Articles
Dry
Valley Streams
in
Antarctica:
Ecosystems
Waitingfor Water
DIANEM. MCKNIGHT,
DEVK. NIYOGI,ALEXANDER
S. ALGER,ARNEBOMBLIES,PETERA. CONOVITZ,
AND CATHYM. TATE
n the McMurdo
DryValleysof SouthVictoriaLand,
Antarctica,the striking features of the stark landscape
includethe wide expansesof barrengroundpatternedwith
largehexagonalshapescreatedby periglacial(freeze-thaw)
processes, the high mountains interspersedwith alpine
glaciers,and the large,permanentlyice-coveredlakesin the
valley floors. For most of the year, it is the cold winds
descendingfrom the polar plateauthat graduallyshapethe
dry valley landscape. The dominant influences of wind
erosion and cold temperatureson the landscape are less
potent during the australsummer,when the temperatures
become warm enough for melting to occur on the surface
and face of the glaciers (Fountain et al. 1999). During
runoff,streamsflow down the sides of the valleys,carrying
water,solutes,and sedimentsto the lakes.
Since the first exploration of the McMurdo Sound
region of Antarctica by Robert Scott's expedition in
1904-1906, the dry valleys, which are located across the
sound from Scott's base, have intrigued scientists from
many disciplines.The McMurdoDry Valleysare the coldest and driestdesertsin the world and arerepresentativeof
the 2% of the Antarcticcontinent that is ice free.The valleys encompass an area of approximately4000 km2. This
region has experienceda warming trend since it was first
explored, and most lakes have risen more than 1 m since
1968, when the New ZealandAntarcticResearchProgram
began monitoring lake levels (Chinn 1993).
The biota that grow in the soils, lakes,and glacialmeltwater streams of this harsh environment are primarily
cyanobacteria,eukaryotic algae, mosses, and heterotrophic and chemoautotrophic bacteria. The multicellular
organismsrepresentinghigher trophic levels are generally
low in diversityand abundance.Aside from the occasional skua gull, the largest animals in the dry valley ecosystems are nematodes.
Streams in the McMurdo Dry Valleys region are very
different from most streams on the planet. Dry valley
streamsare fed by melting glaciersand flow only for up to
10 weeks each year.They vary in length from less than 1
km to 30 km, in the case of the Onyx Riverin WrightValley. Because of the lack of precipitation and terrestrial
runoff,there arelimited interactionsbetween streamsand
the surroundingbarren landscape.For example, because
there are no vascularplants in the region, there is no leaf
litter input to the streams.One might expect these streams
ECOLOGICALLEGACIESALLOWABUNDANT
CYANOBACTERIALMATS TO OCCUR IN DRY
VALLEYSTREAMS
to be relativelybarrenof life, and some of them are (Alger
et al. 1997); however,in many streams diverse microbial
communities persist despite the extremeconditions.
The microbial communities present in the dry valley
streams consist primarily of cyanobacteria, although
chlorophytes and diatoms are also common. The harsh
conditions limit stream fauna to nematodes, rotifers,and
tardigrades.There are no insects or other macroscopic
streamconsumers,which are common in most temperate
streams.Algal communities grow in the streams as mats
and have different colors because of photosynthetic and
accessory pigments. These perennial algal mats can survive long periods of desiccation,allowinghigh biomass to
develop, even by temperate-zone standards (McKnight
and Tate 1997). Consequently, these streams can be
hotspots of life in an otherwise barrenlandscape.
Researchon dry valley streamshas been conducted primarily in Wright Valley,TaylorValley,and Miers Valley.
Canada Stream, a productive stream in the Lake Fryxell
basin of TaylorValley (Figure 1), has been studied most
thoroughly (Vincent and Howard-Williams1986,Vincent
1988). Since 1993, additional research on the stream
ecosystems has been conducted as part of the McMurdo
Dry Valleys Long-TermEcological Research(LTER)project, which is one of a networkof LTERprojectslocated in
is a researchassoDianeMcKnight
([email protected])
ciate and Dev K. Niyogiand ArneBombliesare graduatestudents
at the Institute for Arctic and Alpine Research, Universityof
Colorado,Boulder,CO80309. McKnightis also an associate proand Architectural
fessor in the Departmentof Civil,Environmental,
Engineering,Universityof Colorado,Boulder,CO80309. Alexander
S. Algeris a graduatestudent at the Universityof Michigan,Ann
Arbor,MI 48109. Peter A. Conovitzis a graduate student at
FortCollins,CO80523. CathyM.Tateis
ColoradoState University,
a biologistwith the US GeologicalSurvey,Denver,CO 80223. ?
1999 AmericanInstituteof BiologicalSciences.
December1999 / Vol.49 No. 12 * BioScience 985
University of California Press
is collaborating with JSTOR to digitize, preserve, and extend access to
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Articles
~Iv
Cree
Relict
CA
K0.
S
Channel
,
Figure 1. Map of the LakeFryxellbasin
in TaylorValley,McMurdoDry Valleys
region,Antarctica.Solid circlesindicate
streamgauging stations, and open
circlesindicate the locations of algal
transectstations.
legacies (Moorheadet al. 1999) that control the responseof streamecosystemsto
the changingclimate.
Characteristicsof dry valley
stream habitats
-
'N
wad
2 km
In the extreme environment of the
McMurdo Dry Valleys, physical and
chemical factors determine biological
presence and activity.Therefore,characterizationof the unusualhydrology,geomorphology, and geochemistry of dry
valley streamsprovidesa foundation for
understanding the ecology of these
streams.
Hydrology. Thedrainageof meltwater
Figure2. A dry valley stream (PriscuStream,in the Lake
Bonney Basin of TaylorValley).The dark bands of the watersaturatedhyporheiczone are visiblealong the stream.Stream
width in thephotographis approximately1-3 m.
22 diverseregions acrossthe globe. In our research,we use
descriptive measurements of hydrology, chemistry, and
biology in combination with short- and long-term
stream-scale experiments. In this article, we discuss the
dynamic interactionsbetween cyanobacterialmats in the
dry valley streams and the physical and geochemical
processes of the stream ecosystems. We also discuss the
persistenceof algalmats despite the long periods of desiccation and extreme cold and emphasize the ecological
986 BioScience - December1999 / Vol.49 No. 12
from the alpine glaciers into meltwater
streams supports the brief annual productivity of the
stream ecosystems. For 4-10 weeks during the austral
summer, the meltwaterstreams become a distinctive feature of the barrenlandscape.The land surfaceis composed
of an unconsolidatedsedimentthat has large (5 m), hexagonally patterned ground features caused by periglacial
processes.A layer of active permafrost,which freezes and
thaws annually,begins 0.1 m below the surfacelayer and
extends to approximately0.5 m in depth. The true permafrostlayerthen begins at 0.5 m beneaththe surface.The
streambedsvary in width from 1 m to 30 m. Because the
streambankvegetationis limited to small patchesof moss,
the stream banks consist of steeply sloped, poorly sorted
sediment and are generallyunstable.Undercuttingof the
banks during high streamflowperiods can result in 1-2
cm deposits of sediment in lower reaches of the streams
(Diane M. McKnight,personalobservation).
During the australsummer,darkbands that border the
streams can be seen (Figure 2); these moist areas,which
develop as the summer progresses,are the surficialmanifestations of the hyporheic zone. The term "hyporheic
zone" refers to the area adjacent to and underneath the
stream,in which waterflows through the streambedin the
downstream direction and exchanges with water in the
main channel (Figure3). In Huey Creekin the LakeFryxell basin (Figure 1), the high permeabilityof the hyporheic zone sediment and the steep gradient of the stream
result in exchange rates of water with the main channel
that are more rapid than exchangeratesmeasuredin temperate streams at similar flows (Figure 4; Runkel et al.
Articles
at low flow in GreenCreekin the
1998).Measurements
LakeFryxellbasin(Figure1), whichhas a shallowgradient,yieldedexchangecoefficientsthatareat the high end
of the rangemeasuredin temperatestreams(Figure4).
The rapidexchangerates and the absenceof lateral
inflowsfromshallowgroundwater
resultin a substantial
lateralextentof the hyporheiczone.Forlongstreamsduring low-flowyears,waterstoragein the hyporheiczone
can accountfor a significantportionof the annualmeltwaterfrom the glacier(Bomblies1998,Conovitzet al.
1998).The depthof the hyporheiczone is controlledby
the thawdepthof the activepermafrostlayer(Figure3),
whichextendsto a greaterdepththroughoutthevalleysas
the summerprogresses.Measurement
of the active-layer
thawdepthat 18transectson threedifferentstreamsindicatedthatthedepthwas5-10 cm atthebeginningof summerandincreasedto 50-60 cm by the endof the summer
(Conovitz1999).Both the rapidexchangeratesand the
increasingvolume through the summer indicatethat
hyporheiczone processesare potentiallyimportantin
thesestreamecosystems.
Themeasurement
of streamflowprovidesa contextfor
interannual
understanding
changesin the streams.The
flow of the OnyxRiverin WrightValleyhas been monitoredat two gaugingsitessince1968,anddatafor annual
to LakeVandaindicateconsiderable
interannual
discharge
variation(Figure5a). Becausethe Onyx Riverreceives
which abutsa pieddrainagefrom LakeBrownsworth,
mont glacier,as well as fromtributarystreamsdraining
alpineglaciers,its flowmaybe affectedby climaticconditions in McMurdoSoundin additionto those in Wright
Valley.A networkof streamgaugeswasestablishedin the
LakeFryxellbasinin 1990(von Guerardet al. 1994)and
was extendedto includethe entireTaylorValleyin 1993.
TaylorValleystreamsalsohaveasmuchas a fivefoldinterannualvariationin streamflow (Figure5b;Houseet al.
1995).
In additionto showinginterannualvariation,average
streamflowratescan show considerabledaily variation
duringthe summer,dependingon insolationandairtemperature.Hydrologicdataindicatethat streamflow can
varyas muchas 5-10-foldduringa singleday,depending
on the orientationof the sourceglacierwithrespectto the
i
i
ci
Ma
r
Si
,i i i.
T
iT
Permafrost
a
Hyporheic
9
zone
..
Stream
channel
Permafrost
Figure3. Schematicdiagram illustratingthe hyporheic
zone in dry valley streams. Waterleaves the main stream
channel and passes throughthe substrateon the sides
and below the stream and then may return to the main
channel.Solutesalso pass throughthe streambedand
may undergoreactionsthat affect their transport.The
hyporheiczone may also be a sourceof some nutrients to
the water in the main stream channel.
solar trajectory (Conovitz et al. 1998). For example, the
flow in Huey Creekcan range from a trickleof clearwater
at 5 L/s in the early morning to a surging flow of turbid,
sediment-ladenwater at 80 L/s by the afternoon. During
cold periods, when there is no meltwatersource from the
glacier,the recessionof the hydrographis controlledprimarilyby drainageof the hyporheic zone (Conovitz et al.
1998).
To understand climatic responses of streams throughout the McMurdo Dry Valleys, it is important to know
what causes variation in climate-stream flow relations.
Our analysis(Bomblies 1998) indicatesthat, in additionto
being relatedto variation in meltwatergenerationassociated with characteristicsof the glacier,some of the interstream variation is relatedto stream length and geomorphology,which affectthe storageof waterin the hyporheic
zone. Becausethe physicalprocessesthat control the flow
in the stream channels and hyporheic exchange have a
high degree of temporal variability during a single day,
10-1
Figure4. Comparisonof hyporheicexchangerate among
temperateand dry valley streams.The exchangerate is a
measureof howfast water exchangesbetween the main stream
channel and the hyporheiczone and is estimatedfrom
conservativetracerprofiles.Diamonds indicate values
estimatedfor individual reachesat each stream.LittleLost
Man and Uvas Creeksare in California;Saint Kevin Gulchand
SnakeRiverare in Colorado;and Greenand Huey Creeksare
in TaylorValley,Antarctica(datafrom Runkelet al. 1998,
Diane M. McKnight,RobertL. Runkel,JohnH. Duff, CathyM.
Tate,Daryl L. Moorhead,unpublishedmanuscript).
10-2C-,
1o-
?10-3-
>
o
o 10-4
110-6
LU
Little Uvas Saint Snake Green Huey
Lost Creek Kevin River Creek Creek
Gulch
Man
Stream
December1999 / Vol.49 No. 12 * BioScience 987
Articles
140--
Figure5. Annual variation in
stream dischargefor two of
the McMurdoDry Valley
streams.(a) OnyxRiver,at
VandaStation in the Wright
Valley(north of Taylor
Valley).(b) Canada Stream,
in the LakeFryxellbasin in
TaylorValley(see Figure 1).
a - Onyx River
120
100.......
....
...
80
60
40
E
20
0
o
..
C0) 'It • ,O,r• N"- N- N-,-
CO M
0M
N- N-
CO
Cr
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O
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QD 0'
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O
O
O0
-u
(0350
O
S
b - Canada Stream
S300
.....
250
.
150
0
4
tO
Year
within a summer, and among years,the stream habitat is
dynamic during the brief summer period of flow. Thus,
streamecosystemscontain algalspecies that must not only
survive the long period of desiccation during fall, winter,
and spring but also sustain primary production under a
highly variableflow regime.
featuresof the dry
Geomorphology. Geomorphological
valley streams also control the suitability of streams for
supporting algal mats. Algae are generally abundant in
many moderate-gradient stream reaches, in which the
rocks of the streambedare arrangedin a stone pavement.
These stone pavements are similar to those commonly
found along the shores of alpine lakes, which form
throughperiglacialprocesses.We inferthat the stone pavements in dry valley streamsare also a result of periglacial
processes acting over long periods of time, perhaps centuries or longer.We propose that as the saturatedalluvium
freezes,the expansion of the alluvium surroundinga rock
988 BioScience * December1999 / Vol.49 No. 12
rotates the rock until the
largest flat side is upward.
This process, which occurs
across the streambed,wedges
t C
M, CI3 '•
the rocks together, creating
0") 0') 0) 0) 0") 0')
the appearanceof a flat pavement. The stone pavement
configurationmay limit sediment transportand scour and
create smooth flow conditions, thereby providing a
favorable habitat for algal
..
.
mats (Algeret al. 1997). Thus,
we propose that these stone
pavements are an important
physical legacy from past
summers that promotes the
current growth of cyanobacterial mats and shape the ecological system.
In contrast to their abundance in reaches of moderate
NC
gradient, algae are scarce in
steep reaches of dry valley
streams, where the rocks are
uneven and jumbled, and in
shallow gradient reaches,
where the streambed consists of moving sand. In steep
reaches, algal growth is present as sheets of the chlorophyte Prasiola on the underside of rocks. High stream
velocities, turbulence, and abrasion may limit growth of
other taxa in these steep reaches (Vincent et al. 1993a,
Alger et al. 1997). Stone pavementsprobablydo not form
in steep reaches because the alluvium drains rapidly,
before freeze-thaw processes move the rocks into a stone
pavement formation. Furthermore,high-flow events may
removesand and smallerrocksin these reaches.In shallow
reaches,which are common near the outlet to lakes, the
instabilityof the sand substrateis most likely the limiting
factor for algal growth (Alger et al. 1997). In both steepand shallow-gradientreaches, rivulets of water occur on
the sides of the active channel and are important habitat
for algal mats. The rivulets, which are called parafluvial
seeps, appearto drain the hyporheiczone. In shallow-gradient reaches,the algal mats in the parafluvialseeps may
representmost of the algalbiomass (Algeret al. 1997).
Articles
226 m
50 m
2000
497 m
327 m
(D 1500
0
_00_
00
80
60
0
l..
""
0
40
10
25
aa
20
15
-
21
23
25
27
29 21
23
25
27
29 21
23
25
27
29 21
23
25
27
29
Time (h)
tracer),nitrate,
Figure6. Concentration
profilesatfoursitesbelowan experimental
injectionpointof chloride(conservative
andphosphate.A solutionof nutrientsandchloridesaltwasinjectedinto thestreamfor 4 hours(startingat 21 hours).
Distancedownstream
frominjectionlocationis shownabovepanels.At thesite497 m downstream
fromtheinjectionpoint,
theabsenceof detectableconcentrations
of nitrateandphosphateindicatestherapiduptakeof thesenutrientsbystream
biota.Theopensquaresrepresent
actualdatapoints,whereasthesolidlinesrepresentsimulationsusinga hyporheic
RobertL Runkel,JohnH. Duff,
exchangemodelwithvariableflow andfirst-orderuptakeof nutrients(DianeM.McKnight,
their
CathyM. Tate,DarylL.Moorhead,unpublished
manuscript).
If thenutrientshad behavedconservatively,
concentration
at 50 m at
profileswouldbesimilarto thechlorideprofileat eachsite.Theslightincreasesin concentrations
theendof theinjectionresultfroma decreasein streamflow duringthattime.
Currently active streams in the dry valleys constitute
only a subset of the fluvialfeaturesof the landscape.Satellite images revealwhat appearto be relict streamchannels
in the McMurdoDry Valleysregion.Furthermore,approximately 50% of the streams marked on the topographic
maps of the region do not appearto be flowing duringthe
1990s. Some of these "streams"may representsubglacial
flowpaths that were carved out when terminal glaciers
extended farther down the valleys (David R. Marchant,
Boston University,personal communication). Other relict
channels may have lost their water source afterthe retreat
of alpine glaciers,which now feed new streamchannels,or
they may have been cut off from meltwaterby upstream
sediment deposition that formed a barrierto flow. Future
field studies may revealwhich of these featuresare truly
relict streamchannels,as opposed to subglacialflowpaths,
and may indicatewhetherstone pavementswere common
habitatswhen the channels were active.
Biogeochemistry. Streamsin the dry valleyslink the
glaciersand the lakes.Severalabiotic and biotic processes
in stream ecosystems affect the sources, sinks, and transport of chemicals along these flowpaths.These processes
may occur not only along the streambedbut also in the
hyporheic zone below and alongside the active channel.
A role for hyporheic zones in biogeochemical cycling
has been documented in many streams (e.g., Triskaet al.
1993, Holmes et al. 1994, Jones et al. 1995, Mulhollandet
December1999 / Vol.49 No. 12 * BioScience 989
Articles
al. 1997).As discussedby Vervieret al. (1992),hyporheic
processesarelikelyto playan importantbiogeochemical
As
rolein streamsunderconditionsof highpermeability.
waterin the hyporheiczone exchangeswith waterin the
mainstreamchannel,solutesarealso exchanged(Runkel
et al. 1998).Examination
of majorcationandsilicatedata
for manyTaylorValleystreamsindicatesthatweathering
of silicaterocksandothermineralsoccursin thehyporheic zone at surprisinglyhigh rates (Lyonset al. 1998a,
1998b).Thisconclusionis furthersupportedby observationsthatin vonGuerardStreamin theLakeFryxellbasin,
of majorions, includingsilicate,increase
concentrations
with distance downstream(AlexanderBlum, Michael
Gooseff,W. BerryLyons,Diane McKnight,unpublished
data).Similarly,Greenet al. (1988)proposedthatin dry
valley streams,weatheringof apatiteis the source for
phosphate,andleachingof atmosphericdepositionis the
source for nitrate. Thus, dissolved nutrients can be
released through hyporheic zone reactions. Primary
weatheringreactionsand dissolutionof marineaerosols
probablyoccurin the hyporheiczone,greatlymodifying
the chemicalcompositionof glacialmeltwaterwithinthe
streamsaswaterflowsto the lakes(Lyonset al. 1998a).
Biologicallycontrolledprocessingof streamsolutesis
also evident in dry valley streams. Mats of Nostoc
fix atmosphericnitrogen,accountingfor as
cyanobacteria
muchas 10%of the nitrogeninputsin streams(Howardet al. (1989,1998)
Williamset al. 1989).Howard-Williams
transform
communities
that
reported algal
inorganic
may
nitrogenin dryvalleystreamsto organicformsof nitrogen
to thelakeecosystems.
Monitorthatarethentransported
and
has
shown
that
nitrate
of
stream
phoschemistry
ing
arehigherin streamswithoutalgal
phateconcentrations
mats than in streamswith mats, indicatingthat algal
uptakeof nutrientsin streamsregulatesinorganicnutrient
flux to the lakes(DianeM. McKnight,RobertL. Runkel,
JohnH. Duff,CathyM. Tate,DarylL.Moorhead,unpubet al. (1998) also
lished manuscript).Howard-Williams
acrossthe
concentrations
decreased
that
nitrate
reported
boulderpavementin the OnyxRiver,an areaof extensive
of dissolvedorganicnitrogen
algalmats;concentrations
in streamsthathave
increasedacrossthisreach.Moreover,
in
algalmats,nutrientconcentrations the hyporheiczone
arehigherthanthosein the streamwater,indicatingthat
there is a constant nutrient supply to the mats through
hyporheicweatheringand exchangewith surfacewater.At
the end of the austral summer, drainage from the
hyporheic zone therefore becomes a source of nutrientrich waterto the algal mats.
To examine the controls on dissolved nutrient flux to
the lakes,we conducted a nutrient injection experimentin
Green Creek (Figure 1), a stream that has abundant algal
mats. The injectate concentrations corresponded to
drainageof nutrient-richhyporheicwater into the stream
in late summer (55 iM nitrate and 18 iM phosphate;
Diane M. McKnight, Robert L. Runkel, John H. Duff,
990 BioScience * December1999 / Vol.49 No. 12
Cathy M. Tate, Daryl L. Moorhead, unpublished manuscript). Chloride was injected and monitored as a conservative (i.e., nonreactive) tracer, and chloride data were
used to model hydrologic exchange. Unlike chloride,
nitrateand phosphatedid not behaveconservatively;concentrations of these solutes at the downstreamsites were
much lower than those of chloride (Figure6). During the
arrivalof the chloridetracerat a site 497 m downstreamof
the injection point, nitrateand phosphateremainedbelow
detection (less than 1-2 1iM), indicating rapid nutrient
removal from the stream water (presumablybecause of
algal uptake). Moreover,the production of nitrite at two
intermediatesites (226 m and 327 m downstreamof the
injection point) indicates that other nitrogen cycling
processes, in addition to uptake, were occurring during
the injection of nitrate. Incomplete nitrification or denitrificationcould account for the nitrite production (Diane
M. McKnight,Robert L. Runkel,John H. Duff, Cathy M.
Tate, Daryl L. Moorhead, unpublished manuscript). We
used a solute-transport model (Runkel 1998), in which
nutrient uptakewas representedas a first-orderprocess,to
determine reach-scale parametersfor nitrate and phosphate uptake.The best match to the experimentaldatawas
providedby a model in which phosphateuptakeoccurs in
the main channel and nitrate uptake occurs in the main
channel and the hyporheiczone, possiblyrepresentingloss
due to denitrification(7-16% of total nitrateuptake).
In summary, once the stream ecosystem has been
"turned on" by the arrival of water, there will be some
nutrientsupplyto algalmats through primaryweathering,
leachingof depositedaerosols,and internalnutrienttransformationswithin the hyporheic zone. Algal mats take up
inorganic nutrients from the streambut releasedissolved
organic nutrients, which are then transported to lakes.
Scouringof the algalmats from the streambedduringhigh
flows then providesa source of particulateorganiccarbon,
nitrogen, and phosphorus as well as microbiota to the
lakes,but this flux has not been quantified.
mats in dry
Algal communities. The cyanobacterial
valley streamsare essentiallyperennial,growing at a slow
rate during the summer flow period and overwinteringin
a freeze-dried state. The abundance of cyanobacteriain
dry valley streamsis to be expectedbecause cyanobacteria
dominate the microbial populations of many extreme
environments (Potts 1994). Furthermore,the survival of
cyanobacteriain a freeze-driedstate is not surprising;for
example,freeze-dryingis used for the preservationof bacterialsamples in culture collections (Potts 1994). Previous
ecological studies of dry valley algal communities have
focused on the algal mats and mosses in CanadaStream,
located in TaylorValley(Figure 1; Howard-Williamset al.
1986,Vincent and Howard-Williams1986,Vincent 1988).
One remarkableobservationis that some algal mats begin
photosynthesizingwithin 10-20 minutes of being rewetted afterdesiccation(Vincent and Howard-Williams1986,
Articles
Figure7.Algalmatsgrowingin an areaof stone
pavementin GreenCreekin theLakeFryxellbasin.
Orangematsarepresentin themainchannel
1 m across)and black(Nostoc)mats
(approximately
as
near
theedgesof themainchannel.
grow tufts
Haweset al. 1992).
Comparedto streamecosystemsat other sites in the
LTERnetwork, dry valley streams representseveral
extremes-they lack terrestrialinputsof coarseorganic
matter,they havehigh standingalgalbiomass,they have
low primaryproductivity,
andtheyhavelow grazinglosses (McKnightand Tate1997,Websterand Meyer1997).
The high standingbiomassis a resultof the perennial
natureof the matsaswellasthe lackof macroinvertebrate
grazing,whichin turnis relatedto theharshnessof theclimate.Algalmatsof filamentouscyanobacteria
in the main
channelusuallyconsistof an orangeupperlayerthat is
enrichedin accessory
pigments,anda greenlowerlayerthat
hasgreaterchlorophyll
a contentandgreaterphotosynthetic activity(Vincentet al. 1993b).Therelatively
low ratesof
unit
streambed
area
and
per
primaryproductivity
perunit
reflect
fact
that
in
the
the
mats is
chlorophyll
growth
restricted
to thelayerof algaeon theundersideof themats.
Thedryvalleystreamshavewidelyvaryingproductivity,in partbecauseof variationsin algalmatbiomass.For
example,whereasmatsare abundantin CanadaStream,
in HouseCreek,a stream
is nearlyindetectable
chlorophyll
in theLakeHoarebasinin TaylorValleythatflowsthrough
an ice-boundmoraine(Algeret al. 1997,McKnightet al.
1998).Net primaryproductionratesrangefrom0.6 ? 0.1
of carbon (mean ? 1 SE) for sediment at
tg.cm-2-h-'
House
Creekto 12.2? 1.3pg.cm-2-h-1
of carbonforblack
Nostocmatsin the relictchannelin the LakeFryxellbasin
(Diane M. McKnight,CathyM. Tate,Dev K. Niyogi,
unpublisheddata).Primaryproductionratesreflectdifferencesin wherealgaeoccur.In streamsthathaveabundantalgae,the matsformextensive,blanketlikecoverage
overmostof the streambed(Figure7), whereasin streams
that havesparsemats the algaeoccuras patchesat the
marginsof the activechannelor as smoothsheetson the
sidesandundersidesof jumbledrocks,receivingsunlight
whenthe sun is low on the horizon(Vincent1988,Alger
et al. 1997).Mossesoccurfartherup the streambanks,
wherethey arewetted for short periods of the australsummer, or in flat areas that become damp but where open
waterflow does not occur (Algeret al. 1997).
To document the range of biomass content in the
stream ecosystems, we collected samples of algae at 16
streamsites containing permanentlyestablishedtransects.
At each site, we had delineateda transectacrossthe stream
and mapped a reachof approximately40 m encompassing
the transect; locations of topographic features, stream
banks, thalweg (lowest point on channel bed), stream
edges, wetted-zone edges, and distribution of algal mats
had been recorded (Alger et al. 1997). Within each tran-
sect, algalmatswerevisuallyidentifiedas eitherorange,
red,black,or green,althoughnot all types of algalmat
werefound at everysite. Blackmats are found nearthe
channelmargins,and greenmats are attachedto large
rocksin the mainchannel.Orangeand redmatsoccurin
flowingwaterhabitats,eitherin the main channelor in
rivuletsdrainingthe hyporheiczone at the streammarit is similaritiesin physicalcharacteristics
gins.Therefore,
of the streamhabitat,ratherthan differencesin water
quality,that determinethe occurrenceof the different
algalmatsin streams.
A total of 30 taxa of cyanobacteria
and chlorophytes
werepresentat the 16 streamsites,and the speciescompositionof the differentmattypeswas consistentamong
sites,evensitesthatdifferedwidelyin productivity(Alger
et al. 1997).Evennessvalues(Zar 1996),which indicate
homogeneityexpressedasa ratioof diversityto maximum
possiblediversity,werecalculatedfromthe algalspecies
compositionof each communitytype (McKnightet al.
1998).Theblackalgalmatsweredominatedby Nostocsp.
andhada low evennessof 0.13? 0.07 (mean? SD).The
green algal mats were also made up of essentiallyone
or
species,beingcomposedchieflyof Prasiolacalophylla
Prasiolacrispa,and had an evennessof 0.17 ? 0.10.The
orangeand red algalmatswere composedof speciesof
Oscillatoria
andPhormidium
andhadmorediverseassemof
For
the
blages species.
orangeand red algalmats,the
evennessvalueswere0.55 ? 0.16 and0.48? 0.21,respectively.Theorangeandredmatsalsohad a highdegreeof
in speciesdistribution,
intrasiteheterogeneity
as indicated
data
within
from
five
of
mats
collected
by
samples orange
a 20 m reachin GreenCreek(Figure8). Basedon the uniformityof thealgalspeciesacrossstreamsthatrangewidewe proposethatthe limitedset of algal
ly in productivity,
in dry valleystreamsrepresentsa
that
are
found
species
second majorlegacy(alongwith the first legacy,stone
pavements)forthe streamecosystems.
In additionto cyanobacteria,
38 speciesof diatomswere
identifiedin dry valleystreams(Alger1999). Manyof
these species (e.g., Hantzschiaamphioxys,Stauroneis
anceps,and Pinnulariaborealis)arecosmopolitanandare
December
1999/ Vol.49 No. 12 e BioScience991
Articles
which sustained stream flow was last
recordedin the summer of 1969 (Diane M.
tectorum
E
8B.
spp.
McKnight, Cathy M. Tate, Dev K. Niyogi,
G. kuetzingiana
unpublished data). In January 1995, we
..Nostoc
o. S-i60
UP. frigidum
L
constructed a sandbag control structure
D autumnale J
and routed meltwaterto this relict channel
C 40
0. koettlitziH
.]p.
I
(Figure 1). We discovered that both black
koettlitziF
.-O
O.
and orange algal mats had persisted in the
D 20
EP. crouaniE
channel and that these mats grew rapidly
irriguaD
-O.
a few days of the arrival of flow.
a0
C
within
irrigua
0.
Ei
1
2
3
4
5
EC0. subproboscidea A
Because of the accumulation of aeolian
material on the freeze-dried algae, these
Replicate number
mats had not been apparentbefore flow was
restored. Within 2 weeks, a full microbial
Figure8. Speciescompositionof orangealgal matsfrom GreenCreekin the
was activeover the 1.5 km stream
ecosystem
LakeFryxellbasin. Five replicatesamples,collectedwithin 20 m of each other,
reach.
Both
mat types became quite extenhighlight the intrasite variation in speciescompositionin these communities.
sive
the
second
week of flow. The species
by
Generaare as follows: B., Binuclearia;G.,Gleocapsa;0., Oscillatoria;P.,
the
black
and orange mats were
composing
Phormidium. Lettersafter species referto morphotypes.Figurereprinted
the
same
as
those
commonly found in simfrom Algeret al. (1997).
colored
in other dry valley
mats
ilarly
streams.
consideredto be aerophilicor soil diatoms,whereasothers
Experiments conducted during the second year after
meltwaterwas reroutedto the relictchannel indicatedthat
(Muelleriameridionalisand Muelleriaperaustralis)appear
to be endemic to the Antarcticcontinent (Spauldingand
the black mats in the relict channel had higher net primaStoermer1997). In fact, approximately60% of the diatom
ry productivity than black mats from streams that had
in
streams
be
endemic
to
Antarcticonsistent annual flow (Figure9; Niyogi et al. 1998). The
species dry valley
may
ca (Alger 1999).
relict channel also had higher solutes,including nutrients,
than other dry valley streams,which may account for the
greater productivity of the mats (Diane M. McKnight,
Long-termpersistenceof
Cathy M. Tate, Dev K. Niyogi, unpublished data). This
cyanobacterialmats
To understandthe basis for the long-term persistenceof
ongoing experiment has demonstrated that microbial
stream algal mats, we studied a relict stream channel for
stream ecosystems can persist in a cryptobiotic state for
long periodsand can respondrapidlyto renewed
flow.The toleranceof a cryptobioticstate,which
allows the cyanobacterialmats to overwinter,
Figure9. Rates of net primaryproduction of algal communitytypes in
also providesfor the preservationof algal mats
threestreamsin the LakeFryxellbasin:Canada Stream,von Guerard
in
inactivechannels and their rapidresponseto
Stream,and the relictchannel.Mat types referto colors,as follows:B,
climatic
and geomorphologicalshifts.
black algal mats (Nostoc); O, orangealgal mats (Phormidium); G,green
algal mats (Prasiola). Valuesare means ? 1 SE (fromNiyogi et aL 1998).
100
0,diatoms
80
Anthropogeniceffectson stream
communities
140
o
120
100
8o.
E
c0
80
60
T
20-
B
O
Canada
Stream
G
B
O
Von Guerard
Stream
Algal mat
992 BioScience * December1999 / Vol.49 No. 12
B
O
Relict
Channel
The long-term persistenceof the streamecosystems should be considered in the context of
anthropogenicchanges that may affect the dry
valleys. The current and future effects range
from local, apparently short-term site disturbances associatedwith scientistsand tourists to
the effects of climate and ultraviolet radiation
associatedwith global processes.
The Antarctic Treaty System has adopted
comprehensiveprotocols to limit environmental damage in the Antarcticby providing a system to assess environmentaleffects of planned
activities. The stream research carried out by
the McMurdoDry ValleysLTERhas been permitted after an environmental assessment
Articles
process in which the activitieswere evaluatedin the context of natural processes. The environmental protocols
also apply to the activitiesof tourists visiting the dry valleys. During January1995, the firstvisit of tourists to Taylor Valley--part of an overall increase in Antarctic
tourism-brought several groups to the banks of Green
Creek.Sincethen, touristvisits to the dry valleyshave generallyinvolvedgroups of 10-20 people brought in by helicopterfrom ships in McMurdoSound. The environmental
protocols for the dry valleys recommend several steps to
minimize disturbance to the algal mats; these include
crossing the stream at a perpendicular angle, avoiding
stepping on the mats, and limiting activitieson the banks
that may deposit sediment into the stream.Touristsvisiting the dry valleys may not be aware of the presence of
algal mats and their perennialcharacter,but the protocols
will help them avoid disturbing the mats. While the
streamsareflowing,touristsprobablywould not choose to
walk in the channel for any distance. However,before or
after stream flow, the stream channels present easy pathways for foot travel and convenient flat places for helicopter landings. Extensivetrampling of the freeze-dried
algal mats in the channels could alter the stream ecosystems. The algal mats may be resilient to natural changes,
poised waiting for water; however, they may not be
resilientenough to withstanddisturbanceby modern-day
tourists. Therefore,clear communication between scientists and tour operatorsin the dry valleys is essential for
preservationof these unique ecosystems.
In addition, anthropogenicchanges associatedwith the
modification of the earth's atmosphere may result in
changing temperatures and climate in the dry valleys.
Nevertheless,because the interannualvariationin climate
and flow conditions is alreadyvery large,it is doubtfulthat
the new mean climatewould produceflow conditions outside the range observed in the past two decades. We
hypothesize that the dry valley streams'response to climate is mediated by major ecological legacies,specifically
the stablestone pavementof the streambedand the biodiversity of the algal flora.Although we would expect some
change in spatial distribution of currentmat types in the
streamswith a graduallychanging climate,we would not
expect dramaticshifts, such as loss of mat types.
The Antarctichas alreadyexperienced a change in the
radiative environment associated with the springtime
development of an ozone hole over the continent that
allows penetration of ultraviolet radiation at intensities
greatlyexceedingthose anywhereelse on the planet (Jones
and Shanklin1995). This increasedexposureto ultraviolet
radiationoccurs in August and September,well before the
onset of stream flow. During these months, the freezedried cyanobacterialmats may be directlyexposed to the
increasedultravioletradiation unless they are coveredby
accumulated snow or are shaded in an incised stream
channel.The potential for damageto the mats and impairment of productivity in the following summer months
will depend on the extent to which the radiation penetratesthe upper layersof the mat to the lower layers,where
active growth occurs (Vincent et al. 1993b).Cyanobacteria
do have accessorypigments that limit damagefrom excess
light and ultravioletradiation(Vincent et al. 1993b,Hawes
and Howard-Williams1998), but the ability of these pigments to protect cells during prolonged desiccationis not
known. For relict mats that go for many years without
receiving flow, the damage from increased exposure to
ultravioletradiationmay accumulateover time and could
impair rapid regrowthonce flow returns.
Summaryand conclusions
An axiom of ecology is: "Wherethere is water,there is life."
In dry valley ecosystems of Antarctica,this axiom can be
extended to: "Where there has been and will be water,
there is life."Stream communities in the dry valleys can
withstand desiccation on an annual basis and also for
longer periods-as much as decades or even centuries.
These intact ecosystems,consistingprimarilyof cyanobacteria and eukaryotic algae, spring back to life with the
returnof water.Soil organismsin the dry valleysalso have
remarkablesurvivalcapabilities(Virginiaand Wall 1999),
emerging from dormancywith the arrivalof water.
Streamsin the dry valleyscarrymeltwaterfrom a glacier or ice-field source to the lakes on the valley floors and
generallyflow for 4-10 weeksduring the summer,depending on climatic conditions. Many of these streamscontain
abundant algal mats that are perennial in the sense that
they arein a freeze-driedstateduringthe winter and begin
growing again within minutes of becoming wetted by the
first flow of the season. The algal species present in the
streams are mainly filamentous cyanobacteria(approximately 20 species of the genera Phormidium,Oscillatoria,
and Nostoc),two green algal species of the genus Prasiola,
and numerous diatom taxa that are characteristicof soil
habitats and polar regions. Algal abundancesare greatest
in those streamsin which periglacialprocesses,actingover
periods of perhapsa century,have produced a stable stone
pavement in the streambed.This habitat results in a less
turbulent flow regime and limits sediment scour from the
streambed.
Because dry valley glaciers advance and retreat over
periods of centuriesand millenniaand streamnetworksin
the dry valleys evolve through sediment deposition and
transport,some of the currentlyinactive stream channels
may receive flow again in the future. Insights into the
process of algal persistence and reactivation will come
from long-term experimentsthat study the effectsof reintroducing water flow to channels in which flow has not
occurredfor decades or centuries.
The present work of the McMurdo Dry Valleys LTER
has led us to conclude that the legacy of past conditions
constitutes a dominant influence on present-day ecosystem structureand function in the dry valleys (Moorhead
et al. 1999). For example,Virginia and Wall (1999) have
December1999 / Vol.49 No. 12 * BioScience 993
Articles
found that soil nematodes are partly sustained by relict
organic carbon from algae that grew during the high lake
stands of 8000-10,000 years ago. Similarly,the growth of
current algal populations in the lakes of the dry valleys is
supported by diffusion of nutrients from relict nutrient
pools in the deep bottom waters (Priscu et al. 1999). For
the streamecosystems,abundantalgal mats are presentin
channels that have stable stone pavements,which formed
through freeze-thaw cycles occurring over long periods,
possibly hundreds of years. We hypothesize that these
stone pavements are an important ecological legacy permitting the successful"waitingfor water"strategy.Similarly,the biodiversityof algal species that can survive the
harsh conditions in the streamsof the dry valleys may be
stable for centuriesor more, representinga second important ecologicallegacy.
Acknowledgments
Researchreported in this article as part of the McMurdo
Dry ValleysLTERis supportedby NSF grant no. OPP 92117733.We thank Dr. PaulBrooks,DurelleScott,EarlCassidy,MarthaCrawford,and four anonymous reviewersfor
helpful comments on the manuscript.
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of
Society
21st
Quebec
Scientists
Wetland
Annual
City,
August
meeting
Canada
Qu6bec,
2000
6-12,
ScheduledEvents:
QuBbec 2000
Millennium Wetlands Event
* PlenarySessions
*
andExhibits
Workshops
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andAwards
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The SWS annualmeetingwillbe held in conjunctionwiththe
annual meetings of The International
Society of Ecologists
Peat
International
the
Society (IPS), and the
(INTECOL),
InternationalMire ConservationGroup (IMCG);and will be
knownas theQuebec2000 WetlandMillenniumEvent.
sws
Informationabout the Quebec 2000 Wetland MillenniumEvent's venue, technical
program, partners, cultural events, and your participation,can be viewed at the
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secretariateat (4 18) 657-3853 orby e-mail: cqvb @cqvb.qc.ca.
December1999 / Vol.49 No. 12 * BioScience 995