ECOGRAPHY 20: 585-594. Copenhagen 1997

Airborne dispersal of antarctic terrestrial algae and cyanobacteria

William A. Marshall and Matthew O. Chahners

Marshall, W A and Chalmers, M O 1997 Airborne dispersal of Antarcticterrestnal algae and cyanobactena - Ecography 20 585-594

The dispersal of algae and cyanobacteria at three Antarctic fellfield sites wasinvestigated using microscopic and culture analysis of samples from active andpassive air samplers lntersite vanation in the mean number of large algal propagules( > 5 um diameter) sampled was dependent on the niche space available for algalgrowth and the degree to which soil was exposed to desiccating influences, thesefactors could be related to the degree of matunty of the sue The numbers of largealgal propagules were lowest at sites from which permanent snow cover had recentlydisappeared and highest at sites with developed soil circles but poorly developed mossand lichen flora Mature sites with diverse and developed moss and lichen floraproduced intermediate numbers of algal propagules Propagules of multicellularalgae, cyanobactena and large-celled unicellular algae were found in the air at theend of the growing season of the respective algal groups as the soil surface dned Thiswas the case for Prasiola crispa, Pmnularia borealis, snow algae and filamentouschlorophytes and cyanobactena Dispersal of unicellular chlorophytes was greatestdunng the summer penod and at sites with developed secondary flora, but alsooccurred at other sites and in association with small thaw events dunng winterCultures were obtained from samples collected whilst an air mass that had ongmatedin South Amenca, deposited matenal on Signy Island This suggests that algalpropagules have the ability to survive long-distance transport and potenually providemocula for colonizaUon of Antarctica as regional warming continues to expose freshhabitats

W A Marshall and M O Chalmers, British Antarctic Survey, Natural EnvironmentResearch Council, High Cross, Madmgtey Road, Cambridge, U K CB3 OET

Pristine environments, such as glacial mineral fines, (Williams and Smith 1989) Mineral fines form therequire colonization by autotrophic organisms to form substrata for algal colonization Areas of fines arethe basis of an ecosystem In most ecosystems, these separated from one another by the larger stones andprunary colonizers are photosynthetic organisms that rock which form polygons or circles on flat areas andfix carbon to provide the organic biomolecules essential stnpes on slopes These areas of stone effectively isolateto organisms at higher trophic levels In Antarctic areas of fines and it is therefore unlikely that propag-fellfields, cyanobactena and algae fulfil this role (Wynn- ules can be transported by melt water between theWilliams 1990) In order for a pnstine site to be colo- near-surface regions of adjacent polygons (Davey andnized, propagules must be introduced to the site Rothery 1993) The most likely sources of propagulesthrough wind, water or animal mediated transport are air (Ellis-Evans and Walton 1990, Davey and Roth-

Fellfield soils are formed cryogenically by rock being ery 1993), birds (Schlichting et al 1978) and, morebroken down and sorted into areas of mineral fines and recently, humans (EUis-Evans and Walton 1990,areas of larger stones and rocks by frost action Broady and Smith 1994)

Accepted 5 March 1997

Copyright e ECOGRAPHY 1997ISSN 0906-7590Pnnted in Ireland - all nghts reserved

ECOGRAPHY 20-6 (1997) 5 8 5

Algae have been collected from Antarctic air by antimber of workers using sedimentation methodsRudolph (1970) trapped 148 algal propagules (0 63algal propagules m " - h" ' ) on sticky shdes at CapeHallett in Victona Land, where the frequency of occur-rence in the air was proportional to their local vegeta-tion cover Rudolph (1970) also sampled air in ManeByrd Land at a site seven miles from the nearestvegetation and collected only a few algal cells Broady(1979a) collected samples on agar plates at Signy Is-land, South Orkney Islands on a number of occasionsthroughout the year and found up to 369 algal propag-ules m"^ h~ ' dunng early summer He related thentimber of airborne algae to the extent of lowland snowcover and concluded that, given such large numbers,each algal taxon will be dispersed to, and have theopportunity to colonize, all available habitats Harmataand Olech (1991) collected algae from the deck of aship in the South Shetland Islands at the start of atransect of the Atlantic Ocean

Active samphng methods have also been used, thefirst mstance bemg the use of an Andersen cascadeimpactor, at intervals throughout the year, in 1964-1965, at Mimyy observatory (66''S, 93°E) (Meyer et al1967) similar organisms were found in the air to thoseoccumng in the soil at this locality

More recently, Burkard jet spore traps have beenused alongside Tauber traps on the McMurdo Ice Shelfand in the Taylor Valley, McMurdo Dry Valleys region(Hawes 1991) The work conducted on the Ice Shelfwas to investigate the dispersal of algal propagulesbetween ponds and lakes in an area of undulating iceNo propagules capable of germinating on any of themedia used were detected in 54 000 m^ of air sampledon the Ice Shelf or in 8000 m' air sampled in TaylorValley using the jet spore trap (with the mtake 0 8 mabove the ground) On the Ice shelf, two sets of threeTauber traps (with intakes ca 0 1 m above the ground)were used with media for fresh, brackish and salt waterOne set was sited on Bratina Island and one on theshelf Itself Tauber traps from both sites collected awide vanety of algae in all three types of media, withthose from the Bratina Island traps charactenstic ofsoil, and those in the Ice Shelf traps resembhng algaefound in the ponds on the Ice Shelf It was concludedthat locally denved matenal travelling close to theground was being caught The jet trap was assumed notto have collected any algae because its onfice was 0 8mabove the ground whilst that of the Tauber trap wasonly 0 1 m above the ground

A Burkard jet spore trap was also operated in 1990-91 as part of an investigation into the introduction ofalgae mto Antarctica by htiman activity (Broady andSmith 1994) The sampler was deployed at Scott Base,were it sampled 1800 m-* of air, and, for companson, attwo other pnstme sites Lake Vida in Victona Valley(77°23'S, 161°26'E) and a site on the Ross Ice Shelf

(78''01'S, 169°42'E) where, respectively 4140 m^ and1800 m' of air were sampled Nothing was collected ateither of the pnstine sites and only 0 006 propagulesm"-* were foimd at Scott Base

Work has been conducted in the McMurdo region tolook at the effects of electrostatic charges on the disper-sal and deposition of airborne particles using a vanetyof air sampling devices (Benninghoff and BenninghofT1980, 1983, 1985) No data on the relative abundanceof particle tjqses are given but the samples were domi-nated by clothing fibres and gymnospermous wood cellsindicating much human contamination Single celledalgae, spores, moss fragments and hyphal fragmentswere less common (Benninghoff and Benmnghoff1980) It was suggested that, in the dry conditions ofcontinental Antarctica, saltatory transport near theground is enhanced (BenninghofT and BenninghofT1983) and that sublimation is important in releasingparticles enclosed in snow and tee (BenninghofT andBenninghoff 1985)

The understanding of community development andcolonization in Antarctica can develop without the useof aerobiological samplmg devices Studies in the dryvalleys region investigated the population of algae oc-cumng in cryocomte holes on glaciers (Wharton et al1981) It was supposed that these algae onginated fromthe local terrestnal and aquatic habitats and wereblown onto the glacier and thus cycled between terres-tnal, aquatic and glacial habitats The dominant speciesfound in the cryocomte holes was Phormidtum fngidumSubsequent to this study observational work on LakesChad, Hoare and Fryxell revealed that benthic algalmats (with Phormidium fngidum being the dominantalgae in all cases and the diatom genera Hantzschia andNavtcula also being universally represented) were liftingoff the benthos due to a build up of Oj within the matThe mat floated to the base of the ice where it wastrapjjed by continued ice formation from below Pro-gressive ablation of ice from the lake surface and buildup of ice below, caused the algae to reach the surfaceafter 5-10 yr when it was blown away (Parker et al1982)

The first major attempt to sample, culture and iden-tify airborne algae was undertaken from aircraft overthe Netherlands (Overeem 1937) Nine speaes of algaewere found, of which Chlorococcum spp were mostabundant More algae were found after a dry jjenodSubsequent studies earned out in a wide range oflatitudes have found Chlorococcum spp and Chlorellaspp to be the dominant airborne algae and have indi-cated a strong association between high numbers ofairborne algae and dry conditions (Petterson 1940,Schhchtmg 1961, 1964, Brown et al 1964, Smith 1973Tiberg et al 1983, Rosas et al 1989, Roy-Ocotla andCarrera 1993) Most algal species collected from the airhave been identified as soil algae the lofung of whichwould be enhanced m dry conditions Broady (1979a)

586 ECOGRAPHY 20 6 (I997t

found that algal growth often occurred around particlesof soil or vegetation collected from the air, suggestingthat algae are transported on these panicles

The aims of this study were to investigate the degreeto which airborne transport is implicated in the disper-sal of Antarctic terrestnal algae throughout the year,identify possible factors that influence the numbers ofairborne algae and to consider how this relates tocolonization of ground recently deglaciated by the ef-fects of regional climate change

Materials and methods

A long term aerobiological sampling programme wasearned out on Signy Island, South Orkney Islands,(60°43' S, 45°36' W), in the mantime Antarctic between14 December 1992 and 28 January 1994 Three siteswere chosen, each with varying degrees of fellfielddevelopment (Fig 1) (Marshall 1996a) l) Tioga Hill,the highest point on Signy Island (279 m a s 1), consistsof a rocky peak of frost-shattered quartz-mica schistThe peak accumulates only a thin layer of snow andboth exposed rocks and sparse vegetation frequentlybecome encrusted with nme ice Areas of permanent

^ FaUdand lilands

Fig 1 Map of Signy Island, South Orkney Islands, relative toadjacent land masses, showing the location of sites and areasof permanent snow and ice (shaded)

snow and ice stu-roimd the peak but the summer snow-free area (ca 4 ha) is increasing annually (Convey andSmith 1993) For much of the year (early Apnl-earlyDecember) the lower, more recently exposed, part ofthe peak is covered by deep snow An array of samplerswas sited to the west of the jseak ii) Jane Col (150 ma s 1) starts to accumulate snow in March and is pwr-manently covered by deep snow between early June andlate November Much of the site consists of frost-pat-terned ground with the more stable areas of rockyground and mineral soil colonized by lichens andmosses and the mobile fines remaining free of all but athin algal and cyanobactenal crust The array of sam-plers was sited on a flat area of mineral fines at the topof the col in) Factory Bluffs (111 m a.s 1) remainssnow-free for much of the year with only patchy snowduring the winter However, most of the site is penodi-cally covered by a thick coating of rime ice A samplerarray was sited on the west side of the plateau in anarea of abundant and diverse lichen and moss flora(Anon 1982)

Arrays of rotorod samplers (May et al 1976, Mar-shall 1997) were placed at each site in a line perpendic-ular to the prevailing westerly winds (Pepper 1954) Therotorods were secured so that the centre of the sam-pling surface was 1 m above the ground However,dunng the final 15 samplings collections were made atboth 1 m and 0 15 m Four rotorods (each with twocollecting arms) were operated for two separate 24 hperiods each week at all three sites from 14 December1992 (at Jane Col) or 8 March 1993 (at the other sites)until 28 January 1994 However, some penods weremissed (Fig 2) due to bad weather The samplingsurfaces were prepared and deployed as descnbed inMarshall (1996a) On return to the laboratory, four ofthe eight sampling stnps retneved from each site weremounted for examination by light microscope (Mar-shall 1996a) Of the remaining four stnps, two wereplaced in plastic universal tubes containing Bold's basalmedium (Nichols and Bold 1965) incubated at 15°Cand two were frozen at — 20°C for further possibletreatments

Arrays of "Frisbee" deposition samplers (Hall andUpton 1988) were also placed on all three sites, 1 mabove the ground These were left in place for 1 wk ata time after which the collected matenal was nnsedfrom each Fnsbee into a stenle bottle beneath usingstenle distilled water The bottles containing the sam-ples were then recovered and replaced with freshlystenlized ones The matenal contained in each bottlewas filtered onto aluminium oxide (Anodisk 25, 25 mm,0 2 |xm) filters One filter from each site was desiccatedand stored at — 20°C The second was placed on a glassfibre filter pad in a 45 mm diameter Petn dish andBold's basal medium was added to the Petn dish sothat the top surface of the filter was wet but notimmersed The Petn dishes were then placed in anilluminated incubator at 15°C

ECOGRAPHY 20-6 (1997) 587

004

0 03

0 02

0 011i 000

1 1 r

1 1 1 1 1 1 1

J F M A M J J A S O N D J

Fig 2 Concentrations oflarge algal propagules (> 5nm diameter), per m ' airsampled, using rotorodsamplers at I m aboveground level at three siteson Signy Island a) JaneCol, b) Factory Bluffs, c)Tioga Hill, between 14December 1992 and 28January 1994 Insetsrepresent samples collectedat 0 15 m dunng the finalfifteen samplings (scalesremain identical)• = samples from whichalgae were cultured

Prasiola cnspa '—J Zygnetna sppSnow algae cyst ̂ a Cyanophyte filament

Slides from rotorod samples were examined by lightmicroscopy at 200 x or 400 x magnification and mate-rial of interest recorded, photographed and drawnCultures were kept for a minimum of twelve weeks,during which time they were inspected at regular inter-vals and isolates were subcultured for identificationwhere possible

In total, 162 samples from deposition samplers werereturned to the U K (frozen at -20°C) Of these, 39were examined by scanning electron microscope (SEM)at 500 X magnification, with three transects of 100microscope fields being made of each filter (Chalmers etal 1996). Diatom frustules were photographed at amagnification sufficient to allow identification

ResultsMicroscopic analysis of rotorod samples was only suit-able for larger algae and algal fragments (those with adimension > 5 um) the occurrence of which is shown inFig 2 The mean concentrations of these propagules inair samples were 1 1 x 10-^ 8 7 x 10-^ and 9.1 x lO" '

m"^ for Jane Col, Factory Bluffs and Tioga Hill re-spectively Most peaks on Fig 2, including all of thelargest, represent occurrences of Prasiola crispa (Light-foot) Meneghini, mostly as the unisenate form("Hormtdium"-stsige) but also foliose thallus fragmentsThe proportion of filaments to thallus fragments vanedbetween sites, with filaments comprising 36%, 77% and100% of the total Prasiola catch at Factory Bluffs, JaneCol and Tioga Hill respectively. Other algal propagulesthat could be counted microscopically included redcysts (aplanospores) of the snow algae Chlamydomonasnwahs (Sommerf) Wille and Chlorosphaera antarcticaFntsch, fragments of cyanobactenal filaments and frag-ments of chlorophyte filaments, probably Zygnema sppAlmost all catches of these groups were made dunngthe warmer months Zygnema spp from mid-Novemberthrough to early January, snow algae cysts from Febru-ary through to early Apnl (with one occurrence inMay) and cyanobactenal filaments from late March toearly Apnl

No large propagule types were recorded at 1 m aboveground level at Jane Col dunng the second summer ofsampling Conversely cultures were obtained from Jane

588ECOGRAPHV 20 6 (IW7)

Fig 3 Occurrence ofculturable algae, •unicellular chlorophytes; •filamentous chlorophytes, Ahlamentous cyanobactena,from collections at 1 mabove ground level using a)Fnsbee deposition samplers,b) rotorod samplers, at threesites on Signy Island JC -Jane Col, FB - FactoryBluffs, TH - Tiogd HillSuffix -L indicates rotorodsamples collected at 0 15 mabove ground level dunng(he second summer ofsampling Open symbolsrepresent in a) start timesfor collections (sampling wascontmuous), in b) when 24hr collections were made, atG Jane Col, D FactoryBluffs, A Tioga Hill

TH

TH-L

FB

FB-L

JC

JC-L

1 • • •

* • • • " • •

- • - -

•

• •

• • •

• • •

•t •

• •

- * •

• • •

J F M A M J J A S O N D J

Col samples m January 1994 but not m January 1993(Figs 2a and 3b) There was some vanation betweensites in this respect as large algal propagules were seenon rotorod samples from Factory Bluffs and Tioga Hillm the first and second summer (although records fromTioga Hill are scant) No trend was observed, betweensites, in the numbers of large algal propagules trappedat 1 m and 0 15 m above the ground (Fig 2 msets)

A wide vanety of diatom valves, from marine, fresh-water and terrestnal habitats was trapped The occur-rence of the diatoms Ptnnularia borealis Ehrenberg andPinnularta borealts var rectangulans Carlson (seeChalmers et al. (1996) PI 10 g and h) are doctmientedhere Valves of the diatom Ptnnularta borealts weretrapped on several occasions dunng the early winterpenod (late March-late June), particularly at FactoryBluffs (Fig. 4) In early summer a large peak wasrecorded from Tioga Hill but only low numbers wererecorded from other sites (Fig 4) Ptnnularta borealtsvar rectangularts compnsed 18 5% of all P borealisspp trapped

Although not observed by direct observation of ro-torod stnps, umcellular chlorophytes were a majorcomponent of airborne algal flora as shown by cultureexpenments (Fig 3). Rotorod cultures indicated viablealgae m the air at all sites sampled durmg the beginmngand end of May 1993, the beginning of June 1993, midAugust 1993, occasions dunng November 1993 andDecember 1993-January 1994. No cultures were ob-tained from rotorod samples collected dunng January,Apnl and July 1993 More cultures were obtained fromFactory Bluffs rotorod samples than those at other sites(Table 1), espeaally dunng September and Octoberwhen Jane Col and Tioga Hill samples were consis-tently unproductive (Fig. 3). Filamentous cyano-bactena were found only m cultures from samples

collected dunng December 1993 and January 1994, theexception being singular occurrences from depositionsamples from Tioga Hill and Factory Bluffs in Marchand April 1993 respectively The percentage of samplesthat produced cultures, from collections taken dunngthe final 15 samplings, was no different at 1 m and 0 15m at Factory Bluffs and Tioga Hill, at Jane Col 13.3%fewer cultures were obtained at 1 m than at 0 15 m(Table 1)

The numbers of airborne algae could not be corre-lated to any meteorological variables (temperature,wind sp>eed, relative humidity)

DiscussionThe vanation between sites in total numbers of largealgal propagules sampled can be related to habitatdevelopment Tioga Hill, with the lowest numbers ofairborne algae, includes only a small area of snowfreeground dunng the summer which constitute suitablehabitats for terrestnal algae Factory Bluffs is a moremature fellfield, with a greater dominance of secondaryplant colomsts (mosses and lichens) and, probably, alarger number of micro-invertebrate herbivores JaneCol, with the highest numbers of airborne algae, is anintermediate between the other two sites; pnmary colo-nization IS advanced yet the site lacks the degree ofsecondary cryptogam colonization seen at FactoryBluffs These findings agree with those of Broady(1979b), who found that, of habitats with secondarybryophyte vegetation, only two out of seven examinedon Signy Island had algal populations greater thanmineral soil sites devoid of secondary colonists Thesoils beneath the vascular plants Deschampsta antarc-ttca (6 sites) and Colobanthus qmtensts (1 site) were

ECOGRAPHY 20-6 (1997) 589

J F M A M J J A S O N D J

Jane Col I. I Factory Bluffs ^ H Tioga Hill

Fig 4 Numbers ofPmnutaria borealis fnistulesfound from collections at 1m above the ground usingFnsbee samplers at threesites on Signy IslandSamples examined butdevoid of Pmnulana boreatisat each site are indicated by• Jane Col, • FactoryBluffs, A Tioga Hill

found to have the highest numbers of algae of anyfellfield habitat on Signy Island (Broady 1979b) How-ever, none of these plants were found in the vicinity ofthe samphng sites used and have very restncted distri-butions on Signy Island

Previous studies have found that most airbome algaeare soil algae (Brown et al 1964, Rosas et al 1987)This would suggest the number of algal propagulesfound m the air at each site to be related to the numberof algae in the soil at each site or the niche spaceavailable at each site No data exist conceming thevanation in algal numbers and diversity at these sites atthe time of this study However, Jane Col has muchlarger areas of exposed mineral fines than the othersites, at Tioga Hill frost sorting has yet to provide suchareas and at Factory BlufTs succession has progressedsuch that secondary colomsts predominate Degree ofground cover of macroscopic plants could be an impor-tant factor influencing the airbome algal populationIncreasing numbers of secondary colonists (moss andhchen) will decrease the ability of soil algae to becomeairbome by mcreasing the thickness of the boundarylayer and slowing the rate of drying of the soil surface

Prasiola cnspa is frequently found in aenal habitats(exposed rock surfaces and associated lichens (sensuBroady 1979c)) where the umsenate form is dominant(Broady 1979c) Mature thalloid P cnspa tends to befotmd in subaenal habitats (soils and associated plants(sensu Broady 1979c)), particularly those with a highnutnent content, where it is much more abundant thanthe umsenate form (Broady 1979c) Algae on exposedrock surfaces would be more likely to become dislodgedby the onset of harsh winter condiUons, explaming thelarge peaks observed durmg May and June (Fig 2a,b)Large wind-blown fragments of thalloid P cnspa canfrequently be seen away from natural populations(Broady 1979a). However, the fragments collected m airsamples were much smaller and more numerous thanthose observed in the field and none were large enoughto be visible to the naked eye The comparative rantyof the thalloid form in the air may result from thepresence of short rhizoids attaching it to the substra-tum, and that it adheres to the substratum upon desic-cauon (Kightley unpubl.) On Signy Island, thalloid Pcnspa IS much more abundant m areas with high nutn-

ent input from bird and seal faeces and in more devel-oped soils, such as those found around vascular plants(Broady 1979c) The higher frequency of thalloid Pcnspa at Factory Bluffs may be due to the site's prox-imity to nesting sites but, the degree of soil develop>-ment may also be important

The other algal propagules shown m Fig 2 weretrapped during the penods of senescence of the algaeChlorophytes grow rapidly in spnng until the snowcover melts, when cyanobactena, most notablyPhormtdium spp, commence rapid growth (Davey1991a) Filaments of Zygnema spp were observed onrotorod samples in January and December 1993 andJanuary 1994 at Jane Col, comciding with the end of thefilamentous chlorophyte bloom at Jane Col observed byDavey (1991a) However, Zygnema appears somewhatearlier in the air (mid-November) at Factory Bluffs andTioga Hill where snow clearance occurs earher. Suscep-tibility to desiccation and the effects of high lrradiancesare the most likely causes for the declining growth offilamentous chlorophytes as summer progresses (Davey1991a), desiccation will also increase the ability of soilalgae to become airbome Dunng the early stunmer of1993/94, snow cleared from Factory BlufTs by the sec-ond week of November and had gone from all threesites by the beginning of December Tioga Hill andFactory BlufTs sites are on hill tops, whereas Jane ColIS flanked on two sides by higher ground It is thereforelikely that the soil at Jane Col retains a high watercontent for a longer penod than at the other two sitesShould desiccation be the major factor promoting thelofting of filamentous chlorophytes, this situation wouldexplain the delay seen between the appearance of theseorgamsms m the air at Jane Col relative to FactoryBluffs and Tioga Hill

Filamentous cyanobactena were collected on ro-torods at Jane Col and Factory Bluffs dunng latesummer in 1993, dechnes in the soil populations ofthese orgamsms have been recorded dunng this penodm 1987, 1988 and 1990 (Davey 1991a, Davey andRothery 1992) The growth rates (Davey 1991b) andpercentage soil cover of filamentous cyanobactenathroughout the year (Davey 1991a) demonstrate a typi-cally annual growth pattem and suggest that only smallmocuia are required to initiate spnng growth. The

590 ECOGRAPHY 206 (19971

Table 1 Percentage of aerobiological samples collected from three sites on Signy Island Jane Col (JC), Factory Bluffs (FB),Tioga Hill (TH) from which algal cultures were produced

Site

JC

FB

TH

Height above

10 1510 1510 15

Rotorod samplers(all collections)

23 1-

31 5-

20 8-

Percentage of samples produang cultures

Rotorod samplers(final 15 collections)

46 760606063 663 6

Frisbee samplers(all collections)

51 1

66

444-

low number of recordings of cyanobactena in the airdespite the dommance of this group m fellfield soils iscunous but may be due to transport occumng close tothe ground (Hawes 1991) Such a scenario may beexpected if large aggregates of entangled filaments,glued together with mucilage, with only a few viablecells, are dispersed This seems likely given that thesegroups are rapid colonizers and that there is littledifference in the distnbution of small particles withheight above the ground (between 0 15 m and 1 m)(Marshall 1996a)

Snow algae (Chlamydomonas nwalts and Chloro-sphaera antarctica) occur on most areas of permanentsnow on Signy Island and heavy red coloration devel-ops dunng snow melts (Fogg 1967) The site at TiogaHill IS surrounded by permanent snow and ice yet,despite heavy coloration of the surrounding snow dur-mg summer, no snow algae aplanospores were trappedheie In contrast, snow algae were trapped on FactoryBluffs, although this site has only small f)ermanentsnow patches within 1 km of the site, with the mainareas of red snow on the Island's ice cap 1 6 km distantThus snow algae, despite the size of their spores, and ofclumps of spores, are able to disperse over reasonablylong distances The lack of snow algae aplanospores atTioga Hill IS most likely a consequence of the site beingupwind (in the prevailing westerly winds) of the largestareas where snow algae are abtmdant, and that, unlikecontmental Antarctica regions, melt dominates overablation This would mean that instead of being lofted,following the sublimation of surrounding snow (Ben-nmghoff and Benninghoff 1985), the snow algae cystswould remain m the snow/water until earned away bymelt water At Jane Col water flows from the ice caponto the site where cysts could become desiccated andlofted into the air.

On Signy Island P borealts was only recorded fromsubaenal sites (Broady 1979c), where it is found to-gether with P borealts var rectangularts Ptnnulariaborealts records from deposition samplers are difficultto interpret due to the non-quantitative nature of thecollection method and selective nature of the analysis.Diatoms, whilst undoubtedly present on rotorod collec-tions, are difficult to observe because silicone oil in the

lmpaction adhesive has the same refractive index assilica diatom valves All diatoms sampled appeared tobe dead as they had valves devoid of cell contents Thismay be due to the motile nature of this organism, liveindividuals would be less likely to be blown away dueto the presence of mucilage produced dunng motion(Harper 1977, Edgar and Pickett-Heaps 1983) Fewerthan \0% of individuals are fotmd at the soil surface(Davey and Clarke 1991) further reducing dispersalopportunities of live individuals Frustules of dead indi-viduals would accumulate in the soil over time, out-numbenng live cells Frost action could then raise themto the surface where they would be blown away unhin-dered The very large P borealis p>eak at Tioga HiU isdifficult to interpret since the site offers less stutableniche space for P borealts than the other sites

The appearance of algae in culture from rotorodstnps exposed dunng January 1994 but not in January1993 at Jane Col is cunous (Fig 3) Davey (1991a)found similar inconsistencies in the lnterannual occur-rence of filamentous chlorophytes m soils betweenyears This may also be the case for P crtspa at JaneCol (Fig 2a)

Throughout the winter positive cultures were ob-tained from the Factory Bluffs rotorods more regularlythan from the other sites This may be a consequence ofthe thinner snow cover at Factory Bluffs with morefrequent melting due to black body absorption of solarradiation by the substratum A similar but more pro-nounced effect was noted m the release of lichen sore-dia (vegetative propagules) at the three sites (Marshall1996a) On other occasions algae were found at all sitesAugust and November 1993 (and May and June 1993at Factory Bluffs and Jane Col) The cultures develop-ing from the May and June samples could be due tofrequent thaws providing disp>ersal opportumties atthese times The greater number of cultures obtainedfrom deposition samples, dunng this early part of win-ter (Apnl-July mcltisive), could be due to the continu-ous sampling, all thaws would be intercepted by thesamphng programme

The cultures obtained from rotorod samples in Au-gust are consistent with other records of biologicalactivity at this time m previous years (Hawes 1988,

ECOGRAPHY 2fr6 (1997) 591

Davey 1991a, Convey 1992, Arnold 1996), probablyattnbutable to blackbody absorbtion of solar radiationby the substratum and a subsequent melt at the soil-snow interface Deposition samplers would not revealthis if the cells were killed by high levels of environmen-tal stress (ultra-violet radiation, freeze thaw action ordesiccation) for days pnor to collection (Fig 3)

The cultures obtained from samples collected at allthree sites on 11-12 November coincide with an eventwhere matenal onginating in South Amenca was sam-pled (Marshall 1996b) If the South Amencan origin ofthese algae could be verified the ability of algal propag-ules to survive intercontinental airborne transport intothe Antarctic would be proven (cf van Zanten 1978,van Zanten and Gradstein 1987) The observation ofthis phenomenon provides a possible means by whichthe Antarctic may have been recolonized by certaingroups since the last glacial maximum (LGM) ca 18 000yr ago (Marshall 1996b) A contemporary increase inspecies diversity in the Antarctic also becomes a jwssi-bility in view of the regional warming seen in themantime Antarctic in the last 35 yr (Smith 1990, King1993) Indeed, changes in the process of pnmary colo-nization cannot be ruled out as sjsecies new to thecontinent continue to amve However, it is likely, if thebiota seen in the Antarctic has amved by long-distancetransport smce the LGM, that the contemporary algalpopulations represent those taxa that can survive long-distance transport and therefore do not differ greatlyfrom those taxa that continue to amve Proof of theongins of the algae cultured from the 11-12 November1993 samples would also be impossible using geneticmethods if such transport events occur at frequencies of1.5 yr~' (Marshall 1996b) Direct evidence could onlybe provided by samplmg over the ocean (preferablyfrom aircraft) or by the culture of exotic species such asthose seen around fumaroles in volcanically active re-gions of Antarctica (Broady 1984, Broady et al 1987,Bargagli et al 1996)

Algal propagules must onginate from growth-sup-porting habitats Therefore, when release occurs similarhabitats must be available as sites for deposition ofpropagules This will be the case regardless of seasonfor locally dispersed propagules although, m the sum-mer, greater availabihty of thawed, exposed ground willincrease the chance of deposition in a habitat suitablefor colonization In addition, species with propaguleswhich are able to survive in snow and/or ice and areable to attach to suitable substrata when released dur-mg penods of melt (Davey et al 1991) will provide a"pool" of potenUal colomsts

A small but significant population of larger algalpropagules (Fig 2) is found in the air when the maxi-mum local area is free of snow and ice (November-early Apnl) The abihty of these propagules to becomeairborne is hindered by a number of factors excessivesize and weight, production of mucilage and the ability

to move away from desiccating environments In suchinstances dispersal may depend on extreme environ-mental conditions, such as high wind or the abihty ofthe organism to overcome its own constraints Diatomsfor example are propelled into the air when they dry ona plane surface (Davey p>ers comm ) Similarly a motileorganism may sunply remain at the soil surface whenfaced with desiccating conditions if such conditionsfacilitate dispersal, however, evidence for such a strat-egy would be difficult to obtain

Small unicellular chlorophytes do not appear to beconstrained to the same degree as larger algal propag-ules in their ability to become airborne This is demon-strated by the larger numbers and reduced seasonahtyin the appearance of this type of algae m culturecompared to filamentous forms (Fig 3) This processcould be facilitated by factors such as their poor abilityto form attachments to the substratum (Davey et al1991) Such a tendency to opportunistic dispersal andthe rapid growth seen in this group of algae (Davey1991b) are typical of an r-selected life-history strategy

Avian-mediated transport may also occur (Schhcht-ing et al 1978) but is probably only significant for thosealgal taxa that inhabit the extremely eutrophic omitho-genic habitats around nest sites Most of the sea birdsnesting on Signy Island are rarely seen on maturefellfield away from their nest sites The degree to whichbirds can contnbute to the transport of propagulesdepends on the ability of propagules to adhere to thebirds at source sites and detach at sinks, factors cur-rently unknown

This study demonstrates that airborne transport is animportant means of dispersal for Antarctic terrestnalalgae and cyanobactena However, for propagules de-posited on snow or ice, secondary transport by meltwater may also be important (Broady 1977) As an icefront recedes large amounts of melt water run off ontothe newly exposed substratum Such melt water con-tains much mmeral as well as organic matter (Marshallunpubl) and probably algal propagules which consti-tute pnmary colonists Propagules that accumulate inthe snow and ice propagule bank over a large area, viaairborne transport, may be deposited on a narrow bandof fines at the ice front It may be that much of thebiological activity m this zone is saprophytic and thatfew propagules of typical fellfield algae deposited heresurvive to become pnmary colonizers An investigationof this zone of deposition would identify whether mostpropagules of the pnmary algal colonizers onginatefrom the snow and ice propagule bank or are depositeddirectly onto the soil from the air

Acknowledgements - We would like to thank all the personnelon Signy Island for their help throughout the duration of thisstudy, Paul Broady for identifying many of the algal culturesMargaret Harper for ldenUfying diatoms, Martin Davey andMargaret Harper for invaluable advice and discussion andKen Robinson for advice on and help with scanning electron

592 ECOGRAPHY 20 6 (1997)

microscopy Thanks are also due to P Convey, J Lacey andD D Wynn-Williams for constructive comments on earlierdrafts of this manuscnpt and to Paul Broady for his veryhelpful and clanfying comments

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