The Ross Sea: In a Sea of Change

16 Pages·2016·1.72 MB·English

Old Dominion University
ODU Digital Commons
OEAS Faculty Publications Ocean, Earth & Atmospheric Sciences
9-2012
The Ross Sea: In a Sea of Change
Walker O. Smith
Peter N. Sedwick
Old Dominion University, [email protected]
Kevin R. Arrigo
David G. Ainley
Alejandro H. Orsi
Follow this and additional works at:https://digitalcommons.odu.edu/oeas_fac_pubs
Part of theBiogeochemistry Commons,Climate Commons, and theOceanography Commons
Repository Citation
Smith, Walker O.; Sedwick, Peter N.; Arrigo, Kevin R.; Ainley, David G.; and Orsi, Alejandro H., "The Ross Sea: In a Sea of Change"
(2012).OEAS Faculty Publications. 45.
https://digitalcommons.odu.edu/oeas_fac_pubs/45
Original Publication Citation
Smith, W.O., Sedwick, P.N., Arrigo, K.R., Ainley, D.G., & Orsi, A.H. (2012). The Ross Sea in a sea of change.Oceanography, 25(3),
90-103.
This Article is brought to you for free and open access by the Ocean, Earth & Atmospheric Sciences at ODU Digital Commons. It has been accepted for
inclusion in OEAS Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contact
[email protected].
Oceanography
THE OffICIAl MAGAzINE Of THE OCEANOGRAPHY SOCIETY
CITATION
Smith, W.O. Jr., P.N. Sedwick, K.R. Arrigo, D.G. Ainley, and A.H. Orsi. 2012. The Ross Sea in a sea of
change. Oceanography 25(3):90–103, http://dx.doi.org/10.5670/oceanog.2012.80.
DOI
http://dx.doi.org/10.5670/oceanog.2012.80
COPYRIGHT
This article has been published in Oceanography, Volume 25, Number 3, a quarterly journal of
The Oceanography Society. Copyright 2012 by The Oceanography Society. All rights reserved.
USAGE
Permission is granted to copy this article for use in teaching and research. Republication,
systematic reproduction, or collective redistribution of any portion of this article by photocopy
machine, reposting, or other means is permitted only with the approval of The Oceanography
Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931,
Rockville, MD 20849-1931, USA.
DOWNlOADED fROM HTTP://WWW.TOS.ORG/OCEANOGRAPHY
Special iSS ue oN aN tarctic o ceaNography i N a c haNgi Ng World
roSS Sea
the iN
Sea chaNge
a of
by Walker o . Smith Jr., p eter N. SedWick, k eViN r . a rrigo,
daVid g . a iNley, a Nd a leJ aNdro h . o rSi
abStract. The Ross Sea, the most productive region in the Antarctic, reaches
farther south than any body of water in the world. While its food web is relatively
intact, its oceanography, biogeochemistry, and sea ice coverage have been changing
dramatically, and likely will continue to do so in the future. Sea ice cover and
persistence have been increasing, in contrast to the Amundsen-Bellingshausen sector,
which has resulted in reduced open water duration for its biota. Models predict that as
the ozone hole recovers, ice cover will begin to diminish. Currents on the continental
shelf will likely change in the coming century, with a projected intensification of flow
leading to altered deep ocean ventilation. Such changes in ice and circulation will
lead to altered plankton distributions and composition, but it is difficult at present to
predict the nature of these changes. Iron and irradiance play central roles in regulating
phytoplankton production in the Ross Sea, but the impacts of oceanographic
changes on the biogeochemistry of iron are unclear. Unlike other Southern Ocean
regions, where continental shelves are very narrow and Antarctic krill dominates the
herbivorous fauna, the broad shelf of the Ross Sea is dominated by crystal krill and
silverfish, which are the major prey items for higher trophic levels. At present, the
Ross Sea is considered to be one of the most species-rich areas of the Southern Ocean
and a biodiversity “hotspot” due to its heterogeneous habitats. Despite being among
the best-studied regions in the entire Southern Ocean, accurate predictions of the
impacts of climate change on the oceanography and ecology of the Ross Sea remain
fraught with uncertainty.
9900 OOcceeaannooggrraapphhyy || VVooll.. 2255,, NNoo.. 33
iNtroductioN by the US Navy and Coast Guard. Upon Sound on Ross Island. This station is
The Ross Sea, a large Antarctic embay- launch of the Research Vessel Icebreaker the focus of much regional research and
ment south of New Zealand, is an Nathaniel B. Palmer in 1992, scientific is the staging location for the supply of
unusual region lying within the larger investigations in the Ross Sea greatly other bases and camps on the continent,
domain that is the Southern Ocean. It expanded in frequency, duration, and including the South Pole. As such, sup-
lies to the north of Earth’s largest ice diversity. As a result, today the Ross Sea ply vessels must reach McMurdo Station
shelf, the Ross Ice Shelf (RIS), which is one of the most intensively studied every summer, which has facilitated
covers an area of 520,000 km2, with an regions in the Southern Ocean, with the entrance of research vessels into the
average thickness of 370 m. The outer investigations of geology, sedimentology, area. Facilities at the station also sup-
portion of the ice shelf floats on sea- glaciology, oceanography, and marine port ocean-going research programs,
water that is modified during its transit biology of a number of trophic levels. as well as ocean research activities that
beneath the shelf, and that mixes and The Ross Sea is also considered to be make use of holes drilled in seasonal
exchanges with waters to the north over the least human impacted continental and semipermanent fast ice adjacent to
the continental shelf. The continental shelf on Earth, at least in terms of its the station, where water depths immedi-
by Walker o . Smith Jr., p eter N. SedWick, k eViN r . a rrigo, shelf north of the RIS covers an area of biota (Halpern et al., 2008), displaying ately offshore exceed 400 m. Additional
daVid g . a iNley, a Nd a leJ aNdro h . o rSi 466,000 km2, with an average depth of pristine ecological characteristics that research activities are being conducted
~ 530 m, and the shelf break occurs at are no longer observed in other shelf from the Italian research base Mario
~ 700 m depth (Figure 1). However, of environments. In addition to direct Zucchelli, located in Terra Nova Bay
importance to the biota, small portions observations, substantial advances in our 280 km to the north, and at Scott Base,
of the Ross Sea are shallower than 200 m understanding of the Ross Sea have been operated by New Zealand.
(see Box 1). Numerous troughs run- gained from satellite observations and In this article, we review the essen-
ning roughly in a north-south direction, deployments of in situ instrumentation. tial features of Ross Sea oceanography,
carved by ice streams during past glacial Studies of the Ross Sea have greatly including circulation and water mass
periods, traverse the shelf. These troughs, benefited from the presence of McMurdo characteristics, biogeochemical cycling,
and the intervening banks, influence Station, located adjacent to McMurdo phytoplankton dynamics, and the
regional circulation, sedimentation, and
biogeochemical and biological processes.
–71°
Unlike other Antarctic continental
shelves, the northwest portion of the –72°
Ross Sea shelf was not glaciated during
–73°
the previous glacial maxima, which is an
important characteristic with regard to –74°
the composition of regional biota.
–75°
Following James Clark Ross’s discov-
ery of his namesake sea in 1841, exten-
–76°
sive collections of its benthic biota were
obtained by subsequent British “heroic”
–77°
expeditions; indeed, the Ross Sea is the
type locality for more than 400 marine
–78°
species. Modern scientific investiga- Contour Interval = 100 m
Data from: ETOPO1
tions of the Ross Sea were initiated dur-
164° 168° 172° 176° 180° –176° –172° –168° –164° –160° –156° –152°
ing the 1957 International Geophysical
Year, and were continued through the 0 400 800 1200 1600 2000 2400 2800 3200 3600 5000
Depth (m)
1980s by repeated cruises undertaken figure 1. map of ross Sea bathymetry.
Oceanography | September 2012 91
Unlike in the Southeast Pacific, the major
box 1 | roSS Sea coNtiNeNtal Shelf featureS dynamical obstacle to rising waters is the
abrupt poleward-diving of isopycnals
Summary of features of the ross Sea continental shelf based on the bathymetry used
found over the Ross Sea upper continen-
in orsi and Wiederwohl (2009), where the ross Sea continental shelf is defined by the
tal slope (Ainley and Jacobs, 1981), and
700 m isobath leading to the continental slope and north of the ross ice shelf.
the westward sweep near the shelf break
area: 46.61 x 104 km2 (constituting 18% of the entire shelf region around antarctica) of the Slope Current that carries a thick
layer of cold and fresh Antarctic Surface
Volume: 24.75 x 104 km3
Water (Whitworth et al., 1998).
areas (percentages) of ross Sea continental shelf with water depths less than: Antarctic Surface Water enters the
600 m 31.23 x 104 km2 67.0%
Ross Sea around Cape Colbeck as part
500 m 17.08 x 104 km2 36.6%
of a narrow coastal flow that, in addi-
400 m 1.82 x 104 km2 3.90%
tion to importing sea ice and icebergs,
300 m 1.11 x 104 km2 2.37%
200 m 0.56 x 104 km2 1.20% also incorporates continental meltwater
derived from sea ice-air interactions far-
ther upstream (Jacobs et al., 1985). This
strong boundary current continues west-
ecology of higher trophic levels. We also drawn from the bypassing Antarctic ward along the RIS, bringing low-salinity
discuss evidence of contemporary envi- Circumpolar Current farther to the east surface waters to an area that in winter
ronmental change in the Ross Sea and (Figure 3; Orsi and Whitworth, 2005). develops the giant (25,000 km2) Ross Sea
predict that future changes are likely to Thereafter, the Antarctic Circumpolar Polynya, the most active (390 km3 yr–1)
be substantial within this century. Current hugs the continental slope of “sea ice factory” around Antarctica. The
the Amundsen-Bellingshausen Seas much smaller (~ 3,000 km2) Terra Nova
phySical oceaNography (Orsi et al., 1995), allowing the uninter- Bay Polynya off Victoria Land gener-
Ice concentrations in the Ross Sea rupted surge of Circumpolar Deep Water ates considerably less ice (59.2 km3 yr–1;
decrease rapidly in austral spring (> 1°C) over the bottom layer of the Martin et al., 2007). Atmospheric cool-
(Figure 2). Small areas of reduced ice shelf (Jacobs et al., 1996). That is not the ing sustains sea ice formation, increas-
concentrations are located throughout case over the ~ 466,000 km2 of Ross Sea ing sea surface salinity and producing
the winter near the Ross Ice Shelf, when shelf, where only thin layers of upwelled the dense Shelf Water that fills most
ice is driven northward by strong winds deep water are observed at interme- of the continental shelf bottom layer,
off the ice shelf. This process continues diate depths (Orsi and Wiederwohl, including under the RIS. The product of
through November, and as the solar heat 2009). However, on-shelf transport of topographically controlled circulation
input increases, open waters expand Circumpolar Deep Water is critical to and progressive basal melting emerges
northward. Generally, this open water the heat and salt budgets, regional sea to the north at a few locations and at
extends completely off the shelf during ice cycle, and primary productivity of mid-depths, as less-saline Ice Shelf Water
summer, and the open water region is the Ross Sea, as it is the oceanic source with a depressed (< –2°C) freezing point.
contiguous with the open Pacific. Sea ice of heat and nutrients (Smith et al., 2007). Mostly produced within the Ross Sea
covers the Ross Sea quickly in March,
and continues to thicken and form Walker O. Smith Jr. ([email protected]) is Professor, Virginia Institute of Marine Science,
throughout winter. College of William & Mary, Gloucester Pt., VA, USA. Peter N. Sedwick is Associate Professor,
The stratification of the Ross Gyre, a Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk,
large cyclonic gyre to the north of the VA, USA. Kevin R. Arrigo is Professor, Department of Environmental Earth System Science,
Ross Sea, is dominated by a ~ 2,500 m Stanford University, Palo Alto, CA, USA. David G. Ainley is Senior Ecological Associate,
mid-depth layer of relatively saline and H.T. Harvey and Associates, San Jose, CA, USA. Alejandro H. Orsi is Associate Professor,
warm (> 2°C) Circumpolar Deep Water Department of Oceanography, Texas A&M University, College Station, TX, USA.
92 Oceanography | Vol. 25, No. 3
Polynya, the Shelf Water volume and Water and Circumpolar Deep Water production, macronutrients (nitrate,
salinity decrease seaward and to the east. layers is attributable to Ross Sea exports phosphate, and silicate) are rarely
Its circulation involves northward flow (Orsi et al., 2002). depleted from surface waters during the
against the western flank of troughs, growing season; thus, the region may
and, upon reaching the seaward sills, biogeochemical cycleS be described as a “high nutrient, high
Shelf Water increases the background Waters over the Ross Sea continental chlorophyll” regime (Smith and Gordon,
tilt of isopycnals, thus facilitating inflow shelf are among the most produc- 1997; Arrigo et al., 2008a). Moreover,
of Modified Circumpolar Deep Water tive in the Southern Ocean, sustain- primary production in the southern Ross
(> 0.5°C) available at the base of the ing annual primary production of Sea appears to be characterized by rela-
Slope Front (Ainley and Jacobs, 1981). ca. 23.4 ± 9.98 Tg C yr–1 (Arrigo et al., tively high export efficiencies, perhaps
Poleward extensions of Modified 2008a). Despite high rates of primary facilitated by the formation of organic
Circumpolar Deep Water are apparent
along and over the western side of banks
(Orsi and Wiederwohl, 2009). After
gradual attenuation and thinning along
a ~ 300 km transit toward the RIS, the
most conspicuous and persistent of these
inflows appears to enter the sub-ice cav-
ity near 173°W as a subsurface “warm”
(> –1.5°C) core (Jacobs and Giulivi,
1998). Across the wider western shelf,
shallower Modified Circumpolar Deep
Water inflows mix further with surface
waters, bringing the nutrients that sup-
port observed high levels of primary pro-
ductivity (Smith et al., 2007). Enhanced
mixing of carbon-rich continental waters
with Modified Circumpolar Deep Water
at the Slope Front produces a variety of
dense waters that, upon sinking down
the continental slope, effectively venti-
late adjacent ocean basins. In the Ross
Sea, the export of cold Antarctic Bottom
Water types (< –1°C) is restricted to
the slope regions off Drygalski, Joides,
and Glomar Challenger Troughs, and
supplies the abyssal layer offshore (Orsi
et al., 1999; Gordon et al., 2004). The
lighter mixtures (< 0.5°C) only descend
to mid-depths along much of the Slope
Front, and freshen the voluminous
Circumpolar Deep Water layer of the
Ross Gyre. About 30% of the combined
figure 2. mean ice concentrations in the ross Sea during November, december, January, and february
input of Shelf Water and Antarctic
(a,c,e,g) and composite chlorophyll a concentrations during the same months (b,d,f,h), using data from
Surface Water to the Antarctic Bottom both from the SeaWifS and modiS satellites during 1997–2011. black represents ice or cloud cover.
Oceanography | September 2012 93
aggregates (Asper and Smith, 2003), with production in the Ross Sea also assign the air-sea exchange of CO . The Ross
2
estimated f-ratios of around 50% (Asper the region an important role in the Sea also constitutes a significant regional
and Smith, 1999). (The f-ratio is the ratio regional cycling of other bioelements. sink for silicic acid as a result of the
of nitrate-based production to total pro- Phytoplankton composition has been preferential burial of diatom-derived
duction.) These high export efficiencies, shown to affect the relative concentra- opal, relative to organic carbon, in sedi-
and the importance of the Ross Sea as an tions of dissolved inorganic carbon, ments that accumulate on the inner shelf
area of deepwater formation (see earlier nitrogen, and phosphorus in surface (Nelson et al., 1996). Finally, the Ross
Physical Oceanography section), suggest waters (Arrigo et al., 1999; Sweeney Sea continental shelf is thought to be a
that the Ross Sea plays a significant role et al., 2000). In turn, these factors influ- major source of the climatically active
in the Southern Ocean carbon cycle, spe- ence the carbon and nutrient charac- compound dimethylsulfide to the atmo-
cifically by serving as a major regional teristics of the shelf waters that contrib- sphere (DiTullio and Smith, 1995). This
oceanic CO sink (Arrigo et al., 2008b). ute to oceanic deep waters (Orsi and organic sulfur compound is known to be
2
The high rates of primary and export Weiderwohl, 2009), and, ultimately, to produced by the haptophyte Phaeocystis
antarctica, which is a major phytoplank-
ton species in the Ross Sea (see next sec-
AASW
0 0
km ISWkm tion on Plankton Dynamics).
70°S AABW MCDW
salty fresh CDW There is an important critical nexus in
CDW 1 SW 1 SW
ADW the cycling of each of these bioelements
AASW
75°S MCDW AABW 32A1n9t6Wa0re-c1st9tic80 DeeApA WB32Wat1e9Cr8e0n-t2r0a1l0 A nt. Slo p e C urre nt icinryo ctnlhi.ne I gtR ioosf sn stoh Swee aeg swesneitnehrta italhlly em asicuccpreoppnltyue adtrn itdehn att
low dissolved iron concentrations limit
SW MCDW
salty phytoplankton growth rates and biomass
fresh SW over much of the Southern Ocean (Boyd,
AASW
2002), particularly in the remote surface
ISW ISW
79°S waters of the Antarctic Circumpolar
Current, which exhibit chronic iron
AASW
0
CDW km deficiency (Figure 4). Although there
ISW
70°S CDW
fresh are a number of potential sources of
1 SW
AASW iron to surface waters of the Ross Sea,
75°S ADW 32 2050 AAABDWAWnt. Slo p e C urre nt iaDnnecdelu pgd lWaincaigat esl rei caienfl,to rMouros isdoeindfisiem,d a enCnditr smc, usimenaep rioaclle a r
saltySW aerosols (Sedwick and DiTullio, 1997),
there is ample evidence that availability
freshSW of iron limits primary production in the
AASW
Ross Sea during the growing season.
ISW ISW
79°S Indeed, one of the first examples of “iron
160°E 180° 160°W 140°W limitation” of phytoplankton growth in
figure 3. potential changes in ross Sea circulation and stratification. The upper panel the Southern Ocean was provided by
illustrates the conditions observed during the past 50 years; the lower panel shows
shipboard experiments in the Ross Sea
potential intensification of flow and air-sea-ice interactions leading to an alternative
mode of deep ocean ventilation during the next century. arrow width indicates the (Martin et al., 1990). Results of further
relative strength of each flow, and color represents individual water masses. antarctic field and modeling studies indicated that
Surface Water (aaSW): light blue. Shelf Water (SW): purple. modified circumpolar
the Ross Sea is a seasonally iron-limited
deep Water (mcdW): orange. circumpolar deep Water (cdW): red. New antarctic
deep Water: green. New antarctic bottom Water: dark blue. ecosystem, whereby a “winter reserve”
94 Oceanography | Vol. 25, No. 3
of dissolved iron is depleted from sur- 2004), when mixed layers in the Ross Sea of intact colonies when growth becomes
face waters during the growing season shoal to depths that provide sufficient limited by iron availability (Smith et al.,
(Sedwick et al., 2000; Coale et al., 2003). irradiance for growth. This pattern is 2011b). The life cycle of Phaeocystis
However, more recent observations sug- striking, as few other Southern Ocean involves both solitary and colonial
gest that iron limitation can develop rap- regions, even those some 1,600 km stages, and seasonal changes in the rela-
idly during the late spring, implying that north of the Ross Sea, bloom so early in tive numbers of each have been observed
continued growth and biomass accu- the austral growing season. High, early (Smith et al., 2007). After the decline of
mulation during the summer months season growth rates, coupled with low P. antarctica, phytoplankton assemblages
requires inputs of “new” iron to surface rates of grazing and sinking losses, are are dominated by diverse populations of
waters during summer (Peloquin and responsible for the large accumulation diatoms, which tend to dominate assem-
Smith, 2007; Sedwick et al., 2011). of biomass that is repeatedly observed in blages in austral summer in the shal-
the Ross Sea in late December. lower mixed layers and are often associ-
plaNktoN dyNamicS The temporal dynamics of phy- ated with sea- and glacial-ice melt, and
Studies of phytoplankton in the Ross toplankton assemblages in the Ross with other groups (dinoflagellates, cryp-
Sea have been as intensive as anywhere Sea are well known. The dominance tomonads, silicoflagellates) appearing in
in the Southern Ocean. Smith and of Phaeocystis antarctica (Smith and isolated locations (Arrigo et al., 1999).
Gordon (1997) found Ross Sea biomass Gordon, 1997; Arrigo et al., 1999) in the While this pattern is relatively predict-
to be elevated by mid-November, and spring biomass is generally explained able, significant interannual variations
growth was proceeding rapidly by that by its ability to photosynthesize under in the contribution to total biomass have
time; they extrapolated back in time the reduced irradiances found in spring been noted (Smith et al., 2006, 2011a).
to suggest that growth was initiated in (Smith et al., 2007). Surface mixed layers This regular seasonal succession
late October, which was later confirmed in spring are ~ 50 m, while those in sum- imposes constraints on biogeochemical
by direct observations (Smith et al., mer can be less than 10 m. Chlorophyll cycling because the two dominant func-
2000). Although satellite observations of concentrations in P. antarctica blooms tional groups have markedly different
ocean color during this period are often can exceed 15 µg L–1, but generally elemental ratios and roles in food webs;
obscured by clouds, those data also con- rapidly decrease to low levels over a P. antarctica has a C:N:P ratio of approxi-
firm that chlorophyll begins to increase two-week period, possibly as a result of mately 139:19:1, whereas diatoms have
in November (Arrigo and van Djiken, aggregate formation and rapid sinking ratios of 76:12:1 (Arrigo et al., 2000).
figure 4. interpolated zonal sections of dissolved iron (dfe) concentration in the ross Sea polynya along 76°30’S in spring
(october to November 1996, left panel) and summer (January to february 1997, right panel), showing the seasonal drawdown
of dissolved iron in the upper water column. The shallow area is ross bank. data are replotted from coale et al. (2005).
Oceanography | September 2012 95
Furthermore, these ratios are retained models. The latter have the advantage of important, SAM explains > 64% of the
in material that sinks to depth, thus increased temporal and spatial resolu- interannual variance in chlorophyll a
potentially impacting deepwater con- tion, and are likely the most accurate concentrations on the Ross Sea conti-
centrations of nutrients and sediments means of assessing shelf-wide productiv- nental shelf, with the positive phase of
(Dunbar et al., 2003). Additionally, ity. Arrigo et al. (2008a) estimate that the SAM associated with increased phyto-
P. antarctica is thought to be largely mean net primary productivity for the plankton biomass (Arrigo et al., 2008a).
ungrazed by most mesozooplankton entire Ross Sea sector of the Southern Westerly winds are projected to increase
(Tagliabue and Arrigo, 2003), although Ocean is 69 g C m–2 yr–1, which is less in coming decades (le Quéré et al., 2007),
pteropods may consume both solitary than field-based extrapolations and suggesting that phytoplankton biomass
cells and colonies (Elliott et al., 2009). In earlier remote-sensing estimates but on the Ross Sea shelf could increase in
contrast, diatoms are generally consid- accurately reflects substantial variations the future, albeit constrained ultimately
ered to be grazed at significant rates by in both space and time over this large by iron availability.
zooplankton and incorporated into rap- oceanic region. Annual net primary Finally, micro- and mesozooplankton
idly sinking fecal pellets, hence minimiz- production on the Ross Sea continental in the Ross Sea have received relatively
ing remineralization in the water column shelf is considerably higher, averaging little attention compared to the tempo-
and favoring their export to the benthos. 179 g C m–2 yr–1. Such high productivity ral and spatial scales of phytoplankton
Phytoplankton growth, particularly and export efficiency gives rise to large studies in the region. Caron et al. (2000)
that of diatoms, has been experimentally air-sea gradients in pCO , with sea- conducted dilution experiments to assess
2
shown to be iron limited in summer water CO levels often dropping below microzooplankton grazing, but most
2
under conditions of high irradiance 200 µatm in summer. Recent model (83%) failed to demonstrate significant
(Sedwick and DiTullio, 1997; Sedwick simulations suggest that the Ross Sea ingestion rates. Microzooplankton
et al., 2000). Given the predictability continental shelf is responsible on aver- biomass has been quantified (Dennett
of the seasonal assemblage pattern, as age for more than 25% of the estimated et al., 2001), but microzooplankton graz-
well as the low dissolved iron concen- total CO uptake of the entire Southern ing’s impact on biogeochemical cycling
2
trations in surface waters during much Ocean (Arrigo et al., 2008b). remains unclear. Investigations of meso-
of the growing season (Sedwick et al., Annual net primary production in the zooplankton abundance and feeding
2011), and the likely small differences in Southern Ocean is influenced by large- are similarly limited to defined regions.
taxon-specific iron requirements (Garcia scale climate variations (Lovenduski and Deibel and Daly (2007) concluded that
et al., 2009), it appears that seasonal Gruber, 2005), particularly the Southern overall biomass of mesozooplankton in
changes in the assemblages may involve Annular Mode (SAM). This mode of the Ross Sea, despite high regional pri-
interactions between irradiance and climate variability is characterized by mary production, was ~ 15% that of the
iron (Boyd, 2002). Peloquin and Smith oscillations in the north-south gradient Scotia Sea. Tagliabue and Arrigo (2003)
(2007) also noted the occurrence of sub- in atmospheric pressure that controls the suggested that Ross Sea zooplankton
stantial diatomaceous blooms after the strength of the westerly winds. During were anomalously low as a result of the
decline of P. antarctica, which implies the positive SAM phase, the north- decoupling of grazers and phytoplankton
additions of iron to the euphotic zone, south pressure difference increases and growth, but this hypothesis has not been
although the mechanism for increasing westerly winds intensify, increasing the empirically tested. Sediment traps have
iron remains unknown. Macronutrients rate of Ekman divergence at the ocean collected substantial numbers of meso-
(N, P, Si) rarely drop to limiting lev- surface and stimulating upwelling along zooplankton fecal pellets, which at times
els; indeed, seasonal nitrate removal is the Antarctic Divergence. SAM’s impact comprise 100% of the flux (Smith et al.,
generally 15 µM, or about half of the is particularly strong in the Ross Sea, 2011b), suggesting that mesozooplank-
winter concentrations. where it can explain 73% of the variance ton ingestion in the surface waters can at
Estimates of primary productivity in sea surface temperatures, with cooler times be substantial. Ainley et al. (2006)
have been based on both in situ mea- waters being associated with positive proposed that the low mesozooplankton
surements and satellite-based bio-optical SAM phases (Arrigo et al., 2008a). More abundance was the result of a trophic
96 Oceanography | Vol. 25, No. 3
cascade centered around the unusu- The fish fauna of the Ross Sea is com- nonuniform (Figure 5). The squid,
ally high abundance of meso- and apex posed of 95 species from 16 families and fish, emperor penguins, and Weddell
predators, many of which feed heavily on is dominated by notothenioids (a perch- (Leptonychotes weddellii) and crabeater
krill or on small fish that feed principally like group; 64% of species), with the (Lobodon carcinophagus) seals presum-
on krill. The resulting paucity of krill remainder being mostly liparids (snail- ably remain in the Ross Sea year-round;
leads to reduced grazing on diatoms. fishes) and zoarcids (eelpouts; Eastman, the remainder are present only dur-
Antarctic krill (Euphausia superba) are 2005). Most Ross Sea fish are benthic, ing October to March. During spring/
largely absent from the inner continental epibenthic, or cryopelagic (within the summer, Weddell seals and the two pen-
shelf, but occur in the outer portions of sea ice brash), with the exception of two guin species remain close to the coast for
troughs and near the shelf break; con- very important species, both notothe- breeding, and the squid is found along
versely, the reciprocal pattern is seen in nioids: Antarctic toothfish (Dissostichus the slope; the remaining species use vari-
crystal krill (E. crystallorophias), which mawsoni) and Antarctic silverfish ous parts of the continental shelf with
occur throughout the inner shelf region (Pleuragramma antarcticum). These two habitats largely defined by sea ice, the
(Sala et al., 2002). species, despite their lack of swim blad- marginal ice zone, polynyas, and other
ders, inhabit the mid- to surface waters restricted environments.
higher trophic leVelS as adults; young toothfish (< 100 cm) are This assemblage of apex and meso-
aNd ecology benthic. The extremely large toothfish predators is largely sustained by foraging
The Ross Sea benthic biota is considered (~2 m, > 100 kg as adults) have been on three species: silverfish and crystal
to be one of the most species-rich in called the “sharks of the Antarctic,” krill over the shelf, and Antarctic krill
the Southern Ocean and a biodiversity owing to their large size and voracious over the slope (Ballard et al., 2011).
“hotspot” (Clarke and Johnston, 2003). piscine diet, and the very abundant sil- Hence, close coupling is observed at this
Of the few thousand species known from verfish are known as “the herring of the level of the food web. The vertical and
the Antarctic, more than 400 were first Antarctic,” owing to their abundance and spatial dimensions of foraging lessen the
described from the Ross Sea, 40 of which loose schooling behavior. Silverfish are a extreme competition for prey. The apex
are endemic, including mainly fish and major prey of almost every upper trophic predator in the system is the ecotype B
invertebrates (Ainley et al., 2010a). The level predator over the shelf, including orca (Orca orcinus), which feeds largely
reasons for this richness likely stem from toothfish. Weddell seals and Ross Sea on seals and possibly emperor penguins.
the diversity of habitats as defined by orcas feed upon the toothfish. Slightly below orcas in the trophic
depth and currents (Barry et al., 2003). The upper trophic level predators in pyramid are leopard seals (Hydrurga
In addition, the northwest corner of the the Ross Sea are abundant and diverse, leptonyx), which can be eaten by killer
shelf, unlike other Antarctic shelves, was composed of 16 major species, including whales, but are significant predators on
ice-free during past glaciations. On the one fish (toothfish), Adélie and emperor seals and penguins.
basis of bottom samples, Bullivant (1967) penguins (Pygoscelis adeliae, Aptenodytes
divided the fauna broadly into five com- forsteri), seals, and cetaceans (Ballard the future
munities. The most distinctive of them et al., 2011). High proportions of world Evidence of significant annual, interan-
was the Pennell Bank assemblage and populations for several of these species nual, and decadal variability in Ross Sea
the assemblages around Ross Island; the reside in the Ross Sea (Table 1). The high water properties and sea ice character-
most widespread were those of the deep diversity and abundance is maintained istics emphasizes the region’s sensitivity
shelf (over the deeper parts of the banks) by a mosaic of habitats, defined vertically to changing oceanographic and atmo-
and those in the muds of the troughs. by depth of foraging and horizontally spheric forcing. During the winter of
Barry et al. (2003) suggest that the most by sea ice affinity and seasonal move- 2002, the Ross Sea shelf was anomalously
important factor, other than depth, ments, as well as a temporal component covered with sea ice due to the presence
controlling benthic diversity is the near- (Ballard et al., 2011). Thus, the distri- of a grounded, massive iceberg; develop-
bottom current flow, which regulates bution of apex predators, such as pen- ment of the Ross Sea Polynya was much
the food supply. guins, is both spatially and temporally suppressed (Martin et al., 2007); and the
Oceanography | September 2012 97