doi:10.3402/polar.v35.20703


R E S E A R C H / R E V I E W A R T I C L E

Algal biomass and pigments along a latitudinal gradient in
Victoria Land lakes, East Antarctica
Francesca Borghini, Andrea Colacevich, Tancredi Caruso & Roberto Bargagli

Department of Physical, Earth and Environmental Sciences, University of Siena, Via P.A. Mattioli, 4, IT-53100 Siena, Italy

Keywords

Biodiversity; photosynthetic pigments;

proxy; continental Antarctica; sediments;

biogeography.

Correspondence

Francesca Borghini, Department of

Physical, Earth and Environmental Sciences,

University of Siena, Via P.A. Mattioli, 4, IT-

53100 Siena, Italy. E-mail: borghini@unisi.it

Abstract

It is generally accepted that Antarctic terrestrial diversity decreases as latitude

increases, but latitudinal patterns of several organisms are not always as clear as

expected. The Victoria Land region is rich in lakes and ponds and spans 8 degrees

of latitude that encompasses gradients in factors such as solar radiation,

temperature, ice cover and day length. An understanding of the links between

latitudinally driven environmental and biodiversity changes is essential to the

understanding of the ecology and evolution of Antarctic biota and the

formulation of hypotheses about likely future changes in biodiversity. As several

studies have demonstrated that photosynthetic pigments are an excellent,

although underused, tool for the study of lacustrine algal communities, the aim

of the present study was to investigate variations in algal biomass and

biodiversity across the latitudinal gradient of Victoria Land using sedimentary

pigments. We test the hypothesis that the biodiversity of freshwater environ-

ments decreases as latitude increases. On the basis of our results, we propose

using the number of sedimentary pigments as a proxy for algal diversity and the

sum of chlorophyll a and bacteriochlorophyll a with their degradation

derivatives as an index of biomass. Overall, our data show that biomass and

diversity decrease as latitude increases but local environmental conditions, in

particular, natural levels of eutrophy, can affect both productivity and diversity.

To access the supplementary material for this article, please see the

supplementary files under Article Tools, online.

According to general ecological principles, biodiversity

decreases with latitude as an effect of the harshening of

climate: the more extreme the environment in terms of

temperature, water availability and feeding resources, the

less the terrestrial communities are diverse in terms of

species richness and taxonomic composition. Despite the

recognition of latitudinal diversity gradients (Hillebrand

2004), the effect of latitude on biodiversity is mostly

known for a very limited number of taxonomic groups,

regions and ecosystems. A good example of well-studied

cases is vertebrates from terrestrial or marine ecosystems

in the Northern Hemisphere. However, diversity gradients

for the Northern Hemisphere seem not to apply to the

Southern Hemisphere (Boyero 2002; Vyverman et al.

2007).

The determinant of biological diversity is not latitude

per se but environmental conditions, which vary with

latitude. More than 25 different mechanisms, not mu-

tually exclusive, have been suggested as generating

latitudinal diversity gradients, including the history of

perturbations, environmental stability, habitat heteroge-

neity and interspecific interactions, but no consensus has

been reached yet (Gaston 2000). A recent study concludes

that climatic and productivity-related variables tend to

be the strongest correlates of species richness (Field et al.

2009).

Across Antarctica, many environmental stresses are

severe compared to lower latitudes (Convey 1996), bio-

diversity is generally low and ecosystems are simple.

Several studies showed that many floral and faunal taxa,

Polar Research 2016. # 2016 F. Borghini et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0
International License (http://creativecommons.org/licenses/by-nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the
original work is properly cited.

1

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including higher plants, algae, lichens, protozoa and

arthropods, become less diverse at higher latitude (Smith

1992; Broady 1996; Wynn-Williams 1996). However,

even if it is generally accepted that Antarctic terrestrial

diversity decreases as latitude (and stresses) increases

(Broady 1996; Smith 1996), the latitudinal patterns are

not always so clear.

In a study conducted along a transect from 548 S (sub-
Antarctic) to 828 S (continental Antarctica), the highest
molecular diversity of cyanobacteria was found in the

areas located between 708 S and 808 S, whereas further
south and further north from this zone the diversity

decreased (Namsaraev et al. 2010). Only limited support

was obtained for a systematic decrease in eukaryotic

(primarily fungi, algae and protists) diversity with latitude

(Lawley et al. 2004; Petz et al. 2007), that is, when the

comparisons were made between maritime and conti-

nental Antarctic sites, whereas there was no evidence for

a trend of decreasing diversity across the entire range of

the maritime Antarctic. Similar findings were also re-

ported for tardigrades and rotifers (Sohlenius & Boström

2005) and plants (Peat et al. 2007). In the latter study, a

diversity gradient was found along the Antarctic Peninsula,

but no such gradient was apparent through Victoria

Land and the Transantarctic Mountains. The gradient

along the Antarctic Peninsula was clearer for lichens than

for mosses, and the cline was driven by an increasing

number of species lost from the assemblage with increas-

ing latitude rather than by taxonomically different assem-

blages being found at different latitudes. Moreover, most

studies are generally limited to the maritime Antarctic

and are restricted to measuring macrophyte and animal

diversity. Relatively little is known about microbial diver-

sity, and just a few investigations of diatoms (Maidana

et al. 2005) and bacterioplankton (Schiaffino et al. 2011)

were recently conducted on lacustrine ecosystems across

a Patagonian�maritime Antarctic transect: a decrease of
diatom richness and total bacterioplankton abundance

with increasing latitude was observed.

Nowadays, major research efforts on the continent

focus on understanding how abiotic variables influence

the abundance, richness and fitness of terrestrial animal

and vegetation populations, with projects such as the

Latitudinal Gradient Project assessing how biodiversity

and structure may change with latitude and with climate

change. In Victoria Land, which starts at the Darwin

Glacier (798 S) in the south to Cape Adare (718 S) in
the north, there is a network for monitoring change

in vegetation communities to estimate ties with climate

change (Cannone 2006). The Victoria Land latitudinal

range corresponds to a gradient in factors such as solar

radiation, temperature, ice cover and day length, which

would be expected to underlie differences in biodiversity

(Peterson & Howard-Williams 2000). An understanding

of the links between latitudinally driven environmental

and biodiversity changes is essential to our understand-

ing of the ecology and evolution of Antarctic biota

and the formulation of hypotheses about likely future

changes in biodiversity.

Increasing over the last 40 years, limnological research

in this region has focused on the study of physical,

chemical and biological processes, on the characterization

of biotic communities and the basic interactions driving

the functioning of aquatic ecosystems (Bergstrom et al.

2006). Surprisingly, only a few areas of Victoria Land have

been closely investigated, such as McMurdo Sound in

southern Victoria Land (e.g., Simmons et al. 1993; Vincent

et al. 1993; Vincent & Quesada 1994; Hawes & Schwarz

1999; Nadeau et al. 1999; Taton et al. 2006; Vopel & Hawes

2006). Limnological studies in the rest of Victoria Land

have been conducted at only a few sites (Guilizzoni et al.

1989; Fanzutti et al. 1989; Guilizzoni et al. 1992; Libera

1993; Baudo et al. 2000). More recently, information

regarding the physicochemical characteristics of lacus-

trine waters and sediments has been reported by Borghini

& Bargagli (2004), Borghini et al. (2008) and Malandrino

et al. (2009). These studies showed high diversity in

physical, chemical and biological features of lacustrine

ecosystems in this region.

Several studies (Hodgson et al. 2004; Hodgson et al.

2006; Lami et al. 2009; Borghini et al. 2011; Fernandez-

Carazo et al. 2013) have demonstrated that photosyn-

thetic pigments are an excellent tool for the study of

lacustrine algal communities. However, a challenge in

the study of most Antarctic lakes is that they show large

variations in physicochemical and biological characteris-

tics during spring and summer (Borghini & Bargagli 2004),

implying that reliable evaluations of phytoplankton com-

position and biomass would require repeated sampling,

which the logistical and environmental constraints of

Antarctic fieldwork make difficult.

The aim of the present study was to investigate variations

in algal biomass and biodiversity across the latitudinal

gradient of Victoria Land using sedimentary pigments. The

starting point of this study is that sedimentary pigments

Abbreviations in this article
Bchl a: bacteriochlorophyll a

chl a: chlorophyll a

HPLC: high performance liquid chromatography

PCA: principal component analysis

TN: total nitrogen

TOC: total organic carbon

TP: total phosphorus

Algal biomass and pigments along a latitudinal gradient in Victoria Land F. Borghini et al.

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integrate temporal information since sediments are the

main sinks for solutes and particulate materials from the

lacustrine catchment. Moreover, phytoplankton often

represent less than 2% of total photosynthetic biomass

in polar lakes, and most of the primary production is

confined to the phytobenthos (Bonilla et al. 2005; Rautio

et al. 2011). Overall, lacustrine sediments serve as tem-

poral and spatial indicators of the entire catchment and

are very useful as sentinel systems of environmental and

climatic changes (Quayle et al. 2002; Vincent & Laybourn-

Parry 2008; Leavitt et al. 2009).

The total number of pigments was used as an index of

diversity, while the sum of chl a and Bchl a, with their

degradation derivatives, was utilized as biomass index. It

should be noted that the use of chl a as an estimate for

biomass growth has several limitations as chl a concen-

trations per cell may vary in relation to light acclimation,

nutrient availability and other physiological conditions

(Anning et al. 2000; Kasprzak et al. 2008). However, chl a

is often used in ecological studies as a common proxy for

biomass dynamics (Lizotte 2008). Total chl a is often used

as a proxy of primary productivity as well (Falkowski &

Raven 1997), assuming that the rate of carbon fixation is

positively correlated with the chl a concentration. As rates

of pigment degradation can differ greatly among pigments

and between lakes with different biotic, physical and

environmental features (e.g., Leavitt & Hodgson 2001),

like those of the present study, the sum of chl a, Bchl a

and their degradation derivatives was chosen as biomass

index, instead of chl a alone, in order to integrate possible

differences.

Materials and methods

We sampled 48 lakes and ponds, located in 18 ice-free

areas, with different lithological features and at different

latitudes (spanning about 5 degrees) and altitudes, from

Kar Plateau (76.918 S, 162.548 E) to Helm Point (72.138 S,
170.158 E). Most of the lakes are proglacial and located
in rocky catchments with poorly developed soils. They

lack outlets, are shallow (maximum depth about 5 m)

and have different surface areas (from 10 to 28 630 m
2
,

Supplementary Table S1). Study areas and sampling sites

are shown in Fig. 1, and details of sampling sites and

methods have been provided elsewhere (Borghini et al.

2011). Briefly, surface sediments (1 cm) at a fixed water

depth of 20 cm, free of microbial mats, were collected

in different points of the littoral zone using a hand corer.

The physicochemical features of the water bodies were

reported by Borghini et al. (2008), and sediment TOC, TN

and TP by Borghini et al. (2011). These parameters

are summarized in Supplementary Table S2, along with

the lake ice cover and bird presence, estimated during

the sampling. Sedimentary pigments were determined

by HPLC coupled with a photodiode array and a mass

spectrometer following the extraction and analytical

methods of Borghini et al. (2011). Fresh sediments were

extracted with acetone 90%, utilizing a vortex and a

sonicator at low temperature. Throughout the extraction

procedure, samples were protected from light and high

temperature. Liquid chromatography�mass spectrometry
analysis was performed using a Thermo system compris-

ing a Finnigan Surveyor autosampler, MS Pump and LTQ.

Pigments were detected at their maximum wavelength

and at their characteristic m/z. Peaks were identified by

their absorption and characteristic MH
�

and MS
2

spectra

and their elution order by comparing with literature data

(e.g., Rozema et al. 2002; Whitehead & Hedges 2002;

Yuan et al. 2009). Their concentrations were determined

by comparing HPLC peak areas with those of standard

solutions prepared with commercially available purified

pigments from the International Agency for
14

C Determi-

nation, Water Quality Institute, in Hørsholm, Denmark

(pheophytin a, pheophorbide a, chlorophyllide a, chl c2,

chl c3, divinyl chl a, lutein, peridinin, fucoxanthin,

violaxanthin, canthaxanthin, zeaxanthin, alloxanthin,

neoxanthin, 19’-butanohyloxyfucoxanthin, echinenone,

prasinoxanthin, antheraxanthin, diadinoxanthin, 19’-

hexanoyloxyfucoxanthin, lycopene, myxoxanthophyll)

and from Sigma Aldrich (chl a and b, b,o- and b,b-
carotene). Concentrations of those molecules for which

no standard was available were estimated from peak

area, assuming specific extinction coefficients for each

compound obtained from the literature (Hurley & Watras

1991; Villanueva et al. 1994; Jeffrey et al. 1997). Where

extinction coefficients were not reported or where no

structurally or spectrally similar pigments, the coefficient

of b-carotene was used (Jeffrey et al. 1997).
Results are expressed as ng g

�1
TOC because this

unit of measurement most accurately captures variations

in algal abundance and community composition (Leavitt

et al. 1997).

Data analysis

Prior to multivariate analyses, data and all environmental

variables (except pH, ice cover and bird presence, which

was estimated in percentage), latitude and longitude

were log-transformed to reduce or remove skewness in

the data. Major patterns of variation in the environ-

mental data were explored using PCA, with centring and

standardization of the variables.

The effects of latitude and environmental variables on

the pigment diversity and biomass between sites were

F. Borghini et al. Algal biomass and pigments along a latitudinal gradient in Victoria Land

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investigated also using redundancy analysis. Redundancy

analysis was performed including both geographic and

environmental variables as explanatory variables. The

contribution of both environmental variables and geo-

graphical (latitude, longitude and distance from the sea)

factors in structuring the pigment biomass and diversity

was assessed through the use of variance partitioning

(Borcard et al. 1992). The variance explained by each

of these groups allows us to determine the amount of

variance in the pigment data that is explained by the

environmental data alone, the spatial data alone, by

environmental data that is spatially structured, as well

as the amount of unexplained variance (Borcard et al.

1992).

Fig. 1 Study area and sampling sites. (Modified from the Antarctic Digital Database [www.add.scar.org/home/add7] for the coastline and the Antarctic

Geospatial Information Center, National Science Foundation, for the ground digital model.)

Algal biomass and pigments along a latitudinal gradient in Victoria Land F. Borghini et al.

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Results

A total of 132 compounds were detected in lacustrine

superficial sediments of Victoria Land as reported in

Borghini et al. (2011).

Pigment indices were correlated by simple ordinary

least square linear regression with latitude in order to

reveal how key community features such as biomass

and diversity may vary with latitudinal environmental

gradients. The correlations of these two indices with

the latitude were statistically significant and showed a

negative slope (respectively R � �0.53 and R � �0.65;
PB0.01; Figs. 2, 3). The correlation between biomass and

latitude was confirmed using the sum of carotenoids and

the chl a as indicators of algal biomass. However, a large

part of the data variation remained unexplained by

latitude, suggesting that other factors affect the two

community indices. These results were similar when the

analysis was performed without including the derivative

forms of pigments and the unidentified compounds and

considering just the identified marker pigments (chl a,

chl b, scytonemins, fucoxanthin, violaxanthin, canthax-

anthin, zeaxanthin echinenone, lutein, b,o- and b,b-
carotene, myxoxanthophyll, neoxanthin, Bchl a; Fig. 4).

The algal diversity also showed an increasing (statistically

significant PB0.01, R �0.7) relationship with biomass
(Fig. 5).

The correlation analyses were also conducted using

water and sediment nutrients: TN and TP in the water,

TOC and TN in the sediments (Borghini et al. 2011) as

indicators of the trophic status of the lakes, in view of the

usefulness of a multidisciplinary approach. The correla-

tions of latitude with these parameters were statistically

70

60

50

40

30

Σ
ch

l a
+

B
ch

l a
+

d
e
g
r

20

10

0

71 72 73 74
Latitude

75 76 77 78

Pond E

Fig. 2 Biomass (sum of chl a, Bchl a and their degradation products, in

mg g�1 TOC) versus latitude south.

Fig. 3 Total number of pigments versus latitude south. The circle

highlights lakes with high nutrient concentrations.

Fig. 4 Total number of identified marker pigments (chl a, chl b,

scytonemins, fucoxanthin, violaxanthin, canthaxanthin, zeaxanthin echi-

nenone, lutein, b,o- and b,b-carotene, myxoxanthophyll, neoxanthin,
Bchl a) versus latitude south.

Fig. 5 Species richness (as number of pigments) as function of biomass/

productivity (sum of chl a, Bchl a and their degradation products, in

mg g�1 TOC).

F. Borghini et al. Algal biomass and pigments along a latitudinal gradient in Victoria Land

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significant (PB0.05, R � �0.6) and showed a negative
slope.

Because the two pigment-based indices correlated nega-

tively and significantly (PB0.01) with distance from

the sea and with altitude, in order to better investigate

the relationship with latitude the analysis was repeated

selecting just the coastal lakes*within 1000 m of the sea*
which were at about the same altitude (the lakes further

inland were at a higher altitude). Both diversity and

biomass indices continued to be significantly (PB0.01)

and negatively correlated with latitude, whereas the corre-

lation between the water and sediment nutrients and

latitude was negative but not significant when considering

coastal sites.

PCA identified a subset of environmental variables

that had a significant influence on pigment diversity and

biomass. PCA revealed that the main environmental

gradient (first ordination axis) is associated with sedimen-

tary TN and bird presence. The second PCA axis includes

gradients associated with lake latitude, longitude and

water conductivity (Fig. 6). Together, the first two axes

capture 45.6% of the total variance. In general, even if

no clear cluster is present, it is possible to identify the

following groups: (1) Edmonson Point, Lake 13 (code 39),

characterized by a 50% ice cover and a big penguin colony

around its shores; (2) lakes at 728 latitude (codes 47, 48, 49
and 50), with a partial or total ice cover and with no or few

birds; and (3) Penguin Lagoon (19) and Lagoon (25) at

Inexpressible Island, Lake E (44) at Edmonson Point, Lake

B (38) at Gondwana, and the lake of Depot Island. All

these lakes were without ice cover and with birds.

Redundancy analyses indicated that pigment diversity

and biomass were associated with geography (latitude,

longitude and distance from the sea), sediment chemistry

(TN and TOC) and pH (Fig. 7). The permutation test

performed on all eigenvalues indicated that the above

explanatory variables accounted for a significant (999

permutations, PB0.01) portion of pigment distribution

variations.

Employing the two subsets of environmental and spatial

variables, we used variance partitioning (Borcard et al.

1992) to separate the amount of variance in the pigment

data each group could explain. Spatial and environmental

variables together could account for 67.2% of the varia-

tion in pigment diversity and biomass within our study

lakes. Environmental variation alone accounted for 50.9%

of the variation, pure geographical variation accounted for

Fig. 6 PCA of (a) physical and (b) chemical characteristics of lakes. Individual ion concentrations were omitted from analysis as all are significantly

correlated with conductivity. TNs is sedimentary TN. Supplementary Table S3 provides the identification codes of the lakes.

Fig. 7 Biplot resulting from the redundancy analysis.

Algal biomass and pigments along a latitudinal gradient in Victoria Land F. Borghini et al.

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11.1% and the joint effect of geography and environment

accounted for 38%. Neither the spatial nor the environ-

mental variables considered in the analysis could explain

32.8% of the variation in pigment diversity and biomass.

A classical microscopy-based approach to the diversity

of algal species in the same lakes in Victoria Land was

carried out using the ANTADATA database (Fumanti &

Cavacini 2005). Examining the data in ANTADATA from

the limited number of lakes in the present study that

have also been investigated in previous studies shows a

general increase in species number from Granite Harbour

(11 species, near Kar Plateau) to Inexpressible Island (30

species), Northern Foothills (28 species), Gondwana (35

species) and Edmonson Point (49 species).

Discussion

This study shows that algal biomass and diversity mea-

sured by sedimentary lacustrine pigments decrease with

increasing latitude, though the effect of environmental

variations corresponding to latitude seems to be mediated

by local physical and biological conditions. Multiple

regression models are necessary for a deeper understand-

ing of data variations. For instance, the small Pond E at

Edmonson Point (Fig. 2) was frequented by skuas that

caused eutrophic conditions, explaining this pond’s out-

lier results. Likewise, points within the circle in Fig. 3

(Lake 14 and Lake 15 at Edmonson Point, Gondwana

lakes, Lago Pantano, Lagoon at Inexpressible Island, Pozza

Eneide, Depot Island, and Kar Plateau lakes and Lake F

at Mt. Murray) had more nutrients than those located

at similar latitudes, which may weaken the results from

the classical univariate regression. However, even con-

sidering the high level of data variance, the negative

relationship with latitude seems to be well supported.

This is in accordance with previous studies conducted

along latitudinal gradients in Antarctica. Eukaryotic

organisms recovered in Antarctic soils were influenced

by the environmental conditions at each site (Lawley

et al. 2004), and although bacterial diversity declined with

increasing latitude, habitat-specific patterns appeared to

be important with complex interactions between loca-

tion, vegetation- and weather-related variables (Yergeau,

Bokhurst et al. 2007; Yergeau, Newsham et al. 2007). The

bacterioplankton composition in a latitudinal gradient of

lakes from Argentinean Patagonia to maritime Antarctica

was controlled by a combination of spatial (latitude and

longitude) and environmental (e.g., phosphate) factors

(Schiaffino et al. 2011). Local diversity generally relates to

regional diversity, but local interactions may modify diver-

sity at the community level (Stevens & Willing 2002).

Likewise, in a study recently conducted in the United

States (Stomp et al. 2011), the species diversity of

freshwater phytoplankton sampled from 540 lakes and

reservoirs displayed strong biogeographical variation.

Data analysis indicates that these large-scale biodiversity

gradients were largely mediated by geographic variation in

local environmental factors such as lake productivity (chl a

concentration), lake area, lake depth and water tempera-

ture. Among all of the variables included in this study,

chl a had the largest effect on phytoplankton biodiversity.

Moreover, either a saturating or unimodal productivity

diversity relationship was found, as documented in

many other studies of terrestrial, marine and freshwater

biodiversity (e.g., Dodson et al. 2000).

Lake morphology and conductivity have previously

been shown to strongly influence the microbial mat

structure and composition in the Larsemann Hills region

(Sabbe et al. 2004), Bølingen Islands (Sabbe et al. 2004)

and Rauer Islands (Hodgson et al. 2001). In particular, lake

water depth was the most important variable explaining

pigment composition of the microbial mats, along with

turbidity, presumably on account of their impact on the

light climate. Other variables (such as conductivity and

oxygen) were also significant. The influence of conduc-

tivity on pigment composition may be related to changes

in species composition (Hodgson et al. 2004).

Villaescusa et al. (2010) showed that the trophic

status of the lakes of Byers Peninsula mediates both

the abundance and the composition of bacterioplankton

assemblages. Trophic status, in turn, is regulated both by

external inputs from marine fauna and lake morphometry,

ice and sediment dynamics. In the lakes of Victoria Land

not only does the trophic status differ between lakes of

the same zone but so does the ice cover (Supplementary

Table S2). A study conducted to evaluate the effects on

ecosystem structure of ice loss on several meromictic lakes

along the northern coastline of Ellesmere Island showed

that chl a concentrations and the relative importance of

phytoplankton groups, estimated by photosynthetic mar-

ker pigments and microscopy, differed among the studied

lakes during the open-water period, implying lake-specific

differences in phytoplankton community structure under

ice-free conditions. These differences are likely the result

of the interplay of many factors including seasonal

and interannual variation, catchment and local climatic

characteristics, species colonization, nutrient supply and

trophic interactions (Veillette et al. 2010).

A local community is assembled from a regional pool

of species, the size and structure of which are influenced

by regional processes, including the effects of the geophy-

sical properties of the region, and broad-scale ecological

or evolutionary processes such as species migration,

F. Borghini et al. Algal biomass and pigments along a latitudinal gradient in Victoria Land

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invasions and speciation. Biogeographical variability is

also introduced by differences in historical processes

(Stevens 2006), for example, how long the bare ground

has been exposed to colonization following glacial retreat

or changes in sea level. At the last glacial maximum, most

present ice-free areas were probably almost entirely ice-

covered. Following glacial retreat, algal communities

would have developed from viable propagules deposited

on newly exposed substrata, predominantly through

long-distance dispersal with wind as a fundamental vector

(Muñoz et al. 2004). Favourable movement of air masses

from north to south does occur (Wynn-Williams 1996).

Evidence of these transport propagules is provided, for

example, by the rare occurrence of exotic mosses in cultures

of maritime Antarctic soils. However, the wind direction

in Victoria Land is generally from south/south-east

(Monaghan et al. 2005), with very high speeds recorded

during winter, hindering the dispersal of organisms in a

southward direction across Victoria Land. Microorganisms

may be transported into and around Antarctica via air

(Marshall 1996), ocean currents (Murray et al. 1999)

and animals (Broady & Smith 1994). Others might have

derived from refugia within Antarctica and rock ex-

posures that remained ice-free throughout the Pleistocene

glaciations, for instance, areas of southern Victoria Land

Dry Valleys (Stevens & Hogg 2003). Palaeolimnological

data have demonstrated that some coastal oases of East

Antarctica hosted terrestrial or freshwater faunas for up to

150 000 years (Hodgson et al. 2001).

The timing and extent of resource availability (e.g.,

nutrients and colonizable space) differ across terrestrial

habitats, and these differences influence organisms. Thus,

there are dramatic differences between organisms living

a few tens of metres apart (Peck et al. 2006).

The ice-free areas in Antarctica have different sizes,

are widely dispersed and can be considered islands

surrounded by large areas of uninhabitable environment.

Like islands, the ice-free areas have peculiar character-

istics in terms of isolation and abiotic factors, so differences

in assemblage between the ice-free areas are expected

either in relation to isolation [the distance of separation

could be considered as a surrogate of dispersal rate

(Conrad et al. 1999)] and size of ice-free areas or because

of intrinsic differences in the relevant processes operating

in the different sites. In one study, the size of the nunatak

and oasis influenced the abundance and taxonomic

composition of microfauna in East Antarctica (Sohlenius

& Boström 2005). The findings of Vyverman et al. (2007)

suggest that historical processes constrain global patterns

in regional and local diatom diversity, consistent with the

theory of island biogeography and metacommunity con-

cepts and probably underlying the endemism observed in

the diatom floras in the Southern Hemisphere. The results

of Vyverman et al. showed that there are latitudinal

gradients in local and regional genus richness in the

Southern Hemisphere. Patterns at a regional scale were

explained by the degree of isolation of lake districts: the

number of locally coexisting diatoms was constrained by

the size of the regional diatom pool, habitat availa-

bility and the connectivity between habitats within lake

districts.

As inland water bodies can be considered to be islands

within a sea of land (Reche et al. 2005) and lakes also show

a patchy distribution likely to generate dispersal con-

straints (Barbèran & Casamayor 2010), the strong struc-

ture we observed in the biogeographic patterns of Victoria

Land lacustrine biota may be the norm in Antarctica.

Conclusions

Freshwater biomass and diversity measured by sedimen-

tary lacustrine pigments decrease at increasing latitude.

The outliers in the regression analysis and PCA indicate

that the local environmental conditions could dramati-

cally affect both biomass and diversity. The present

study confirms that lakes are ideal sentinel ecosystems of

environmental conditions. Moreover, since spatial (lati-

tude and longitude) gradients are surrogates for temporal

changes and can link temporal changes to climate, the

study indicates that lakes are early and good indicators

of environmental and climatic changes. The chemical

approach based on pigment determination is relatively

simple and rapid for preliminary studies at large spatial

scales. Further investigations involving also phytoplank-

ton and more detailed taxonomic approaches are neces-

sary to validate pigment indices in the future.

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