F E A T U R E

THE PALMER LTER: A LONG-TERMECOLOGICAL RESEARCH PROGRAMAT PALMER STATION, ANTARCTICA

By Raymond C. Smith, Karen S. Baker,William R. Fraser, Eileen E. Hofmann,David M. Karl, John M. Klinck,Langcion B. Quetin, Barbara B. Prbzelin,Robin M. Ross, Wayne Z. Trivelpieceand Maria Vernet

THE ANTARCTIC marine ecosystem-the assem-blage of plants, animals, ocean, sea ice, and islandcomponents south of the Antarctic Convergence-is among the largest readily defined ecosystems onEarth (36 X 106 km2) (Hedgpeth, 1977; Petit et aZ.,199 1). This ecosystem is composed of an intercon-nected system of functionally distinct hydro-graphic and biogeochemical subdivisions (Treguerand Jacques, 1992) and includes open ocean,frontal regions, shelf-slope waters, sea ice, andmarginal ice zones. Oceanic, atmospheric, and bio-geochemical processes within this system arethought to be globally significant, have been infre-quently studied, and are poorly understood relativeto more accessible marine ecosystems (Harris andStonehouse, 1991; Johannessen et al, 1994). ThePalmer Long-Term Ecological Research (PalmerLTER) area west of the Antarctic Peninsula (Fig.la) is a complex combination of a coastal/conti-nental shelf zone and a seasonal sea ice zone9 be-cause this area is swept by the yearly advance andretreat of sea ice. The Palmer LTER program is a

R.C. Smith (bio-optics, remote sensing), University of Cal-ifornia, Santa Barbara, Institute for Computational Earth Sys-tem Science (ICESS), Geography Department, Santa Barbara,CA 93106, USA; KS. Baker (data management), M. Vernet(primary production), University of California, San Diego,Scripps Institution of Oceanography, Marine Research Divi-sion, La Jolla, CA 92093-0218, USA; W.R. Fraser (apexpredators), W.Z. Trivelpiece (apex predators), Montana StateUniversity, Polar Oceans Research Group, Department of Biol-ogy, 3 10 Lewis Hall, Bozeman, Montana 59717, USA; E.E.Hofmann (biophysical modeling), J.M. Klinck (physical oceanmodeling), Old Dominion University, Center for Coastal Phys-ical Oceanography, 768 West - 52nd Street, Norfolk, VA23529-2076, USA; D.M. Karl (microbial ecology), Universityof Hawaii at Manoa, Department of Oceanography, 1000 PopeRoad, Honolulu, Hawaii 96822, USA; L.B. Quetin (secondaryproducers), R.M. Ross (secondary producers), B.B. Prezelin(primary production), University of California, Santa Barbara,Marine Science Institute, Santa Barbara, CA 93106, USA.

multidisciplinary program established to study thispolar marine ecosystem. The Palmer LTER

Long-term ecological research addresses thetemporal gap between short-term process-orientedstudies and paleo-oceanography by studyingprocesses on time scales from months to decades tocenturies. Such studies provide opportunities forcontemporaneous hypothesis testing across interre-lated phenomena, where cause and effect can onlybe detected on longer time scales. For example,there is now widespread recognition that there arenumerous natural and anthropogen icall y forcedphenomena, such as atmospheric ozone depletion,green-house gas accumulations, El Nifio-SouthernOscillation (ENSO), global deforestation and sunspot cycles, with potentially significant ecologicalimpacts that will require long-term studies to re-solve. Further, long-term research has been shownto be especially important in the study of a varietyof ecological phenomena like the following(Strayer et al, 1986): 1) slow processes, such aslong life cycles of key species, 2) episodic phe-nomena, such as relatively rare and intense physi-cal forcing, 3) processes with high levels of year-to-year variability, such as climate in polar regions,and 4) elusive and/or complex processes where it isdifficult to distinguish meaningful patterns fromrandom events, such as interannual variability of

program is a

multidisciplinary

program established to

study this polar marine

ecosystem.

recruitment success of keystone species.The National Science Foundation (NSF) created

the Long-Term Ecological Research (LTER) Net-work in 198 1 by establishing six sites in the conti-nental United States (Brenneman, 1989). From itsinception, the LTER program has endeavored tocoordinate research from each site for possible in-tersite and transdisciplinary interpretation of long-term ecological phenomena (Franklin et aZ., 1990).The LTER Network supports a comprehensive setof core measurements across the sites, in addition

OCEANOGRAPHY,401. 8, No. 3.1995 77

Palmer LTERLarge-scale Grid

cw* -*m Is..-

Iv-’ Weddell Sea

\I I

29@’ 29S’ 3000

295’ 3oo”

Palmer LTERSeabird Foraging Ranges-------_____*-- --_ -.

“,Bimark Strait

-600

-66O295” 300*

Fig. 1: (a) The Palmer LTER region located west of the Antarctica Penin-sula, Antarctica. Points show the location of cardinal sampling stationswithin the large-scale peninsula grid which covers an area of 900 km(roughly parallel to the peninsula) by 200 km (on to off-shore). Sea ice extentfor low, average, and high ice years is indicated by fine dotted lines and cor-responds to the color plate shown on the back cover. (b) The Palmer Basinarea. Palmer Station is located at the southern end of Anvers Island. The for-aging ranges of the seabirds during the reproductive season are indicated bythe concentric circles at 50 and 100 km from Palmer Station. A jme-scalesampling grid exists within ~3 km of Palmer Station. Automatic WeatherStation (AWS) installations within the area are labeled AWS where B =Bonaparte, H = Hugo, and R = Racer Rock.

78 OCEANOGRAPHY+• ~. 8, No. 3.1995

to project-specific field experiments designed totest site-specific hypotheses (Callahan, 1984). Theset of core measurements include: primary produc-tion, nutrient concentrations and cycling rates, or-ganic matter transport, trophic interactions, and ef-fects of natural and anthropogenic disturbances.Currently, the LTER Network includes 18 siteswith diverse ecosystems, including Arctic tundra,hardwood forests, lakes, streams, prairie grass-lands, high deserts, tropical rainforests, as well asAntarctic coastal and terrestrial desert regions. ThePalmer LTER program was established in 1990,was the first LTER site in the Southern Hemi-sphere, and is the only marine (from coastal topelagic) site in the LTER Network. We share withother sites similar core measurements, as well asthe unifying ecological concept that physical fac- ~tors force the biology of the system.

Over the past 15 years there have been severalmajor multiyear studies of the Antarctic marineecosystem. Although many studies were conductedmostly north and east of the Antarctic Peninsula,there were three BIOMASS (Biological Investiga-tions of Marine Antarctic Systems and Stocks) re-search cruises which were carried out in the south-west Atlantic Ocean, including the northwestextent of the Palmer LTER study region (El-Sayed,1994). BIOMASS was an international multidisci-plinary program conducted from 1980 to 1985,which studied and assessed the standing stocks ofmicro- and macroorganisms in the Antarcticecosystem, making significant contributions to ourunderstanding of Southern Ocean ecology.AMERIEZ (Antarctic Marine Ecosystem Researchon Ice Edge Zones) concentrated on seasonal ice-edge dynamics through a series of research cruisesfrom 1983 to 1986 in the Weddell Sea (Ainley etaZ., 1986). The RACER (Research on AntarcticCoastal Ecosystem Rates) program conducted acomprehensive summer study of coastal processesin the western Bransfield Strait (1986-87) and inthe Gerlache Strait (1989) (Karl, 1991), and insubsequent years revisited the study area and ex-panded their temporal (austral winter) and spatial[Grandidier Channel, Crystal Sound and Mar-guerite Bay which are in the present LTER area(Fig. 1)] coverage. The AMLR (Antarctic MarineLiving Resources) program, sponsored by the Na-tional Oceanic and Atmospheric Administration(NOAA), maintains an annual ecosystem studywhich focuses on trophic linkages in the vicinity ofElephant Island, an area northwest of the PalmerLTER study region. AMLR also supports someseabird work in the Palmer LTER study region.Recently, as part of the Southern Ocean JointGlobal Ocean Flux Study (JGOFS), the U.K.“Sterna” study was carried out in the southeastPacific sector of the Bellingshausen Sea (Turnerand Owens, 1995). This program evaluated themagnitude and variability of biogeochemical fluxesand investigated primary production in different

environments and communities of direct relevance phy, primary and secondary production and seato the Palmer LTER such as: 1) the microbial com- bird nesting sites. Sea ice, in situ bio-optical watermunities growing in or on the ice, 2) the marginal properties, temperature, conductivity, dissolvedice zone (MIZ) determined by the persistence of CO2 concentrations and estimated air-sea fluxes,meltwater density structure, and 3) the open-water photosynthetic pigments, nutrient concentrations,pelagic areas. All of these studies made significant phytoplankton and bacterioplankton abundancecontributions to our overall understanding of the and production rates, zooplankton abundance andAntarctic marine ecosystem and the early ones composition, secondary production, particle sedi-were instrumental in the design of the Palmer mentation and seabird ecology are routinely sam-LTER program. pled and/or monitored within the grids.

A central tenet of the Palmer LTER is that theannual advance and retreat of sea ice is a majorphysical determinant of spatial and temporalchanges in the structure and function of theAntarctic marine ecosystem, from total annual pri-mary production to breeding success in seabirds.We are working with a number of testable hy-potheses linking sea ice to 1) the timing and mag-nitude of seasonal primary production, 2) the dy-namics of the microbial loop and particlesedimentation, 3) krill abundance, distribution, andrecruitment, and 4) the survivorship and reproduc-tive success of apex predators. The overall objec-tives of the Palmer LTER are to 1) document theinterannual variability of annual sea ice and thecorresponding physics, chemistry, optics, and pri-mary production within the study area, as well asthe life-history parameters of secondary producersand apex predators, 2) identify the processes thatcause variation in physical forcing and the subse-quent biological response among the representa-tive trophic levels, 3) construct models that linkecosystem processes to environmental variablesand that simulate spatial/temporal ecosystem rela-tionships, and 4) employ such models to predictand validate ice-ecosystem dynamics. A key chal-lenge for the Palmer LTER project is to character-ize and understand the linkages between the differ-ent spatial and temporal scales of the variousphysical and biological components of the Antarc-tic ecosystem.

The Antarctic Marine Ecosystem

To structure our long-term regional observa-tions, we created a sampling grid along the westcoast of the Antarctic Peninsula (Waters andSmith, 1992), analogous to the well-known Cal-COFI (California Cooperative Oceanic FisheriesInvestigations) grid along the west coast of NorthAmerica. Establishing a fixed sampling grid was

k

As noted by many previous studies, a dominantand distinguishing characteristic of SouthernOcean marine ecology is sea ice. Hunt (199 1) andAinley et a2. (1994) discuss the influence of vary-ing sea ice coverage on seabird environments andtrophic level coupling. According to these studies,different springtime seabird habitats associatedwith varying sea ice coverage, which thereforehave different implications on trophic level cou-pling, are the following: 1) open leads andpolynyas, through which seabirds can gain accessto the water column and underside of sea ice, 2)the ice edge, which can either be compact orhighly diffuse, and which is a major ecologicalboundary, and 3) the outer marginal ice zone(MU), where meltwater contributes to stabiliza-tion of the water column. The MI2 is an areabounded on the open ocean side by the stabilizinginfluence of meltwater and on the pack ice side bythe penetration of ocean swell (Stammerjohn andSmith, 1995). The MI2 can be up to 250 km inwidth and is often an area of high biological pro-ductivity (Smith and Nelson, 1985). It is also anecosystem boundary where the flow of energy, thecycling of nutrients and the structure of biologicalcommunities change dramatically, both tempo-rally and spatially. The area1 extent of sea icecover and the associated timing of the MIZ in re-lation to specific geographic areas (e.g., seabirdrookeries) have high interannual variability. Therange for variable ice cover is shown schemati-cally in Fig. la. Sea ice extent for low, average,and high ice years are indicated (Stammerjohnand Smith, 1996).

. . . the annual

advance and retreat of

sea ice is a major

physical determinant

of spatial and temporal

changes in the

structure and function

of the Antarctic marine

ecosystem . . .

motivated by the need for station locations thatcould be visited repeatedly over time scales ofmany years. This geographically fixed study areawill also simplify seasonal and interannual com-parisons and facilitate modeling of multidiscipli-nary data sets. We have embedded within thelarge-scale peninsula grid smaller sampling gridsin the vicinity of Palmer Station (Fig. lb) to focuson the foraging ranges of our apex predators(roughly 100 km from Palmer Station). In addi-tion, we have a fine-scale grid in the immediatevicinity of Palmer Station (within the 3.7 km boat-ing limit), which is specific to the local hydrogra-

Mesoscale oceanic circulation patterns in thelarge-scale LTER grid are not well known (Hof-mann et aZ., 1996). The barotropic circulation isunknown, and there are no direct current measure-ments anywhere in the region. Even tidal charac-teristics are poorly known (Amos, 1993). Thereare some suggestions of the direction of flow onthe shelf from dynamic calculations and other in-direct indications. The Antarctic Circumpolar Cur-rent (ACC) stands out in all calculations and flowsnortheasterly off the shelf. There are a number ofindications of southward flow on the inner part ofthe shelf (Fig. 2). Some studies suggest the exis-tence of two gyres (Stein, 1992), but it is not cer-tain whether permanent eddies are part of the

OCEANOGRAPHYWO~. 8, No. 3.1995 79

295’ 3oo”I I 3OY

Palmer LTER_# Oceanographic Features

Drake Passage

Fig. 2: Schematic showing the circulation in the upper 200-300 m to thewest of the Antarctic Peninsula. The Polar Slope Current is indicated by thelong arrow north of Bransjield Strait, and the two smaller wide arrows in theBellingshausen Sea represent the possibility of a single cyclonic gyre in thisregion. The Palmer LTER large-scale peninsula grid is shown by fine dots.

coastal circulation, or if the coastal flow is merelyconvoluted and not sampled sufficiently to resolveits features. The most prominent water mass westof the Antarctic Peninsula is Circumpolar DeepWater (CDW), which is characterized by tempera-tures greater than OS’C and by salinities of 34.75(Hofmann et aZ., 1996). The CDW brings macro-nutrients onto the shelf, as well as warm saltywater. The presence of a warm, salty water massnear the bottom of the mixed layer has consider-able implications for heat and salt budgets in thisregion. In addition, the circulation patterns and thepresence of CDW may affect the timing and extentof sea ice, as well as the transport and/or retentionof physical and biological properties in the PalmerLTER area.

Smith et aZ. (1996b) have recently reviewedhistorical phytoplankton biomass and productivitydata for the Palmer LTER region and concludethat the average productivity of the region is onthe order of a few hundred grams C m-2 y-l whichis roughly about a factor of 5 lower than other pro-ductive coastal areas of the world’s oceans(Chavez and Barber, 1987). Factors that regulateprimary production within this environment in-clude those that control cell growth (light, temper-ature, and nutrients) and those that control the ac-cumulation rate of cells and hence populationgrowth (water column stability, grazing, sinking,

and advection) Sea ice mediates several of thesefactors and often, but not always, conditions thewater column for an initial bloom which is charac-terized by a pulse of production restricted in bothspace and time. The relationship of the relativecontribution of bloom versus nonbloom primaryproductivity to the timing and balance of the re-sulting food web dynamics is currently unknown.

Like most marine food webs studied so far, thetrophic relationships in Antarctica are complex.However, the links between primary producers,grazers and apex predators (seabirds, seals, andwhales) are often short and may involve fewerthan four key species (Fig. 3). Predators tend toconcentrate on a core group of prey species, forexample, the abundant euphausiids and fish close

-to the base of the food web. The general samplingapproach in our study capitalizes on the close cou-pling between trophic levels, the limited numberof species involved, and the fact that the dominantpredators are seabirds that nest on land and arethus easily accessible during the spring and sum-mer breeding season. The two key apex predatorswe study are the Adelie penguins (Pygoscelisadeziae), which dominate the seabird assemblagenear Palmer Station in terms of abundance andbiomass, and the south polar skuas (Catharactamaccormicki), which inhabit numerous small is-lands in the vicinity of Palmer Station. During thesummer breeding season, these seabirds depend onresources that can be found within their respective

salps)

t jxcretion

,b Phytoplankton Pr&ztion

(biomass and prodtic&@ IPropertfes ~ 6 0.4qm, p 2D jimI

LA c-frfadiance M

Inorganic

Deathnutrients

ti;+ ;Particles Partick Oynamics microbial

(fecal pellets, snow) (POG DOW loop

Flux bwtetia

Emphasized processes andspecies

Fig. 3: Conceptual diagram of the ecosystem (foodweb and environmental factors) investigated by thePalmer LTER. Boxes indicate various key compo-nents within this Antarctic marine ecosystem.

80 OCEANOGRAPHY+• ~. 8, No. 3.1995

foraging ranges, which are believed to be within100 km of the breeding sites. Finding sufficientprey within these foraging ranges is critical to thereproductive success of these birds. Preferred preyfor Adelie penguins is Antarctic krill (Euphausiasuperba) and for south polar skuas, silverfish(Pleuragramma antarcticurn). Both prey are foundwithin the various marine habitats inside these for-aging ranges, but the timing of prey availabilityand abundance is highly variable from year toyear. Potential mechanisms controlling prey avail-ability within the foraging ranges may include 1)changes in water mass distribution and oceaniccurrents (meanders and eddies), 2) variability inreproductive and recruitment success of the prey,3) patchiness, abundance and type of prey food,and 4) prey behavior and predation.

Conceptual Model of Physical and BiologicalLinkages

Solar radiation, atmospheric and oceanic circu-lation as well as sea ice cover are the physicalforcing mechanisms driving variability in biologi-cal processes at all trophic levels in Antarctica.Analyzing the extreme seasonality of these forcingmechanisms in conjunction with the seasonal tim-ing of ecologically important events and life histo-ries of key species from each trophic level pro-vides a conceptual model for the discussion oftrophic linkages (Fig. 4). Some of the componentsin Figure 4 are known with relative certainty,while others are proposed based on our currentknowledge and hypotheses.

Weather in the Palmer LTER study area is ex-tremely variable, both seasonally and interannu-ally. Solar radiation has a strong seasonal cycle atthese latitudes, with shortwave (0.3 to 3.0 pm)input varying from close to zero in winter to 390W m-2 under cloud-free conditions near summersolstice. Daylength, a key biological parameter,varies from 9 hours in early spring to more than20 hours in midsummer. It also has different regu-lating effects on phytoplankton growth, as doestotal photosynthetically available radiation (PAR),another key parameter. Anthropogenic ozone-re-lated increases in ultraviolet B (UVB) radiation(280-320 nm) are well documented, and springand early summer UVB values are dramaticallyhigher today than they were under preozone holeconditions (Smith et aZ., 1992). Cloud cover ispersistent and strongly influences the heat budget,incident PAR and ultraviolet radiation. In general,weather in the Palmer LTER study area is typi-cally maritime Antarctic, with snow (70 cm y-l)and rain (40 cm y-l) occurring any time of theyear. Temperatures at Palmer Station are relativelymild, averaging about -6’C in July and 3’C inJanuary, with temperature extremes recorded at-25’C and 1O’C. Weather, while mild by compari-son to the interior of the Antarctic, has a strongannual cycle, is punctuated by episodic storm

events, and shows an interannual variability incontinental influence, especially during winter(Smith et aZ., 1996a). Automatic weather station(AWS) units (Bromwich and Stearns, 1993) havenow been installed at the end of Bonaparte Point(64’46’S 64’04’W) near Palmer Station and inthe Hugo archipelago (64’57’S 65’4l’W). Thesetogether with the AWS on Racer Rock (64’lO’S61’32’W) in the Gerlache Strait, make a suite ofthree AWS installations within the Palmer LTERstudy area, in addition to the Palmer Stationweather records (Baker and Stammerjohn, 1995).

The timing and extent of the annual advanceand retreat of sea ice in the vicinity of Palmer Sta-tion are highly variable. Analysis of satellite pas-sive microwave data, available from the past 17years, shows that the Palmer LTER study area hasa mean annual ice cycle that, in contrast to meanannual cycles of other areas of the SouthernOcean, typically involves a relatively short periodof ice advance (5 mo), followed by a relativelylong period of ice retreat (7 mo) (Stammerjohn,1993; Stammerjohn and Smith, 1996). In addition,the satellite data show some evidence of a recur-rent 6 to 8 year cycle, where several consecutivehigh ice years (1979-82, 1986-87, and 1991-92)alternate with consecutive low ice years (1983-85,1988-90, and 1993-94). This time series is stilltoo short to resolve any persistent low frequencyperiods statistically, but evidence is accumulatingto show that high interannual variability in magni-tude, timing of ice advance and retreat, duration ofnear maximum and minimum ice coverage, andapparent clumping of high or low ice years have asignificant impact on the survival rates, distribu-tions and/or life histories of key indicator species(Ross and Quetin, 1991; Fraser et aZ., 1992; Fraserand Trivelpiece, 1996; Siegel and Loeb, 1995).Further, we are beginning to define the physicalforcings (e.g., thermodynamics versus advectiveprocesses) controlling variability in sea ice cover-age and the resulting ecological consequences.

There are hydrographic seasonal variations oftemperature and salinity in the upper 0 to 100-150m of the water column. In winter, temperature andsalinity are uniform down to - 100 m (Hofmann etaZ., 1996). Beginning in spring and during sum-mer, the ocean warms from the surface, creatingone or more layers of warmer water that are a fewtens of meters thick. Episodic storms mix theselayers so that the vertical temperature structurenear the surface can be quite variable. There isalso some freshening of the surface layer duringsummer due to sea ice and glacier melt, which canplay a key role in stabilizing the water column andhence increase rates of primary production. De-pending on the intensity of macrozooplanktongrazing, phytoplankton biomass can accumulatenear the surface. Although variations in salinityare small, the density and stability of the upperwater column are more a function of salinity than

. . . the seasonal tim-

ing of ecologically im-

portant events and life

histories of key

species from each

trophic level provides

a conceptual model for

the discussion of

trophic linkages . . .

OCEANOGRAPHY+• ~. 8, No. 34995 81

temperature. It appears that the CDW floods theshelf throughout the year, characterizing the deepwaters in the LTER study area.

. . . we must view the Key factors controlling phytoplankton cell and

microbial loop modelspopulation growth in waters of the SouthernOcean are 1) extreme seasonal variability in PAR

as hypotheses that de- (mediated by daylength and clouds), 2) low tern-

serve a thorough,peratures, 3) generally high nutrient levels, and 4)springtime water column stability (mediated by

quantitative field eval- sea ice and glacier melt and surface heating). Therelative contribution of MIZ-related production to

uation. overall production continues to be debated. Smithand Nelson (1985) and Smith et a2. (1987) showevidence that the MIZ, especially in spring, sup-ports high phytoplankton biomass and/or high pro-duction rates. On the other hand, recent observa-tions (Lancelot et al., 1993; Boyd et al., 1995)suggest that speci .fic meteorological conditionsinfluence whether blooms do or do n.ot occur inthe MIZ. Nonetheless, total annual productivity isthought to be dominated by the high productionrates associated with spring blooms, whose devel-opment may be timed and paced by ice-drivenwater column stability and/or favorable meteoro-logical conditions. The timing of this burst of pro-duction and consequent food availability for preyand thus predator, as well as the habitat considera-tions associated with these environmental condi-tions, affect a variable mix of trophic level interac-tions (Fig. 4). These couplings are subject not onlyto the progression of the seasons, but also toepisodic events that disrupt and/or reset the cycleof water column stability, phytoplankton produc-tivity, and subsequent linkages.

Our models of the trophic organization ofAntarctic marine ecosystems have evolved consid-erably during the past decade. Prior to 1980, en-ergy flow in Southern Ocean habitats was thoughtto be dominated by relatively short and, therefore,efficient transfers from large (>20 pm) phyto-plankton cells to krill and, subsequently, to apexpredators. More recently, our concept of the ma-rine food web has been expanded to reflect the po-tential roles of heterotrophic microorganisms in-cluding bacteria, protozoans and small (< 150 pm)nonkrill crustaceans. Heterotrophic microorgan-ism-based food webs, also referred t,o as microbialloops (Azam et aZ., 1983) are present in all aquaticenvironments including Antarctic habitats. Thesedetritus-driven systems are fueled by nonrespira-tory community carbon losses, including dissolvedand particulate organic matter released by excre-tion, predation, and mortality. Because microbialloops require several trophic levels to transfer car-bon and energy to apex predators, most detritus-based food webs are inherently inefficient andsometimes constitute major energy sinks. It is im-portant to emphasize that comprehensive, quantita-tive ecosystem studies of energy and carbon flowin Antarctic food webs do not exist. At best, thereare order of magnitude estimates for a few se-

lected regions. A major, unexpected result of t,heAntarctic field studies conducted so far is the ap-parent short-term uncoupling of algal and bacterialmetabolic processes (Cota et aZ., 1990; Karl et aZ.,1991; Karl and Bird, 1993). The reasons for thisuncoupling are not well understood. Consequently,we must view the microbial loop models as hy-potheses that deserve a thorough, quantitative fieldevaluation.

Sea ice provides one of the major habitats formicroorganisms in Antarctic marine ecosystems,and it is possible that some microbial food websmay be entirely ice-associated (Palmisano andGarrison, 1993). In the Palmer LTER program, wehave just begun to investigate these unique habi-tats systematically. Preliminary results suggestthat the bacterial community goes through several Ias yet poorly-defined stages of succession as theannual pack ice recedes (Christian and Karl,1994). The water is initially seeded with bacteria,as well as substantial inputs of organic matter thatmay be quite different in chemical compositionfrom the organic matter in the water column. Phy-toplankton blooms at the receding ice edge pro-vide additional sources of organic matter, throughexcretion, lysis, and grazing, that are likely to bepresent only for a short time. Furthermore, photo-chemical alteration of dissolved organic mattermay be accelerated in the sea ice habitat, becauseit is immobilized in a region of high UV and visi-ble light flux. These processes are likely to play animportant role in the adaptation of marine bacteriato this seasonally variable environment, and thebiochemical characteristics of the bacteria maychange rapidly with time. Future studies will focuson the quantitative role that variations in ice covermay have on the presence and intensity of micro-bial loop processes.

Associated with the increase in daylength andt,he melting of sea ice, both contributing to watercolumn stratification, phytoplankton biomass (chl-a) in the Palmer LTER area starts to accumulatenear the end of November in an average ice year.The average chl-a in the top 30 m can increasefrom CO.5 mg m-3 in a pre-bloom period, to higherthan 15 mg m-3 during a spring bloom, with aver-age values between 1 and 3 mg m-3 (Smith et aZ.,1995b). Blooms are mostly dominated by cells>20 pm, which are typical1Y large or chain- form-ing diato ms, although ccl 1s <20 pm also growduring a bloom (i.e., cryptomonads and prasino-phytes). On occasion, the bloom is dominated by -the colonial prymnesiophyte Hzaeocystis puchet-tii. Smaller diatoms and flagellates dominate dur-ing nonbloom periods (Holm-Hansen et al, 1989).In the open ocean a massive sedimentation of phy-toplankton often accompanies the termination ofthe spring bloom. The termination could be causedby cell advection, sinking and/or zooplanktongrazing, which leads to the recycling of materialsin the euphotic zone and reduces export particle

82 OCEANOGRAPHY+• ~. 8, No. 34995

flux. Large fluxes of organic matter have been ob-served in sediment traps, sometimes after a periodof intense erosion of the mixed layer, as mightoccur during a storm. In contrast, the developmentof a massive coastal bloom, even for a short pe-riod of time (days or weeks), can significantly de-crease inorganic nutrients which then terminatesthe bloom. Surface nitrate concentrations incoastal waters of the western Antarctic Peninsularegion can decrease from 25 PM, when nonbloomchl-a concentrations are typically 0.5 mg m-3, tonear depleted levels (CO. 1 PM), when bloom chl-a concentrations can be >35 mg m-3 (Kocmur et

. aZ., 1990). The coastal bloom, in terms of contin-ued new/export production, crashes unless nutri-ents are replenished by mixing and erosion of themixed layer or by advection of richer offshore wa-ters. However, production can continue without al-lochthonous nutrients, if nutrients are locally re-generated. During summer chl-a concentrationsremain intermediate (1-2 mg m-3) in both openocean and coastal areas, although periodic blooms

, may still occur nearshore. Fall blooms may appearin late February and March.

Palmer LTER predator/prey time-lines includethe critical periods during reproduction of the twochosen prey/predator pairs (krill/Adklie penguinsand silverfish/south polar skuas). All four speciesare relatively long-lived. Antarctic krill live for 5to 7 y, reproducing as early as their third summer.Ovarian development begins in austral spring, andthe rate of ovarian development is dependent onfood availability. A prolonged spawning seasonruns from late December to early March. Both theproportion of the population reproducing and thelength of the spawning season are dependent onfood availability during spring and summer. Ice-edge blooms, occurring prior to open-waterblooms, are thought to be one source of food es-sential to high reproductive output throughout thesummer (Quetin et aZ., 1994). After spawning theembryos sink, hatch at depth, and the early non-feeding larvae ascend through the water column.The first critical period occurs when early larvaearrive at the surface, about three weeks after re-lease, and need food. Winter is the second criticalperiod, because unlike adults, larvae lack energystores. The 6-mo long fall and winter period of

. low food availability in the water column can cre-ate starvation conditions for the young-of-the-year(Ross and Quetin, 1991). The essential wintergrazing ground for larvae is the under-ice habitatwhere they feed on ice algae. Thus, larval survivaland recruitment are hypothesized to be enhancedby the presence and duration of winter ice. Graz-ing pressure from the three dominant macrozoo-plankton groups @ill, salps, and copepods) varieswith season. Krill are present all year round in sur-face layers. Grazing pressure by adults is lower inlate fall and winter, although larvae continue tofeed on ice algae if sea ice is present. Salps gener-

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low

New P r o d u c t i o n

POM IZxporllowmax

0 n e a r z e r o I I I IMicrob ia l Ac t iv i ty I tow . I

CO nsumers

Graz ing ( w a t e r c o l u m n )

krill I ”

SdpS 1

copepods 1A n t a r c t i c krill

Apex P reda to r s

Ad6lie p e n g u i n s

c r i t i c a l p e r i o da d u l t sch icks

South Polar sku

c r i t i c a l p e r i o d

Fig. 4: Annual time-lines of key physical and biological components in thevicinity of Palmer Station (within 100 km) based on generalized knowledge ofAntarctic waters and preliminary $ndings from the Palmer LTER field sea-sons (see text). Timing associated with climatology, sea ice cover, and lifehistories of consumers and apex predators is relatively well known. Develop-ment of the seasonal thermocline and the corresponding strength and depthof stratijcation, nutrient history and phytoplankton development varies inter-annually with the timing, and strength of environmental forcings and areshown schematically to illustrate the potential temporal linkages.

ally bloom in late summer or fall if at all. Adultcopepods migrate into surface layers in spring/sum-mer, while the new year class appears in summerand intensifies grazing pressure. Grazing pressurethen decreases in fall and winter as many cope-pods migrate into deeper layers.

Adklie penguins have a circumpolar distributionand a breeding season that passes through a seriesof stages in which their energetic requirementsgreatly vary. The season begins with a three week

OCEANOGRAPHY~O~. 8, No. 3.1995 83

0 ur sampling

strategy combines begin with the male on the nest requiring him to

courtship period, during which both members ofthe pair remain ashore fasting. This is followed bya month long incubation period from late Novem-ber through December.T h e incub ation duties

seasonal time series

data from the fine-

scale Palmer grid,

fast for an additional two weeks, while the femalegoes to sea to feed. During this first critical periodthe female is believed to return to the ice edge insearch of a predictable source of krill to restoreher body condition (W. Trivelpiece, personal ob-

including the sea bird servation). If the female fails to replenish her sup-

nesting sites, with dataply of fat and return to the nest within two weeks,the male abandons the nest to forage and the eggs

collected from re- are lost. If the female is successful in finding food,she relieves her mate at the nest, and he spends the

search cruises which following two weeks at sea recovering from his

cover the large-scale

peninsula grid.

five weeks of fasting. Upon the return of the male,the pair alternate between attending the eggs andforaging at sea on progressively shorter time inter-vals, until they are switching duties daily by thetime the eggs (usually two) hatch. Followinghatching, the pair continue alternating betweenguarding the chicks at the nest and foraging forfood for their young until the chicks reach approx-imately three weeks of age. A second critical pe-riod affecting breeding success occurs in mid tolate January, when the chicks are between threeand seven weeks old. During this “creche stage,”the food/energy demands of the chicks are at theirhighest. The parents must find predictable suppliesof krill within a foraging area of about 100 km, orpreferably much closer, otherwise breeding suc-cess may be significantly reduced.

South polar skuas (Catharacta maccormicki),like Adelie penguins, have a circumpolar breedingdistribution. In high latitude areas, south polarskuas typically breed near Adelie penguin breed-ing colonies, and indeed a significant portion ofthe south polar skua’s diet consists of Adelie eggsand chicks. However, in the region of the Antarc-tic Peninsula, including the breedin g sites nearPalmer station, the south polar skua’ s range over-laps with that of its larger congener, the brownskua (Catharacta lonnbergi). Here, brown skuascontrol the penguin resource, forcing the smallersouth polar skuas to feed exclusively at sea. Thus,in the area of the Palmer LTER, the south polarskua’s breeding success is dependent on the distri-bution, availability, and abundance of food (krill,fish or scavenged penguin eggs and chicks) withinits foraging range and the variable energy de-mands that are directly coupled to foraging. Pairsarrive at their nesting territories from mid-Octoberto November, and males courtship-feed the fe-males to bring them into reproductive condition.Annual variations in laying dates, number of eggsproduced (1 or 2), and egg sizes all reflect theavailability and abundance of food during thiscourtship period. The survival of south polar skuachicks also varies from year to year and appears tobe specifically linked to the presence of silverfish

in the chicks’ diet. Years of low silverfish abun-dance are associated with total reproductive failure(Trivelpiece et aZ., 1990). High silverfish abun-dance, and thus reproductive success in skuas,have been correlated with years of lower than av-erage ice cover.

Sampling Strategies and Data ManagementOur sampling strategy combines seasonal time

series data from the fine-scale Palmer grid, includ-ing the sea bird nesting sites, with data collectedfrom research cruises which cover the large-scalepeninsula grid. The relatively continuous weeklydata obtained on the fine-scale grid from approxi- 1mately October through March provide informa-tion on the seasonal progression of hydrography,nutrients, pigment biomass, primary productivity, *and near-shore abundance and distribution of krilland their larvae, as well as yearly information onthe timing and reproductive fate of the apex preda-tors. These near-shore data also allow the large-scale cruise data to be placed into a seasonal con-text to estimate interannual variability better.During the first four field seasons, there were sixresearch cruises, each of which covered a sig-nificant portion of the large-scale peninsula gridduring various times of the year (AugustBeptem-her, November, January, March/April) to addresslarge scale seasonal variability. In addition, therehave been a series of three January cruises chosento coincide with the critical period of Adelie pen-guin chick growth and survival. These Januarycruises typically include 1) intensive sampling ofall LTER components within the Adelie penguinforaging area and 2) large-scale sampling withinthe peninsula grid. The large-scale sampling re-solves physical and biological variables whichcannot be detected in the fine scale sampling andthus places the fine-scale observations in a moreaccurate environmental context, while the oppositeis true for variables detected only in the fine scalesampling.

Data management and computer networking arekey elements of all LTER sites. The LTER NetworkOffice for intersite communications encourageselectronic connectivity u s i n g t h e Internet(http://www.lternet.edu) to facilitate communica-tions among distributed researchers (Andrews et aZ.,1992). In addition to the sharing of information anddata within the LTER computer network, this ap-proach has facilitated the sharing of computer re-sources and technologyI as well. Each site designatesa data manager, who works with the other re-searchers to create on-line documentation and tomanage and archive project data. Data managementfor the Palmer LTER is organized using a decentral-ized model, where each researcher is responsible fora subset of core and noncore data. For the PalmerLTER the documentation and data storage are orga-nized through an electronic hub at the Institute forComputational Earth System Science (ICESS) at the

84 OCEANOGRAPHYWO~. 8, No. 3.1995

University of California at Santa Barbara, whichalso serves as a data archive as needed(http://www.icess.ucsb.edu/lter). There are setdefinitions of core datasets and metadata, providinga common vocabulary and archive structure acrossthe diverse LTER programs. The data archive struc-ture has been organized to facilitate rapid informa-tion exchange and online data documentation.

SummaryThe conceptual diagram of the ecosystem (Fig.

3) and the annual time-lines of key physical andbiological components (Fig. 4) provide an initialconstruct for defining air-ocean-ice-ecosystem andtrophic-level linkages. The interannual variabilityin sea ice coverage allows us to “conduct” naturalexperiments on the impact sea ice has on varioustrophic levels by monitoring parameters andprocesses during and after seasons/years of differ-ent sea ice coverage. Prior to the Palmer LTER,there were few systematic long-term, large-scalestudies in Antarctica that would allow one to dis-tinguish secular trends from cycles and to evaluateinterannual variability inherent in the system.Palmer LTER research encompasses short andlong temporal scales as well as fine and regionalspatial scales and thus is poised to resolve ecosys-tem dynamics and linkages, in addition to provid-ing a historical long-term data base for futureecosystem studies in this region.

A further motivation for this long-term ap-proach is the recognition that the underlying nat-ural variability in this marine ecosystem cannot beunderstood, evaluated nor distinguished from po-tential anthropogenic impacts in the Antarcticwithout long-term study. Some climate modelsplace high latitude changes in sea ice cover as anindicator of global warming (Stouffer et aZ., 1989)and the seasonal ice zone as a region which mayshow a relatively large and early response to pre-dicted greenhouse-gas warming. A statisticallysignificant warming trend over the past SO yearshas been shown for the mid-Antarctic Peninsulaarea (King, 1994; Stark, 1994; Smith et aZ., 1996a)and the significant correlations between air tem-perature, sea ice and the Southern OscillationIndex (Smith et aZ., 1996a; White and Peterson,personal communication) suggest that the Palmer

* LTER area may be uniquely well located to studyecological responses to climate variability.

Separating long-term (decadal) systematic trendsfrom large interannual variability in populations isvital to measure the effects of increased human pres-sure on living resources and uphold the agreementof the Convention for the Conservation of AntarcticMarine Living Resources (CCAMLR). CCAMLRstates that any living resource (e.g., krill) should notbe harvested to such an extent that either the popula-tion itself or consumers depending on that food itemare affected. With the continuation of the PalmerLTER we expect to understand the processes under-

OCEANOGRAPHYWOL 8, No. 3.1995

I

lying interannual variability, and thus be in a posi-tion to separate changes dependent on natural cyclesfrom those due to long-term trends. Understandingnatural variability within the Antarctic marine envi-ronment is fundamental to our understanding of themore subtle impacts of humankind.

AcknowledgementThe Palmer LTER is funded by the National Sci-

ence Foundation Office of Polar Programs, GrantOPP90-11927 and DPP91-18439 (D.K.). Field sam-pling is done at Palmer Station and on board the R/VPolar Duke and R/V Nathaniel B. Palmer, under theadministration of Antarctic Support Associates.

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