Algal concentration and abundance in sea ice will not
depend on sea ice extent, but will be a function of the
degree of open water, the floe size and thickness, and
may also be dependent on when the ice formed and
under what conditions. The complexity of the habitat
for krill has been highlighted by Daly (2004), who
showed that larval grazing on sea ice algae in southern
areas of the WAP is low in winter, but becomes more
important in spring as the ice melts and the light levels
increase. The successful survival of krill through
various critical stages of the life cycle will therefore be
a complex function of interaction between sea ice
habitats in winter and open ocean regions in summer
(Quetin & Ross 2001, 2003; Siegel 2005). Final
recruitment success, when animals are at least 1 year
old, will reflect conditions over at least the previous 2
years, which would have affected maturation and
spawning of mature animals and larval survival in
summer and winter.
An apparent consistency of year class recruitment at
the Antarctic Peninsula and across the Scotia Sea has
been noted and a number of explanations have been
proposed (Priddle et al. 1988; Murphy et al. 1998b;
Brierley et al. 1999; Quetin & Ross 2001, 2003; Siegel
Scotia Sea ecosystem E. J. Murphy et al. 123
Phil. Trans. R. Soc. B (2007)
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et al. 2003; Atkinson et al. 2004). Spawning may be
occurring right across the region, under similar large-
scale physical and growth conditions, generating
successful regional recruitment (figure 6). Alterna-
tively, spawning and larval survival may be occurring
mainly in a central area in the south with drift taking the
older individuals into a larger habitat over the next 1–2
years (figures 5c and 6; Priddle et al. 1988; Hofmann
et al. 1998; Murphy et al . 1998b; Quetin & Ross 2003;
Hofmann & Murphy 2004; Fach et al. 2006). Analyses
of sea ice data indicate that conditions show marked
variation across the region, suggesting that recruitment
success will also vary. We also know that large animals
occur in off-shelf areas in regions of rapid current flow,
indicating that there is significant transport of both
larvae and adult krill, and a general oceanic mixing of
year classes across the region (Hofmann & Murphy
2004; Murphy et al. 2004a,b). These observations tend
to support the view of a central spawning region with
dispersal into the larger habitat (figure 6), but this is
based on limited information. At local scales, around
islands, how the krill get onto the shelf from off-shelf
areas is unknown, although vertical migration
strategies may be important in areas where exchanges
of water occur at depth (Murphy et al. 2004a). In some
areas close to the Scotia Arc, larvae may be entrained
back onto the shelf by cross-shelf transfers of water
associated with upwelling or surface water mass
exchange (Dinniman & Klinck 2004; Klinck et al.
2004). These exchange mechanisms are likely to be
important in larval retention along the Antarctic
Peninsula and around the South Orkneys. However,
there is no evidence that there is a significant larval
recruitment onto the shelf in more northern regions
such as South Georgia (Ward et al. 1990; Watkins et al.
1999; Atkinson et al. 2001).
The implication of this view of krill dynamics is that
a large proportion of the young produced will be
immediately lost from the regional system (Murphy
et al. 2004a,b). This can be a viable strategy for a
species as long as some animals are retained in the
major spawning zones or there is some reverse mixing
against the flow towards the south and west. However,
this view of a broadcast spawner, in which the majority
of the larvae drift away from a central favoured habitat
and are lost from the population, may not be
appropriate for this species. The chances of successful
development of larvae in the pelagic areas of the Scotia
Sea will depend on food availability (Meyer et al. 2002,
2003; Ross et al. 2000; Meyer & Oettl 2005). As
previously mentioned, the southern Scotia Sea and Arc
show variable but moderate chlorophyll a concen-
trations that are likely to be adequate for krill growth
(Atkinson et al. 2004).
(d) K rill–habitat interactions in the Scotia Sea
This large-scale view of krill dynamics indicates that the
more northerly regions of the Scotia Arc will be
unfavourable areas for krill with an apparent lack of
larval recruitment and low growth rates as a result of
high temperatures (Atkinson et al. 2006). A longer
growingseasonatlowlatitudesmayoffsetthis
situation, but it raises the question of whether these
northern areas are part of a linked system where
animals are returned south to the major spawning areas
or whether they are effectively a dead end, where krill
are consumed, starve or are transported out of the
system. A direct active migration (Siegel 1988; Nicol
2006), towards favoured spawning areas in the south,
would be successful even in the rapid flow of the ACC
with a sustained swimming speed of 15 cm s
K1
. Such a
sustained swimming speed may be possible for krill
(Marr 1962; Kils 1982; Miller & Hampton 1989),
although there is no evidence of a large-scale migratory
strategy (Marr 1962). The proposed evidence of active
directed horizontal migration of krill over extended
distances could also be largely explained by small-scale
interaction effects and interactions with larger scale
environmental structure. One such small-scale strategy
would be a vertical migration to exploit changes in flow
speed and direction with depth (Hardy 1967).
Simulation studies have shown that diurnal vertical
migration in surface waters (less than 200 m) can
modify the direction that krill are transported within
the main current flow (Murphy et al. 2004a). Krill do
general
spawning and
recruitment
(a)(b)
central spawning
and dispersal
recruitment
year 0
larvae
year 0
larvae
year 1
year 1
year 2
year 2
year 3
year 3
Figure 6. Schematic of two alternative spawning and
recruitment scenarios that can both generate concordance
in recruitment across the region. (a) Spawning occurs
generally across the region and then recruitment is main-
tained in all shelf regions. (b) Spawning and successful
survival during the first year occur mainly in central and
southern areas of the Scotia Sea, and the year class is
dispersed through interactions with the ocean and sea ice over
the next 1–2 years. Intermediate scenarios between these
extremes can also be envisaged.
124 E. J. Murphy et al. Scotia Sea ecosystem
Phil. Trans. R. Soc. B (2007)
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not however appear to undertake a deep (greater than
300 m) migration during winter, although there are
suggestions that vertical migration during winter in
shelf and slope regions may be more important than
first recognized (Siegel 2005; Taki et al. 2005). Even if
such a seasonal vertical migration does occur, there is
little southward flow at any depth in the Scotia Sea
sector as intermediate water masses enter through
Drake Passage and not from the Atlantic Sector, so a
deep winter migration will not move krill south in the
Scotia Sea.
A change in vertical distribution in the water column
through the year does however occur owing to the
winter association with the sea ice, which is a crucial
part of the life history. The association with the sea ice
has so far been assumed to be a strategy for accessing
alternative sources of food and for the avoidance of
predators (Loeb et al. 1997). However, the association
may also be a strategy for retention and life cycle
closure. The direction of the drift of ice is different from
the underlying ocean circulation because the motion of
ice is mainly wind driven (Thorndike & Colony 1982;
Steele et al. 1997). A recent modelling study (Thorpe
et al. submitted) has suggested that drifting with the sea
ice over winter can generate retention of krill in
southern regions where conditions for larval growth
over the whole year are most favourable. A strong
physical association of the krill with the sea ice could
lead to a rapid southward redistribution as the retreat of
sea ice in spring is often wind driven. This process may
be particularly important in the Antarctic Peninsula
region where the sea ice tends to move towards the
continent from the Bellingshausen Sea region rather than
offshore and northwards (Stammerjohn et al.2003).
The link with the sea ice will also be important in
generating the large-scale distribution of krill.
Simulations of the growth and development of larval
krill (Fach et al. 2002) showed that krill drifting east
from the Antarctic Peninsula region would encounter
sea ice advancing north across the region (Murphy et al.
1998b). Thus, larvae would be entrained in the west
and central Scotia Sea during autumn. Modelling
studies (Thorpe et al. submitted) indicate that krill
entrained with the sea ice in the southern Scotia Sea in
autumn would drift east and north with it during
winter. During the spring ice retreat, the krill would
either be entrained into the water column in the eastern
or southern Scotia Sea or remain with the ice as it
retreats and become entrained in the eastern Weddell.
Further drift and entrainment in sea ice in the following
season may release the krill into the favourable growth
conditions of the Scotia Sea or in the Antarctic Coastal
Current in the following year (figure 5c). The sea ice
interaction is therefore potentially important in gener-
ating the distribution of krill in the Scotia Sea (figure 5;
Murphy et al. 2004a).
During winter, the sea ice zone across the Scotia Sea
system will also provide a very different habitat to that of
the WAP region and probably favours ice algae growth
even during mid-winter (Garrison & Close 1993). Day
length in mid-winter across the southern Scotia Sea is
more than 5 h, whereas there is no daylight in areas
further south in the Weddell Sea and along the WAP.
The sea ice zone will be an area of ice divergence where
leads and floes are consistently changing, generating a
MIZ system right across the southern Scotia Sea.
During winter, the mean concentration of sea ice is
approximately 42%. Even for the peak months of July,
August and September, the concentration averages
below 50%. An area of approximately 0.5!10
6
km
2
is
covered by sea ice at this time and over 0.17!10
6
km
2
will be covered by sea ice less than 30% in concentration.
These characteristics of low-concentration sea ice and
relatively high irradiance are likely to favour the growth
of sea ice algae and other components of the microbial
community across the southern Scotia Sea during
winter (Garrison & Close 1993). These are areas
where krill are known to occur during spring and are
likely to be an important habitat for krill and the whole
food web during winter (Marr 1962; Hopkins et al.
1993b; Brierley et al. 2002a).
Although the South Georgia population depends on
inputs from areas further south, there does appear to be
some local retention of krill over a number of years
(Reid et al. 1999b; Murphy & Reid 2001). There are
consistent changes over weeks to months and between
summer and winter and also between years in krill
length in the diet of Antarctic fur seals (Reid et al.
1999a). This raises key questions about how krill
overwinter in these more northern regions. At South
Georgia, krill overwinter on shelf where they are the
target of a fishery that operates over a series of banks off
the north coast (Murphy et al. 1997). However, we do
not know what these krill feed on during winter, nor do
we have much information on the winter dynamics of
local plankton populations. We do know from studies
around the Antarctic Peninsula that krill can feed on
benthic material, so a benthic food source may be
available (Ligowski 2000; Daly 2004). We also know
that krill occur near the sea bed in other regions.
Activity recorder studies on penguins foraging from
Signy Island have shown that at times they are
feeding close to the bottom around 200 m and
consuming krill (Takahashi et al. 2003). Krill can also
feed on a range of planktonic species and groups other
than large diatoms, including microbial species and
meso-zooplankton (Marr 1962; Quetin & Ross 1991;
Hopkins et al. 1993b; Huntley et al. 1994; Pakhomov
et al. 1997a, 2004; Atkinson et al. 2002; Meyer et al .
2002, 2003; Daly 2004). These alternative food
sources will be important in allowing the krill to survive
in winter away from the sea ice, and further infor-
mation on krill diet in winter in areas outside the sea ice
is required.
There have also been suggestions that the Scotia Sea
krill stock is maintained by two separate inputs of krill
from populations in the Weddell Sea and the WAP
regions (Siegel 2005). There are some indications of an
east–west split in krill dynamics with different-sized
krill dominating in the east or west in some years.
However, there is no physical or planktonic community
distinction between these areas, indicating that there is
no simple ecological distinction (Marin 1987; Ward
et al. 2004, 2006). It is also likely that krill are produced
right across the region in areas of the WAP, the
southern Scotia Arc, Weddell Sea and possibly right
across the Scotia Sea (Hofmann & Husrevoglu 2003;
Tarling et al. in press). However, we do not observe
Scotia Sea ecosystem E. J. Murphy et al. 125
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large numbers of larval krill regularly around South
Georgia, and those that are present do not appear to
recruit successfully to the local population (Ward et al.
1990). It is possible that larval development may be
constrained by high temperatures in these more
northerly regions. The view of a mixed Scotia Sea
population is also supported by the observed consist-
ency of recruitment success across the region and
indicates that a discrete two-source view is inappropri-
ate (Fach et al. 2006). Modelling studies have also
suggested that krill will recruit into the Scotia Sea from
right across the southern region (around the Antarctic
Peninsula and northern Weddell Sea) and that
successful recruitment will be a complex function of
krill life cycle and feeding interactions (Fach et al.
2006). The apparent east–west split might therefore be
the result of a combination of oceanic and sea ice
interactions (Siegel et al. 2004; Siegel 2005). Krill
larvae generated around the Antarctic Peninsula (east
or west) will be moved eastwards over winter and will
emerge in areas in the eastern Scotia Sea in spring
(Fach et al. 2002, 2006; figure 5c). This could result in
a separation in the distribution of different year classes,
with annual waves of recruitment moving east,
associated with local year class retention in shelf areas
(figures 5c and 6).
Larger scale closure of the life cycle of krill from eggs
to mature adult and spawning may involve connections
between krill in areas that occur outside the Scotia Sea.
Simulation studies indicate that after the ice retreats, a
lot of the krill in the Scotia Sea would be transported
out of the region to the east around the South Sandwich
Islands (Murphy et al. 2004a). Such eastward move-
ment may facilitate transport to areas further south in
the eastern Weddell Gyre and Lazerev Sea (Thorpe
et al. submitted). The link with sea ice areas in the
southern Scotia Sea requires more specific study,
focusing on larvae in ice edge regions encompassing
both oceanic and neritic waters particularly during
spring and autumn. Understanding the links and
potential sources in this highly distributed system
requires large-scale coupled simulations of the life
cycle in association with oceanic and sea ice dynamics.
These analyses of population dynamics indicate that
the central southern Scotia Sea and Arc may be a much
more important habitat for maintenance of the krill
population across the whole area than previously
considered. The habitat of the central southern Scotia
Sea appears particularly crucial in both winter and
summer and will be a valuable focus for studies to
determine larger scale controls on the distribution
of krill.
(e) Krill population variability and chan ge
in the Scotia Sea
A number of studies have developed integrated
analyses of krill population dynamics across the Scotia
Sea. These built on earlier studies of variability of the
ecosystem such as those of Maslennikov & Solyankin
(1988) and Priddle et al. (1988). Together these have
shown that fluctuations in the numbers of larval krill
produced and their subsequent survival is the major
driver of variation in the abundance of krill across the
Scotia Sea (Murphy et al. 1998b; Murphy & Reid 2001;
Reid et al. 2002). The importance of year class strength
in driving changes in abundance in krill populations
in the WAP and Elephant Island regions has been
known for some time (Quetin & Ross 1991, 2003;
Siegel & Loeb 1995; Ross & Quetin 1999; Quetin &
Ross 2001). However, despite the evidence of a large-
scale relationship between krill density and sea ice
extent (Atkinson et al. 2004), the situation at South
Georgia and across the northern Scotia Sea is more
complicated. Smaller size/age (less than 30 mm and
1-year-old) classes of krill are generally not observed at
South Georgia (Watkins et al. 1999). Size classes of
older age groups merge together as the animals increase
in size owing to the asymptotic nature of krill growth
(Priddle et al. 1988). As noted earlier, this has made it
difficult to determine whether abundance changes are
driven by individual year class variations or bulk
changes across all year classes. Initial studies suggested
that bulk shifts in distribution of all age groups, linked
to large-scale atmospherically driven changes in ocean
currents, were generating the observed variation
(Priddle et al. 1988). Subsequently, Murphy et al.
(1995) and Fedulov et al. (1996) showed that these
changes were also linked to sea ice changes further
south and that they affected the availability of krill to
the fishery. Model studies showed how the observed
rapid reductions and recoveries in abundance could be
the result of year class fluctuations in a system where
older age groups dominated (Murphy et al. 1998b).
Further analyses of krill size in the diet of Antarctic fur
seals at South Georgia showed consistent changes in
length frequency between years (Reid et al. 1999a),
indicating that year class fluctuations were generating
the observed abundance and biomass changes at South
Georgia (Murphy & Reid 2001; Reid et al. 2002). The
abundance changes are therefore driven by the influx of
a large cohort of young krill which dominate the
population and maintain regional biomass for 1–2
years. The biomass then declines until the next influx
event. For the northern Scotia Sea, there is therefore a
second-stage distributional effect on top of the original
recruitment variation occurring elsewhere (Murphy
et al. 1998b). The two effects are however linked, i.e.
cold periods favour recruitment success and disperse
krill further north, so it is unlikely to be possible to
simply separate physical and biological effects.
These events of influx of young krill into the northern
Scotia Sea are strongly related to the physical conditions
across the Scotia Sea. At South Georgia, it is krill that
have overwintered at least once under the ice that are
transported to the island during the early summer
(Murphy et al. 1998b). The further north the sea ice
extends across the region, the colder the conditions in
the north (Fedulov et al. 1996; Whitehouse et al. 1996a).
Analyses of recruitment of krill into the population at
South Georgia (Murphy et al . submitted) and particle-
tracking studies, including interactions with sea ice
(Thorpe et al. submitted), indicate that more extensive
winter sea ice leads to enhanced dispersal and transport
of young krill into northern regions. The effect is that
during cold periods, influx recruitment is enhanced,
while there is little or no influx during warm periods. In
years of little or no flux, mortality rates will also increase
as the predators attempt to maintain food supply,
126 E. J. Murphy et al. Scotia Sea ecosystem
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reducing abundance more quickly (Murphy & Reid
2001; Constable et al. 2003). These interactive effects of
varying krill abundance and predator demand mean that
mortality will be a key process in determining the
interannual variability and may enhance the amplitude
of the observed variation. However, although we can
estimate the mortality rates of older krill, rates for larval
and juvenile krill are unknown (Murphy & Reid 2001;
Siegel 2005). Higher temperatures are likely to exacer-
bate the decline in krill biomass through reduced rates of
growth (Atkinson et al. 2006), and may also affect
survival. As the duration of the warm period extends
over 2–3 years, biomass declines further so that the
lowest biomasses occur at the end of the warm period
(Reid et al. 1999a; Murphy & Reid 2001). A recent study
of population changes at South Georgia (Murphy et al.
submitted) indicates that influx events are most clearly
detected after the warmest, lowest biomass years.
During the colder periods, influx events are less obvious
as the biomass is generally higher due to consecutive
years of reasonable or high recruitment.
The influx of krill to the South Georgia area depends
on transport from the southern Scotia Sea in spring
(Murphy et al. 2004a,b). Some of the transport is
associated with the SACCF which has been shown to be
important in advecting krill (Hofmann et al. 1998; Fach
et al. 2002, 2006; Murphy et al . 2004b). Further analyses
of fluctuations in the position of the SACCF have also
indicated that this may affect the large-scale transport of
krill across the Scotia Sea (Thorpe et al. 2002; Trathan
et al. 2003; see also Priddle et al. 1988).
The sea ice and SST variation in the Scotia Sea is
related to larger scale atmospherically driven changes
(Murphy et al. 1995; Turner 2004; Murphy et al .
submitted). ENSO variation influences the region
oceanically through a signal that propagates across
the southern Pacific sector and through Drake Passage
into the South Atlantic region, 2–3 years after the
variation in the ENSO region (figure 7; Murphy et al.
1995; Trathan & Murphy 2002; Murphy et al.
submitted). During the most intense ENSO periods,
the signal can be modified by direct, short-term,
atmospherically driven changes (Meredith et al. 2005;
Murphy et al. submitted). Low SST in the South
Atlantic is also associated with greater sea ice in winter
(Trathan et al. 2006; Murphy et al. submitted). As
previously noted, these changes in sea and SST affect
the recruitment in southern ice-covered regions and
dispersal of older age groups to the north. This will
introduce biological lags that affect the northern Scotia
Sea through dispersal of krill 2–3 years after the
recruitment in the south. The coherent nature of the
physical variability provides the potential for prediction
of physical and biological changes in the Scotia Sea.
A lack of information on seasonal changes in krill
abundance limits our understanding of these inter-
annual fluctuations. Seasonal variation in krill abun-
dance has been recorded in the Antarctic Peninsula
region and may be a key aspect of the interannual
fluctuations (Siegel 2005). Data from krill predators at
South Georgia also indicate that there are marked
seasonal changes in krill population structure in
South
America
Weddell
Sea
warm/cold anomalies
are propagated in the
ACC through Drake
passage 2–3 years
after ENSO variation in
equatorial Pacific
Krill population recruitment is related to regional sea ice
conditions. The dispersal and recruitment of older age groups into
northern regions is also enhanced during cold periods.
South Georgia
sea ice extent in winter
sea ice extent is
further north in
winter, and summer
SST is cooler, when
region is dominated
by cold anomalies
atmospheric
influences of ENSO
(lag < 6 months)
∼
atmospheric
influences of ENSO
(lag < 6 months)
∼
Scotia Sea
Figure 7. Schematic of the main physical processes generating variation in the Scotia Sea ecosystem. These factors also affect krill
recruitment and dispersal across the region, generating observed correlations of changes in krill density and biomass and higher
trophic level predator foraging and breeding performance with sea ice and larger scale indices of oceanic and climatic variation.
Scotia Sea ecosystem E. J. Murphy et al. 127
Phil. Trans. R. Soc. B (2007)
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northern regions (Reid et al. 1999b) and biomass peaks
during the summer (Brierley et al. 2002b;BAS
unpublished data). In the northern regions, changes
in timing of influx, growth and mortality during the
season will all affect local krill abundance and hence
their availability to predators. There is little knowledge
of these processes and it is important that further
information on seasonal changes in krill abundance
across the Scotia Sea is obtained.
On longer time-scales, analyses of historical net data
have shown a decline (50–80%) in the abundance of
krill across the Scotia Sea over the last 30 years
(Atkinson et al. 2004). Across the region, annual krill
density is related positively to the previous winter sea
ice cover in the Scotia Sea (Atkinson et al. 2004).
Although it is tempting to infer a causal relationship
from these observations, sea ice changes over the last
three decades have been complex and show marked
interannual, sub-decadal and decadal changes
(Murphy et al. 1995; Murphy et al. submitted). The
long-term decline in overall abundance could be
interpreted as a stock that has become more dependent
on fewer years of successful recruitment and which are
consequently subject to high mortality rates (Murphy
et al . 1998b; Reid et al.1999b; Murphy et al.
submitted). The effect on population dynamics will
therefore be expressed more clearly than in higher
biomass periods that dominated two to three decades
ago. When the population size was larger, single year
class fluctuations would have had less effect on biomass
(Murphy et al. 1998b). With this view, we would expect
that across the Scotia Sea, correlations between
environmental variation and krill abundance fluctu-
ations will be stronger now than in previous periods
when biomass was higher. However, as both recruit-
ment and dispersal of krill are related to sea ice, it is
difficult to determine whether the changes in density in
the Scotia Sea are due to reductions in overall
abundance or changes in distribution (Murphy et al.
1998b; Constable et al. 2003).
(f ) Krill in the Scotia Sea food web
The analysis of the life cycle of krill highlights the
spatial operation of the krill population in the Scotia
Sea. Krill are therefore a variable and dynamic
component of the food web across the Scotia Sea
(figure 8). As we have noted, krill are omnivorous and
consume other zooplankton or microbial groups. In sea
ice-covered regions, they can consume sea ice algae
and, in shelf areas, have been found to be feeding on
benthic algae (Ligowski 2000). However, a major
source of energy for krill during spring and summer is
diatoms (Cadee et al. 1992; Pond et al. 2005). Each
year, as the ice retreats, large blooms of diatoms occur
across the Scotia Sea, particularly in the regions of the
Scotia Arc and downstream (Holm-Hansen et al.
2004b), and these are exploited by krill (figure 8).
Later in the spring, as the ice retreats southward, large
blooms of diatoms in southern regions will fuel the
growth of post-larval krill in areas where they are
retained over the shelf or as they drift north across the
Scotia Sea (figure 8). In the more northern regions,
krill will benefit from the occurrence of large blooms
associated with the Scotia Arc that are maintained for
South North
extensive MIZ
MIZ
winter
spring
summer
Southern Scotia Arc Northern Scotia Arc
phytoplankton
krill
advective
dispersal
increasing demand and production
intense demand and production
influx of krill into north
predator demand
low demand and production
Figure 8. Schematic of seasonal development (winter, spring and summer) of the Scotia Sea ecosystem, highlighting major
spatial connections and the development of the spatial distribution of krill.
128 E. J. Murphy et al. Scotia Sea ecosystem
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extended periods during summer (Atkinson et al. 2001;
Korb et al. 2005).
The resultant krill distribution is highly hetero-
geneous with the highest densities occurring in areas of
shelf around the Scotia Arc, but with a significant
biomass in areas off-shelf, particularly at the ice edge
during spring (Marr 1962; Atkinson et al. 2004; Hewitt
et al. 2004; Siegel 2005). Large numbers of predators
require land-based breeding sites during summer, from
where they operate over a restricted local area as
centrally placed foragers (Croxall et al. 1988). These
areas occur on islands around the Scotia Arc where
enhanced concentrations of krill are found on the shelf.
These are also the regions where most of the
commercial fishing for krill occurs (Murphy et al.
1997). There is therefore a spatially heterogeneous
demand for krill, with intense hotspots, where krill
concentrations are high and predator demand is
greatest (figure 8; Murphy & Reid 2001). In areas of
low production or retention of recruits, the demand for
prey is maintained by the advective influx of krill. Rates
of mortality of krill across the Scotia Sea will therefore
be highly heterogeneous (Murphy & Reid 2001).
Within these regions of high concentration, krill
show further spatial structure associated with meso-
scale (tens to hundreds of kilometres) physical features,
such as frontal regions, plumes and the edges of
submarine canyons (Brinton 1985; Wat kins et al.
1986; Murphy et al. 1988; Witek et al. 1988; Miller &
Hampton 1989; Watkins & Murray 1998). Biological
and physical process interactions within these aggrega-
tions generate swarms in response to predatory and
feeding stimuli (Murphy et al. 1988). This patchiness
across a range of scales allows a variety of predator
species with very different foraging strategies to exploit
krill (Murphy et al. 1988). The longevity of krill is also
an important factor in their role in the food web. The
long lifespan of krill (5–7 years) means that at a
population level, they are able to cope with the strong
seasonal and interannual variability (Murphy et al.
1998b; Fraser & Hofmann 2003). By exploiting a wide
range of food sources, they can survive periods of
starvation. Long lifespan allows for the potential of
spawning several times during their life and means they
can cope with unfavourable years (Fraser & Hofmann
2003). This combination of high abundance, longevity,
dispersal and heterogeneity is why krill have such a
central role in the food web of the Scotia Sea (figure 9).
5. FOOD-WEB OPERATION
The Scotia Sea ecosystem encompasses high pro-
duction regions around areas of shelf and sea ice retreat
during spring, and low production regions in the west
near Drake Passage. The advective nature of the system
also makes a narrow geographical view inappropriate.
Some locally detailed studies, mainly based on summer
seabird and seal predation in the Scotia Sea
(a)(b)
krill
amphipods
copepods
fish
squid
4% 1% 70% 8% 16%
fish and squid (?)
krill
biomass = 44.3 (CV = 11.38%)
(alternative range = 109.4 – 192.4)
macaroni penguin (3.8 – 8.1)
antarctic fur seal (1.1 – 3.8)
chinstrap penguin (3.8)
crabeater seal (4.5)
dove prion (1.4)
whales (1.6 – 2.7)
antarctic fulmar (0.54)
adelie penguin (0.46)
total demand for krill
~ 17.2 to >> 25.3 × 10
6
tonnes
Figure 9. Predator links in the Scotia Sea food web. (a) Proportional consumption of different groups of prey by the major
predators. (b) Estimates of annual consumption of krill (10
6
tonnes yr
K1
) by the main krill predators. Where available a range of
estimates are given to illustrate the uncertainty. Information on predator diet and consumption is from Croxall et al.(1984,
1985), Boyd (2002a) and Reilly et al. (2004). Estimates of krill standing stock are from Hewitt et al. (2004) and values in
parentheses are from Demer & Conti (2005). Estimates are based mainly on summer studies and are likely to overestimate the
importance of krill in the diet (Croxall et al. 1985). Values for fish and squid consumption have not been included, but may be
very large (Kock 1985; Pusch et al. 2004).
Scotia Sea ecosystem E. J. Murphy et al. 129
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data, have been undertaken on various aspects of the
ecosystem (Croxall et al. 1985; Gowing & Garrison
1992; Hopkins et al. 1993a,b; Bathmann et al. 2000;
Atkinson et al. 2001), but there have been few attempts
at a broader synthesis (Everson 1977; Hempel 1985a,b).
The focus for carbon flow analyses has been on primary
production and lower trophic level interactions where
the flows are greatest (Bathmann et al.2000). In
contrast, analyses of variability have focused mainly on
higher predator interactions with their prey, which has
generated datasets extending back almost 30 years
(Croxall et al. 1988; Reid & Croxall 2001). Here, we
highlight some of the key features of the food web,
considering trophic and spatial links.
(a) Trophic links
Within the lower trophic levels, a particular focus has
been on the role of copepods and their interactions with
krill. Copepods in the northern Scotia Sea, around South
Georgia, can at times be the dominant grazers (Ward et al.
1995; Atkinson & Snyder 1997; Pakhomov et al.1997b;
Atkinson et al.1996, 1999). Shreeve et al.(2005)found
that daily gross krill production was 0.022 g C m
K2
d
K1
compared with 0.026 g C m
K2
d
K1
of the older stages
(CIV and CV) of the copepod Calanoides acutus (which
represented approx. 25% of the total copepod biomass at
South Georgia). These analyses give a valuable view
of the summer situation and indicate that copepods
may be the dominant zooplankton secondary producers
across the Southern Ocean (Shreeve et al.2005).
Developing more detailed analyses of the relative impact
of krill and copepods on production requires a seasonal
view. These are highly dynamic systems where the timing
of interactionswill generate multiple and varying controls
that are not easily resolved by short-term sampling
programmes. The situation is complicated by associated
spatial changes in the operation of the food web. Krill
biomass around South Georgia is greatest on the shelf
where local temperatures and chlorophyll a concen-
trations are generally low. In contrast, copepods tend to
dominate in warmer waters, off-shelf to the west, where
chlorophyll a concentrations are higher. Both competi-
tive interactions and ‘bottom-up’ processes (physical
and chemical) in the food web have been invoked to
explain these relationships between copepods and
krill (Atkinson & Snyder 1997; Priddle et al.1997,
2003; Atkinson et al. 1999; Shreeve et al. 2005; Ward et al.
2005).
The extended lifespan of the larger zooplankton
species may be an adaptation to survive in highly
seasonal and variable systems. However, they may also
be an important factor in determining the influence of
zooplankton on lower trophic levels (Priddle et al.
2003; Shreeve et al. 2005). Overwintering zooplankton
will affect the development of phytoplankton popu-
lations during spring. Large numbers of zooplankton
rising from depth in spring (copepods; Atkinson et al.
1997; Ward et al. 1997; Tarling et al. 2004), or already
present in surface waters (krill), will have an instan-
taneously high grazing impact, affecting the net growth
rates of phytoplankton (Lancelot et al. 1991, 1993).
Krill grazing impacts may therefore affect bloom
development even though their impact on overall
productivity during the summer is low. This may be
particularly important in the Scotia Sea where cope-
pods and krill are a dominant part of the food web.
The relative importance of grazing through cope-
pods and krill will impact the fate of carbon (Cadee
et al.1992; Gonzalez 1992; Ross et al.1998;
Schnack-Schiel & Isla 2005). Krill generate large faecal
pellets that sink quickly (rapidly removing carbon to
depth), whereas copepod waste material is smaller and
likely to sink less rapidly. However, copepods spend
a large part of the year in diapause at depths of greater
than 1000 m where they may die, generating a direct
carbon flux at depth, whereas krill remain in surface
waters. The complexities of such vertical interaction
effects in the food web, including links to microbial
systems, are largely unknown but are likely to be
important in determining vertical fluxes of carbon
(Tarling & Johnson 2006). These life cycle and
behavioural effects demonstrate that analyses of carbon
fluxes in the Scotia Sea, and indeed biogeochemical
cycles generally, will require detailed knowledge of the
life cycles of key planktonic species, particularly krill
and copepods (Giesenhagen et al. 1999).
Of the other zooplankton groups, two in particular
deserve further study in the Scotia Sea. Firstly, salps are
distributed across the Scotia Sea and have an important
role in regional biogeochemical cycles and food webs
(Foxton 1966; Fortier et al. 1994; Pakhomov et al.
2002). However, it is unclear whether they are as
important as in other regions around the Southern
Ocean, particularly in the WAP region and in warmer
areas to the north (Marchant & Murphy 1994; Loeb
et al. 1997; Atkinson et al. 2004; Kawaguchi et al. 2004;
Smetacek et al. 2004). This may be the result of high
concentrations of diatoms that adversely affect salp
feeding, or the result of competitive or predatory
interactions with other species (Smetacek et al. 2004).
Recent suggestions that their abundance is increasing
in high-latitude Southern Ocean ecosystems
(Pakhomov et al. 2002; Atkinson et al. 2004) make
the need to improve understanding more urgent.
The second group for which information is very
limited are the predatory amphipods, particularly
Themisto gaudichaudi (Pakhomov & Perissinotto
1996). These amphipods can become a major com-
ponent (more than 50%) in the diet of mackerel icefish
around South Georgia in years of low krill abundance
and are also consumed by a range of other pelagic and
seabird predators (Rodhouse et al. 1992; Kock et al.
1994; Pakhomov & Perissinotto 1996; Reid et al.
1997b; Bocher et al. 2001). It is likely that they are a
significant predator on a wide range of species,
including krill and copepods (Pakhomov & Perissinotto
1996) and may exert predatory control on the
dynamics and interactions of lower level species
(Bocher et al. 2001).
Top-down controls are important in Scotia Sea
planktonic ecosystems (Atkinson & Snyder 1997;
Priddle et al. 2003; Shreeve et al. 2005), but much
more specific studies of plankton interaction effects are
required to elucidate mechanisms and generate
dynamic models. Feedbacks within the food web
will also affect the dynamics of phytoplankton pro-
ductivity (Priddle et al. 2003; Shreeve et al. 2005). For
example, large aggregations of krill grazing on
130 E. J. Murphy et al. Scotia Sea ecosystem
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phytoplankton can generate locally high concentrations
of ammonium through excretion (Priddle et al. 1997,
2003; Atkinson & Whitehouse 2000). As ammonium is
a preferred nitrogen source for many phytoplankton
species, this will lead to enhanced phytoplankton growth
rates (Priddle et al. 1998b). Ammonium levels can also
be enhanced around local predator colonies, again
increasing potential growth rates of phytoplankton
(Whitehouse et al. 1999). Such interaction and feedback
effects are likely to be particularly significant in the
Scotia Sea, which has such high concentrations of
copepods, krill and higher predators.
Across the Scotia Sea, there is no simple relationship
between the structure of plankton communities and any
major physical features, such as frontal boundaries
between water masses. In recent analyses of zooplank-
ton community structure, Ward et al.(2004, 2006)
found that the major Scotia Sea frontal systems did not
act as significant boundaries for species or between
communities. Instead, the variation of community
structure in spring was dominated by north–south
differences in the state of development of the different
zooplankton species. Species in communities further
north were generally in a more advanced state of
development than areas to the south during mid-
summer. This development-related variation has been
generally linked to timing changes in water temperature
and sea ice cover related to latitude and seasonal
variation in production and planktonic system develop-
ment (Hempel 1985a; Marin 1987; Atkinson & Sinclair
2000; Ward et al. 2004, 2006). This difference was
further shown in analysis and modelling of the
development status of key copepod species (Tarling
et al. 2004). Overwintering stages reach the surface
waters earlier in the season in the more northern regions
(Voronina 1970). The short and later season also
impacts the overall life cycle, with animals in the north
completing their life cycle in a single year, whereas
further south, more of the population take 2 years
(Tarling et al. 2004). These variations in timing, growth
and development will affect interactions with krill and
modify the dynamics of the food web and will be a
valuable focus for the next generation of model studies.
Even though krill dominate the energy flows to higher
trophic levels (figure 9), the pathways of energy transfer
through the food web are complex (Croxall et al. 1984,
1985; Hopkins et al. 1993a,b). Most of the studies of
krill consumption are from analyses of the diet of
land-based predators (figure 9). However, krill are
also consumed by a wide range of pelagic species,
especially squid and fish, although consumption esti-
mates are extremely uncertain (Kock 1985; Rodhouse &
Nigmatullin 1996; Pakhomov et al. 1996; Pusch et al.
2004). The importance of copepods as prey items in the
food web is even less well quantified than for krill.
Shreeve et al. (2005) recently discussed the potential
importance of the copepods in the food web and noted
that C. acutus is an important part of the diet of the
mesopelagic fish, Electrona antarctica. Another copepod
species, Drepanopus forcipatus, is consumed by the larvae
of the commercially exploited icefish (Champsocephalus
gunnari). Copepods are also important to the flying
seabirds such as Antarctic prions (Pachyptila desolata)
and diving petrels (Pelecanoides sp.; Reid et al. 1997a,b).
Groups and species other than copepods and krill can
also be locally or seasonally important in the diet of a
range of pelagic and land-based predators. For example,
the diet of icefish varies markedly around South
Georgia; euphausids other than krill (e.g. Thysanoessea
spp.), the amphipod T. gaudichaudi or mysids (Siegel &
Muhlenhardtsiegel 1988) are a significant component
of the diet in different areas. These groups and species
are also an important, but variable, component of the
diet of other fish, seabird and squid predators (Kock
et al. 1994; Croxall et al. 1999; Everson et al. 1999;
Bocher et al. 2001; Dickson et al. 2004). Variations in
trophic links as zooplankton and fish species grow and
develop are largely unknown. The impact of chaetog-
naths, jellyfish and other pelagic species on young stages
of fish and krill is also unknown, but may be an
important component of variability of recruitment of
fish and krill.
Around South Georgia and across the Scotia Arc,
demersal and pelagic predators such as fish and squid
provide alternatives to krill as prey for higher predators
during the summer (figure 9). For example, the
mackerel icefish, C. gunnari, which is semi-demersal
on the shelf, is an important prey item for Antarctic fur
seals at South Georgia (Reid & Arnould 1996; Everson
et al. 1999). Equally, squid are an important prey
for several groups of higher predators including sea-
birds, seals and toothed whales (Rodhouse et al. 2001;
Collins & Rodhouse 2006). Mesopelagic species of
myctophid fish are also important in the diet of various
predators, including Antarctic fur seals (Reid &
Arnould 1996), squid (Rodhouse et al. 1992)and
king penguins (Olsson & North 1997). A number of
these predator species forage in the vicinity of the PF
and Rodhouse et al.(1992, 1994) considered explicitly
some of the trophic links in this region. Further south
in winter in MIZs, E. antarctica can be a more
important prey item in the diet of flying seabirds
(Ainley et al. 1991; Hopkins et al. 1993b). Salps and
jellyfish also appear to be a potentially significant
dietary component in a range of pelagic and land-based
predators but their importance is unknown (Catry et al.
2004). As well as being key components of the diet of
many of the predators, squid and myctophid fish
species will also be important in links between pelagic
and mesopelagic communities (Collins & Rodhouse
2006). There will also be important pelagic–benthic
links, and a number of species develop in shallow
waters and migrate deeper to shelf–slope regions as
they grow. For example, in coastal ecosystems along the
northern Scotia Arc, the commercially exploited
Patagonian toothfish (Dissostichus eleginoides) will be a
significant predator with dynamic trophic interactions
that vary between pelagic and benthic systems as it
grows. Short-term variations in trophic interactions
involving diurnal and seasonal changes in depth are
also known to be important. However, although
descriptive analyses are available, quantitative studies
of abundance and fluxes associated with most of these
interactions are not available.
These less well-known alternative pathways to the
traditionally studied krill links will be important in
maintaining the ecosystem structure and determining
the dynamics of individual species. These alternative
Scotia Sea ecosystem E. J. Murphy et al. 131
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pathways cannot however support the same level of
predator demand as the krill–predator pathway,
because more complex pathways involve more trophic
transfers and associated energy losses at each step
(figures 10 and 11).
The upper trophic level structure of the food web is
dominated by different predator species across the
Scotia Sea. In the north, there are extensive land areas
that are not covered by snow during the summer which
allow access to suitable breeding sites for macaroni
penguins and Antarctic fur seals, which are the major
krill predators (Croxall et al. 1984, 1985; Boyd 2002a).
Further south, in regions covered by sea ice for much of
the year, it is chinstrap or adelie penguins and Weddell
or crabeater seals that are the major krill consumers
(Croxall et al. 1985; Trathan et al. 1996; Priddle et al.
1998a; Boyd 2002b; Takahashi et al. 2003; Lynnes et al.
2004). The dietary differences and specific habitat
requirements that generate niche separation of the
many predator species across the Scotia Sea have been
described in detail. Krill predators often dominate local
Scotia Sea food webs, but large numbers of other
seabird species, such as petrels and albatrosses, are
dependent on groups other than krill, particularly
copepods, amphipods, fishes and squid (Croxall et al.
1984, 1985; Reid et al. 1996a; Lynnes & Rodhouse
2002; Xavier et al. 2003a,b, 2004). We have some
knowledge of the diet and foraging of many of these
species, particularly in one or two localities such as
South Georgia, Signy Island and around the Antarctic
Peninsula, but little information for much of the area
(Croxall et al. 1985; Reid et al. 1996a, 2004).
There is also very little data on the overall
abundance and distribution of most of these predator
species across the Scotia Sea. Much more detailed
information is required on trophic links at local and
regional scales combined with data on geographical
distribution and abundance. There are major gaps in
our knowledge about the operation of the mesopelagic
systems across the Scotia Sea and the links between
pelagic and benthic systems. These systems will be
crucial, both in terms of the fate of upper ocean
production (and hence carbon) and in terms of the
effects on the long-term dynamics of pelagic food webs.
krill
-life cycle
- spatial dynamics
zooplankton
phytoplankton
and microbial
physical and chemical processes
intermediate
predators
predators
12%
3.5%
12%
10%
10%
2.5%
10%
Figure 11. Estimated transfer efficiencies in the Scotia Sea
krill-based food web with transfer efficiencies for alternative
routes through other zooplankton species and intermediate
predators. Based on Priddle et al. (1998a).
krill
copepods
amphipods
myctophids
icefish
seals penguins other predators
phytoplankton
krill
copepods
amphipods
myctophids
icefish
seals
(a)(b)
penguins other predators
p
h
y
to
p
lankton
Figure 10. Schematic illustration of alternative pathways in part of the Scotia Sea food web, showing shifts between (a) years
when krill are abundant across the Scotia Sea and (b) years when krill are scarce. Major pathways shown as black arrows.
132 E. J. Murphy et al. Scotia Sea ecosystem
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It is not possible to examine all pathways in the food
web, so we need to focus effort. Specific studies of the
role of key predator groups for which we have very little
information, such as the whales and crabeater seals,
will be important. With a changing climate, copepod
and mesopelagic fish interactions, that are crucial in
more sub-Antarctic ecosystems, are likely to become
more important across the northern Scotia Sea.
Focusing effort on these pathways as alternatives to
energy flow through krill will be a valuable basis for
future research on trophic links.
(b) Sp atial operation of the food web
In a highly advective physical environment, the food-
web structure can only be maintained by a combination
of local and externally generated production. That both
local (see also Atkinson et al. 2001; Gilpin et al. 2002)
and external production are important in krill popu-
lation processes and in maintaining food webs was
recognized as far back as the Discovery Investigations
(Marr 1962; Hardy 1967; Mackintosh 1972, 1973).
This view has been supported by field analyses that
have shown that large amounts of krill do occur in high-
flow areas off South Georgia (Ward et al. 2003; Murphy
et al. 2004b). These flows will be important in taking
krill around the island and onto the shelf, although the
exact pathways and connections will be complex and
variable (Thorpe et al. 2002; Meredith et al. 2003b,c).
Field measurements have shown that the flows will be
variable, but at times their influence will dominate the
local food-web structure (Murphy et al. 2004a).
Even conservative estimates suggest that local krill
production around islands such as South Georgia is
rapidly consumed through predation. Calculations of
predator demand are however complex, and it is
especially difficult to account for all the potential
demand (Boyd 2002a; Murphy et al. 2004b; Shreeve
et al. 2005). The relatively low growth rate of krill in
warm regions of high predator demand around South
Georgia during summer further indicates that pro-
duction will not supply local demand in key regions of
the Scotia Sea (Atkinson et al. 2006). Such a large
impact should generate rapid changes in numbers that
would need to be replenished to maintain local krill
populations (Murphy et al. 1998b). The lack of very
young krill on the shelf at South Georgia, combined
with the strong flow of the ACC past the island, further
indicates that the population in the northern regions is
maintained by recruitment from further south (Ward
et al. 1990; Thorpe et al. 2002; Meredith et al. 2003c;
Murphy et al. 2004b). Variations in these inputs will
have profound effects on the krill population dynamics
and the local system operation. Input of krill into more
northern regions probably peaks during spring and
early summer following the retreat of the sea ice in the
southern Scotia Sea, and is likely to be lowest in winter
(Murphy & Reid 2001; Murphy et al. 2004a,b). On top
of these input effects, further variation will be
introduced through fluctuations in rates of growth
and mortality. Growth rates may be highest in early
spring and summer, while mortality rates are likely to
peak in mid to late summer. Together these factors will
generate the apparent mid-season peak in the biomass
of krill, the timing of which is likely to vary.
Although it is difficult to estimate the total predator
demand for krill, it is clear that top-down control on
plankton communities will be important in areas
around the major predator colonies in the Scotia Sea
(Reid et al. 2004; Murphy et al. 2004b). At South
Georgia, in areas of shelf to the east of Cumberland
Bay, the concentration of krill is variable, but usually
exceeds that observed in areas further west around Bird
Island, where the variation is lower (Brierley et al.1997).
The majority of seabird and seal demand for krill around
South Georgia is concentrated in the west around
Bird Island (Croxall et al. 1985; Trathan et al. 1996).
Long-term data on predator breeding performance has
demonstrated that there are periods where krill
availability in the western regions has been sufficiently
low that they result in catastrophic mortalities of
predator offspring (Croxall et al. 1988; Reid & Croxall
2001; Reid et al. 2005; Trathan et al. 2006). During
these periods of reduced krill biomass, foraging trips
become extended as the predators forage further
offshore in the search for prey (Boyd & Murray
2001). These factors indicate that predator demand is
at times sufficient to deplete local krill biomass. This
top-down influence will in turn modify the dynamics of
the planktonic communities on the shelf, although the
actual effects are unknown. The processes of replen-
ishment of plankton on the shelf through cross-shelf
exchange are unknown as we have little real under-
standing of the detailed circulation on the shelf
(Brandon et al. 1999; Meredith et al. 2003b; Meredith
et al. 2005). This is also true in terms of understanding
the links along the shelf around South Georgia between
areas that are fished in winter, which are mainly in the
east, and areas further west where the predators forage
during spring and summer (Murphy et al. 1997).
Development of high-resolution models of on-shelf
circulation will be an important component in
generating the required understanding.
The role of fronts in transferring material across the
Scotia Sea is likely to be important, as these are areas of
high flow rate (Hofmann et al. 1998; Nicol 2006;
Murphy et al. 2004b). However, analyses of the large-
scale distribution of krill during summer indicate that a
simple view of a ‘conveyor belt’ of krill across the Scotia
Sea connecting the Antarctic Peninsula region to South
Georgia is not appropriate (Hewitt et al. 2004; Murphy
et al. 2004a). The distribution of krill and other
plankton in spring will be a function of the timing
and pattern of sea ice retreat, affecting local pro-
duction, krill emergence from the MIZ and copepod
migration from depth. Thus, the summer distribution
of plankton across the Scotia Sea will be strongly
dependent on the system development during spring
(Murphy et al. 2004a). The importance of advective
transfers and dispersal of species other than krill, many
of which are more planktonic, such as copepods, has
not been the focus of as much study, but is likely to be
important. The importance of advection in generating
the observed distribution of phytoplankton was
demonstrated in a detailed analysis of the planktonic
system on the northern side of South Georgia (Ward
et al. 2002). The analysis showed that the planktonic
system around the island was strongly influenced by
inflows associated with the SACCF. Enhanced
Scotia Sea ecosystem E. J. Murphy et al. 133
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chlorophyll a concentrations coincided with the centre
of the high-flow region of the SACCF. It was estimated
that the chlorophyll a and depleted nutrient concen-
trations in the region would have taken two to three
months to develop. In this same area, copepods were
found to be more advanced in development state
compared to surrounding communities and krill
abundance was also high. Model simulations of
transport pathways indicated that the system would
have been associated with areas of the southern Scotia
Sea during early spring. At that time, the planktonic
communities would have been within the MIZ,
indicating spatial connections between the planktonic
systems of the ice edge during spring and South
Georgia during summer.
Predator foraging is also important in the spatial
transfer of energy in the ecosystem and the operation of
the food web (Croxall et al. 1984; Reid et al. 2004).
During summer, many of the predator species forage
over large distances with foraging bouts taking
generally a few days to a week (Croxall et al. 1984,
1985). Of the dominant krill foragers, the macaroni
penguins forage directly across the shelf areas and the
major direction of the flow, potentially maximizing the
volume of water sampled. Fur seals forage further out
over the shelf, spending more time in the off-shelf
regions, where krill are advected in the current flow
(Boyd et al. 1994, 2002 Boyd 1996, 1999; Staniland &
Boyd 2002, 2003; Staniland et al. 2004). The flying
seabirds also travel large distances (more than
1000 km) to obtain prey (Croxall & Briggs 1991;
Croxall et al. 1997, 2005; Xavier et al. 2003c; Catry
et al.2004; Phillips et al.2005). Black-browed
albatrosses, which consume large amounts of krill,
forage on the shelf and in areas of the southern Scotia
Sea as far as the South Orkney Islands. Species such as
evolutionary
change
copepod–krill shifts in dominance
interannual shifts in1° production
and food web structure
seasonality
Sea-ice
SST
overwintering of krill
penguin PD
seal PD
whale
PD
physical
processes
10 km 10
2
km 10
3
km 10
4
km 10
5
km
sub-decadal
fluctuations
ACC interannual
variability
ice and SST
rapid Scotia
Sea change
1 year
10 years
100 years
variation in pattern
of 1° production
biological
processes
shifts in upper trophic
level food web
krill PD
1 month
ice declines
WAP warming
global climate
change
copepod PD
ENSO
circumpolar
processes
krill decline
harvesting
driven changes
fish PD
flow across the
Scotia Sea
migration and
dispersal
shifts in1°
production and plankton
community structure
Figure 12. Schematic of the temporal and spatial scales of the main physical and biological processes important in determining the
dynamics of the Scotia Sea ecosystem (based on Murphy et al.1988). The 1 : 1 relationship is based on the scales of physical mixing in
the oceans. Note the physical and biological processes are illustrated offset above and below this line respectively for clarity. The
shaded grey block illustrates the natural spatial and temporal scale of Scotia Sea processes. We include processes above and below this
scale to approximately 10
5
km and a few hundred years and down to approximately 200 km and approximately two to three months.
PD, population dynamics.
134 E. J. Murphy et al. Scotia Sea ecosystem
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the grey-headed albatross and king penguins, which
consume more squid and fish, operate more widely
across the Scotia Sea or in areas further north around
the PF (Collins & Rodhouse 2006).
The predators in the South Georgia ecosystem, and
at other islands across the Scotia Sea, are therefore not
only dependent on local production, but also forage
out across the Scotia Sea and surrounding areas,
bringing energy back to feed their young in the large
and spatially restricted colonies (Croxall et al. 1984,
1985). This concept was developed in predator–prey
modelling by Murphy (1995) and it generates a
distance–demand relationship away from the centre
of foraging. The result is intense heterogeneity in the
demand for prey, which is centred around the islands
with most of the demand within approximately
200–250 km of the islands. This concept has been
further developed by Trathan et al. (1998b) and Reid
et al. (2004), who used data from large-scale at-sea
observations to derive more specific species-based
relationships of foraging in relation to distance from
land in the Scotia Sea. Such analyses provide the
basis for generating spatially distributed demand
maps, and hence prey mortality distributions, across
the Scotia Sea.
Food webs in the Scotia Sea also show marked
seasonal changes with major shifts in structure between
summer and winter. Thus, for example, around South
Georgia the diet of fur seals changes during late
summer and autumn, from being dominated by krill
to one where myctophid and other fish species are
proportionately more important (Reid & Arnould
1996). Many of the higher trophic level species also
show shifts in foraging areas or disperse across the
region during winter (Boyd et al. 1998). This includes
many of the krill-eating species of penguins, such as the
adelie and chinstrap penguins in the southern regions,
which disperse as the sea ice advances north from about
May. In the north, fur seals disperse south towards the
advancing sea ice, but also north across the PF to areas
of the Patagonian Shelf (Boyd et al. 1998, 2002).
However, a significant number of animals remain in
areas around the islands such as South Georgia during
winter (Boyd et al. 1998, 2002). Macaroni penguins
also disperse away from South Georgia during the
winter months, but there is little available information
on winter distribution. Many of the seabirds also leave
the region, as illustrated by the wandering albatross
which forages right around the Southern Ocean. Other
species and groups leave the Southern Ocean
completely and move north across the southern
hemisphere. This large-scale dispersal is most well
known for the whales, which migrate north along the
east coast of South America and the west coast of
Africa. There is therefore a movement out of the region
of a potentially significant, but unknown, proportion of
the upper trophic level predators. This dispersal across
the Scotia Sea, the Southern Ocean and southern
hemisphere is crucial in understanding the operation of
the food web. The effect is to reduce the upper trophic
level demand for energy during the low production
winter season, so that a significant proportion of the
potential demand leaves the ecosystem during the
winter (figure 8). More broadly, it also means that these
predators connect the dynamic operation of the Scotia
Sea ecosystem with ocean ecosystems across the South
Atlantic.
Improving our understanding of the spatial and
temporal operation of Scotia Sea food webs requires a
multi-scale approach. In particular, we need to under-
stand how localized food webs, such as around the
island areas of the Scotia Arc, interact as part of the
larger scale Scotia Sea and Southern Ocean ecosys-
tems. Understanding how physical and biological
processes and interactions operating over different
scales impact the regional food web will be crucial in
analyses of the long-term dynamics of the ecosystem
(figure 12).
6. ECOSYSTEM VARIABILITY AND LONG-TERM
CHANGE
As we have noted, the major focus of studies aimed at
understanding the factors generating interannual
variability in Scotia Sea ecosystems has been on
changes in the distribution and population dynamics
of krill. However, the development of that focus has
been generated in part through studies of the impact of
variability on other trophic levels in the food web
(Croxall et al. 1988; Priddle et al. 1988; Constable et al.
2003). Unique long-term monitoring datasets of the
breeding biology and population size of upper trophic
level predators across the Scotia Sea highlighted that
there were years in which availability of krill was very
low and predator breeding performance was signi-
ficantly reduced (Boyd & Murray 2001; Fraser &
Hofmann 2003). These impacts have been shown
across a range of predators for which krill are a
significant component of their diet, including the
land-based breeding predators, such as macaroni
penguins, gentoo penguins, Antarctic fur seals, black-
browed and grey-headed albatrosses and Antarctic
prions at South Georgia, and chinstrap and adelie
penguins at Signy Island and on the Antarctic
Peninsula (Priddle et al. 1988; Boyd & Murray 2001;
Reid et al. 1997a, 2005). Pelagic predators are also
affected as shown in the earliest observations of
interannual variability, which were revealed by changes
in the distribution and feeding of fin and blue whale
species between years (Priddle et al. 1988 and
references therein). Growth and condition indices of
the mackerel icefish also show marked interannual
changes (Everson et al. 1997, 2000b; Everson & Kock
2001). These changes have shown that predator
performance can be used to monitor changes in krill
availability (Boyd & Murray 2001; Reid & Croxall
2001). However, although indices of predator per-
formance do identify periods of low krill abundance,
they cannot resolve changes in abundance above
approximately 25–30 g m
K2
(Boyd & Murray 2001).
Above this level, increases in density do not generate
improved performance of predators (Reid et al. 2005).
It is likely that above this concentration foraging
constraints, competitive effects and other density-
dependent factors dominate the dynamics. More
recently, relationships have been revealed between the
variation in predator performance and local and large-
scale indices of physical variation (Forcada et al. 2006;
Scotia Sea ecosystem E. J. Murphy et al. 135
Phil. Trans. R. Soc. B (2007)
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Trathan et al. 2006; Trathan et al. in press). At South
Georgia, warm conditions in the previous season
precede low reproductive success in penguins and
seals (Forcada et al. 2006; Trathan et al. 2006; Trathan
et al. in press). Detailed studies of krill population
dynamics have shown that these relationships reflect
changes in krill availability which are linked to changes
in the ocean and sea ice regimes (Murphy et al. 1998b;
Murphy & Reid 2001; Constable et al. 2003; Murphy
et al. submitted).
The variability reveals shifts in food-web structure
between years of high and low krill availability
(figure 10). At South Georgia, when krill are scarce,
the diet of the large numbers of seals, penguins and fish
shifts. Fur seals consume mackeral icefish and mycto-
phids, penguins consume fishes and amphipods, and
icefish consume more amphipods (Croxall et al. 1999).
Energy flows through these alternative pathways are
insufficient to support the demand required to generate
a large number of offspring, so we see failures in
reproductive performance (figure 11; Croxall et al.
1988). However, the switching does allow the survival
of adults and hence maintains the populations during
years of low krill abundance. The switching therefore
reveals a property of the food web that buffers the
system response to variability. These weak pathways
can appear unimportant in terms of energy flow
compared to the main krill-related flows, but they are
crucial in maintaining the system in the longer term.
These alternative, weaker interaction pathways linking
production to highest trophic level predators are likely
to be crucial in determining the dynamics of the food
web and its stability properties (Rooney et al. 2006).
There are also direct physical effects that modify the
food-web operation when the region is dominated by
warm or cold conditions. During colder, longer
duration winters, the sea ice extends further north
and more ice obligate species will occur across the
Scotia Sea. Leopard seals (Hydrurga leptonyx) usually
occur in ice-associated environments, but they are
present around South Georgia in winter although their
abundance varies between years. Jessopp et al. (2004)
analysed a time-series of leopard seal occurrence from
South Georgia. They found that in winters which are
cooler and of longer duration, when winter sea ice
extends further north, leopard seals occur in greater
numbers at South Georgia, arriving earlier and leaving
later. It is also likely that at these times, the general
influence of ice-associated species will extend further
north across the region. Of the predator species, the
role of crabeater seals is likely to be particularly
significant across the region at these times.
The physical and biological process interactions
underpinning the structure of the food web make it
sensitive to regional and hemisphere scale changes in
climate (Trathan & Murphy 2002; Murphy et al.
submitted). As we have noted, changes in this region
not only reflect local processes, but are also linked to
global scale processes. This has given us a short-term
predictive capacity that will be tested over the coming
years.
The rapid regional reductions in sea ice concen-
tration and SST associated with increases in air
temperature are generating major changes in the Scotia
Sea ecosystem (Murphy et al. 1995; Vaughan et al.
2003; Meredith & King 2005). Along the Antarctic
Peninsula, shifts in the breeding distributions of
penguins have been related to reduced ice extents
(Fraser et al. 1992; Smith et al. 1999). Across the Scotia
Sea, the abundance of krill has declined by between 50
and 80% over the last 30 years (Atkinson et al. 2004).
These declines have been linked to changes in sea ice
distribution and, as we have discussed, probably reflect
regional variations in recruitment and dispersal
(Murphy et al. 1998b). At South Georgia, there have
also been significant changes in penguin and albatross
populations (Reid & Croxall 2001; Barlow et al. 2002).
However, although the changes have been linked to a
general reduction in krill abundance over the last 20
years (Reid & Croxall 2001), other effects are also likely
to be important. Although there have been clear
regional changes in sea ice and ocean temperatures,
the effects on the ecological system are complicated by
the long-term dynamics of the food web (May 1979;
May et al. 1979). The regional food web has been
strongly perturbed over the last two centuries, as a
result of harvesting of seals, whales, fishes and krill
(Everson 1977; Murphy 1995). It has been suggested
that the reductions in whales and seals may have
generated a ‘krill surplus’ and that this will have been
used by other groups of predators (Laws 1985).
Although the extent to which this occurred has been
debated, the exploitation will have had long-term
consequences (Everson 1977; Croxall 1992; Murphy
1995). Populations of many of the exploited whale
species have not recovered to pre-exploitation levels. In
contrast, Antarctic fur seal populations have recovered
rapidly from very low numbers (less than hundreds) in
the first half of the twentieth century to around 2–3
million just around South Georgia, while their range
has expanded across the Scotia Sea. These large-scale
and rapid ecological changes are likely to be generating
intense competition for krill (Barlow et al. 2002) and be
a major factor influencing the dynamics of the food
web. It is also worth noting that two species whose
populations are expanding in the northern Scotia Sea,
the Antarctic fur seal and king penguin, both exploit
the myctophid P. choriodon as part of their diet. This
myctophid is the major component of the diet of both
predators in sub-Antarctic areas, outside the main krill
zones. The success of these predator species may reflect
a shifting competitive balance favouring species that
can exploit prey other than krill.
Over the last 30 years, the Scotia Sea and
surrounding regions have also been the major area of
exploitation of living resources in the Southern Ocean
(Everson 2001). It is currently the main area where krill
are harvested and is the focus for the development of
ecosystem-based approaches to the management of
exploited fish and krill stocks (Constable et al. 2000).
The long-term effects of the disturbance are unclear as
we have little knowledge of the population size of the
major predator species across the Scotia Sea (Everson
1977; Everson 1984). If we are to predict the long-term
dynamics of key species in the ecosystem, we need
much better information on the size of populations,
their distribution and their dynamics across the Scotia
Sea. We also need to consider the larger scale physical
136 E. J. Murphy et al. Scotia Sea ecosystem
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processes affecting the region (such as ACC, ENSO
and Southern Annular Mode variability), as well as the
biological processes (such as krill transport processes
and predator migration) and their interactions
(figure 12; Murphy et al. 1988, 1998b, submitted;
Constable et al. 2003; Fraser & Hofmann 2003; Ainley
et al. 2005; see also Clarke et al. 2007a,b). This will be
crucial in developing models of the response of the
system to change and in developing long-term sustain-
able approaches to management of exploitation. This is
a key region in global fisheries and an area where
the potential for expansion of the demand of fisheries in
the coming years is generating real concern, particu-
larly among scientists involved in developing manage-
ment procedures within the Commission for the
Conservation of Antarctic Marine Living Resources
(CCAMLR). It is urgent that a comprehensive under-
standing of the operation of this regional ecosystem is
developed. The Scotia Sea system is changing so
quickly that in a short time regions in the north may
not be influenced by polar waters and could undergo
rapid changes in ecosystem structure.
7. CONCLUDING COMMENTS
The food web of the Scotia Sea is highly heterogeneous,
widely distributed but dynamically connected. The
fundamental determinant of the operation of the Scotia
Sea ecosystem is the ocean circulation and its
interaction with the Scotia Arc. The interaction of the
flow with the regional topography generates a highly
energetic environment of intense vertical and hori-
zontal mixing. The ecosystem is therefore dominated
by the flows of the major current systems (the ACC and
the WSC), but it is also strongly influenced by the
seasonality manifest most clearly by the advance of sea
ice across the region during winter. This combination
of mixing and seasonality generates a unique environ-
ment in the ACC which is high in both nutrients and
chlorophyll a. The generation of megablooms of large
diatoms is the result of the flow of the ACC across the
Scotia Arc, affecting macro- and micro-nutrient
concentrations. These blooms are a particularly
consistent feature across the northern Scotia Sea
towards the PF. This production fuels the food web,
and krill are particularly reliant on the development of
these large diatom blooms during spring. Krill are the
major link between lower trophic level production and
consumption by higher trophic level predators across
the Scotia Sea. The regional food web, which has
developed in this dispersive and seasonal context, is
therefore highly distributed with production at higher
trophic levels maintained by advective flows. The
advective regime that disperses krill is therefore a
fundamental factor in determining the structure of the
whole ecosystem. Energy is generated in restricted
regions in spring, particularly in areas of the retreating
ice edge in the south. This energy is dispersed across
the region and over the summer to be used in more
northern regions in areas where large numbers of
predators are concentrated during breeding.
The role of krill in the ecosystem is crucial, not only
owing to their high abundance, but also because of a
number of biological characteristics that make them a
major prey item for many of the predators. Krill are
heterogeneously distributed over a wide range of scales
from tens of metres to hundreds of kilometres. This
makes them available as prey over very different scales,
so that predators of very different size and foraging
strategies can all exploit krill as food. This hetero-
geneity generates a highly spatially structured and
variable ecosystem in which food-web connections
involve complex spatial as well as temporal and trophic
interactions. Relative to other species of zooplankton,
krill attain a large maximum size (approx. 60 mm) and
are long lived (5–7 years). This makes them available
to large-bodied predators over extended periods,
especially during periods of reduced primary pro-
duction. This longevity is therefore important in
allowing them to be dispersed across the Scotia Sea,
surviving through winter and across low production
regions, connecting regions of production with remote
areas of consumption. Krill production and develop-
ment are not limited to the shelf areas of the Scotia Sea
and the central Scotia Sea area is a key region for
overwintering of post-larval and larval krill. The
interaction with the sea ice during winter and spring
will be crucial in determining survival and dispersal
during summer. The retreating ice edge across the
Scotia Sea generates high productivity which drives
the regional production of krill. The MIZ across the
southern Scotia Sea in winter and spring is therefore a
key habitat for krill, where production of both sea ice
algae and phytoplankton will occur during winter and
spring. The sea ice is therefore a link between the
southern and northern Scotia Sea and between winter
and summer.
The importance of krill to the higher trophic level
predators means that any examination of Scotia Sea
food webs requires not only descriptions of distri-
bution, abundance and production, but also detailed
knowledge of the life cycles of key species. The debate
about whether advection is important in transporting
krill has developed to focus on quantifying the relative
roles of these different processes and the factors
controlling their operation. This requires a specific
focus on key stages of the life history of krill,
particularly the larval and juvenile phases in oceanic
waters during spring. Developing the models of krill
requires detailed analyses of the operation of the life
cycle. This will require coupled physical–biological
models that involve not only the oceanic system but
also the sea ice. Taking account of the interactions of
krill at different scales will be important and will require
a multi-scale modelling strategy that links behavioural
and population processes with physical models which
can resolve appropriate physical processes. In particu-
lar, this will require high-resolution shelf models
embedded in lower resolution oceanic models.
To develop a wider understanding of the dynamics
of the food web, the detailed life cycle operation of krill
and other key species will need to be analysed in the
context of the regional food-web interactions. Improv-
ing knowledge of winter processes across the Scotia Sea
is crucial, as it is gaining more information on poorly
studied species or groups such as the fishes (especially
mesopelagics), squid, crabeater seals and whales.
Developing studies of the importance of alternative
Scotia Sea ecosystem E. J. Murphy et al. 137
Phil. Trans. R. Soc. B (2007)
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pathways involving groups such as copepods and
myctophid fish will also be valuable. The importance
of large-scale physical and biological interactions in
determining the dynamics of the Scotia Sea ecosystem
has also highlighted that any analyses and modelling
must take account of the wider circumpolar and
southern hemisphere atmospheric, oceanic and bio-
spheric connections (figure 12). The Scotia Sea
ecosystem is highly spatially and temporally variable,
so future research effort aimed at understanding the
effects of a changing environment must be focused on
aspects of the food web that determine long-term
behaviour of the system. To deal with the complexity,
we need to focus effort on analyses of food-web
structure that centre on the dominant energy flow
pathway (through krill) and some of the alternative
weaker pathways to higher predators. These links will
be crucial in determining the operation of the food web
and its long-term dynamics (Rooney et al. 2006).
In analysing the Scotia Sea ecosystem, the biggest
challenge we face is to determine what has happened in
the last two centuries of major ecological change. This
requires detailed analyses of long-term datasets and the
development of models that account for the spatial and
temporal complexity of the ecosystem operation. This
will allow the development of models to predict the
responses of Southern Ocean ecosystems to change and
procedures for the sustainable exploitation of resources.
Developing such models is an urgent requirement
because we know that there is rapid regional change
occurring in the ocean and sea ice. There is clear
evidence that there was a stepwise change in the physical
regime of the Scotia Sea during the last century. This
change has occurred simultaneously with the ecological
changes driven by harvesting. Ecological systems are
dynamic and particular states represent dynamic
equilibria or transient effects, so they are always varying
and changing. However, the rapid changes over the last
20–30 years on top of the changes that have occurred in
the last two centuries may already be driving the Scotia
Sea ecosystem into a very different operational state. It is
likely that we will see major changes over the next 50
years, with the potential for extremely rapid and locally
catastrophic changes in species distribution and abun-
dance across the northern Scotia Sea.
8. SUMMARY
(i) The ecosystem of the Scotia Sea and the
Antarctic Peninsula region is undergoing
some of the most rapid regional environmental
and ecological change in any area of the ocean.
(ii) Advection and the interaction of the circulation
with the regional bathymetry generate intense
mixing and are major factors determining the
structure of the Scotia Sea ecosystem.
(iii) Interactions of the circulation with the regional
topography probably generate elevated
concentrations of iron in regions of high
macronutrients. This fuels regular blooms of
extended duration (more than one month)
across areas around the northern Scotia Sea. In
the south, blooms are more irregular and
associated with areas of melt water stabil-
ization, following the spring retreat of the sea
ice, or with shallow areas of the Scotia Arc. The
variability of the sea ice system and its spring
retreat results in highly variable productivity.
(iv) Krill are a long-lived and key species in the
food web, maintaining the majority of higher
trophic level production. The dynamics of their
population operates across the Scotia Sea and
is linked to adjacent regions of the Weddell Sea
and Antarctic Peninsula. Analyses of the
operation of the Scotia Sea food web require
a detailed understanding of the spatial and
temporal dynamics of krill populations.
(v) Krill occur in predictable densities across the
Scotia Arc. High concentrations of krill also
occur across the central oceanic regions of the
Scotia Sea in areas seasonally covered by sea
ice. The drifting MIZ of the central Scotia Sea
is likely to favour ice-associated production,
and its relatively low latitude for a polar region
means that the light cycle will fuel production
even during winter. These central sea ice-
covered regions will be a key overwintering and
spring habitat for krill, connecting areas further
south with regions to the north.
(vi) There is marked variability in the spatial
structure of the food web across the Scotia
Sea. There is also marked temporal variation in
the connections within the food web. During
winter many of the higher predators disperse or
leave the region, reducing the energetic
requirements at higher trophic levels.
(vii) The food-web structure is maintained by
horizontal advection of energy, for which krill
are the key vector. Biological dispersals and
active movements further maintain the
regional food-web structure.
(viii) There is clear evidence of top-down control
effects of grazers on the primary production
systems. Grazing and predatory impacts on the
plankton affect the dynamics of the plankton
community, and shifts between krill and
copepods affect the regional production and
phytoplankton community development.
(ix) There is also evidence that top-down control is
exerted by higher predators on macro-plankton
in shelf regions. High local demand for krill
reduces density and variance in the distribution
of krill, which in turn affects plankton dynamics.
(x) There is marked interannual variability in the
operation of the Scotia Sea ecosystem that is
driven by changes in regional sea ice and SST
conditions that are linked to hemispheric-scale
variations (linked to ENSO).
(xi) These variations affect the population dynamics
and dispersal of krill across the Scotia Sea during
spring and summer. This generates a reduction
in the recruitment of krill into northern regions
during warm periods.
(xii) The removal of the large seal and whale
predators over the past two centuries has
undoubtedly generated long-term top-down
cascade effects and modified the local plankton
138 E. J. Murphy et al. Scotia Sea ecosystem
Phil. Trans. R. Soc. B (2007)
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populations. These effects are probably continu-
ing today and will affect the interpretation of the
ecosystem responses to change.
(xiii) There has been marked regional climate and
oceanic change over the last 20 years. A rapid
change occurred in the duration of winter sea ice
across the Scotia Sea between the first and the
second half of the last century.
(xiv) A decline in krill abundance has been linked to
changes in sea ice, but these changes are
confounded by ecological shifts in the predators.
(xv) The Scotia Sea ecosystem has many key features
that make it ideal for examining the effects
of harvesting and climate change on processes
in large-scale oceanic ecosystems, from primary
production through to the highest level
predators.
We are grateful to our many colleagues and national and
international collaborators who have worked with us on many
of these topics. We thank the BAS logistical and technical
support teams, the officers and crew of the RRS James Clark
Ross and all those involved with Bird Island, South Georgia.
This paper is a contribution to the BAS DYNAMOE and
DISCOVERY 2010 Programmes. We are also grateful for the
comments of two anonymous referees, which greatly
improved the manuscript.
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