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Middle and late Holocene climate change and human impact inferred from diatoms,
algae and aquatic macrophyte pollen in sediments from Lake Montcortès (NE Iberian
Peninsula)
Scussolini, P.; Vegas-Vilarrúbia, T.; Rull, V.; Corella, J.P.; Valero-Garcés, B.; Gomà, J.
published in
Journal of Paleolimnology
2011
DOI (link to publisher)
10.1007/s10933-011-9524-y
document version
Publisher's PDF, also known as Version of record
Link to publication in VU Research Portal
citation for published version (APA)
Scussolini, P., Vegas-Vilarrúbia, T., Rull, V., Corella, J. P., Valero-Garcés, B., & Gomà, J. (2011). Middle and
late Holocene climate change and human impact inferred from diatoms, algae and aquatic macrophyte pollen in
sediments from Lake Montcortès (NE Iberian Peninsula). Journal of Paleolimnology, 46(3), 369-385.
https://doi.org/10.1007/s10933-011-9524-y
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ORIGINAL PAPER
Middle and late Holocene climate change and human impact
inferred from diatoms, algae and aquatic macrophyte pollen
in sediments from Lake Montcorte
`
s (NE Iberian Peninsula)
Paolo Scussolini
•
Teresa Vegas-Vilarru
´
bia
•
Valentı
´
Rull
•
Juan Pablo Corella
•
Blas Valero-Garce
´
s
•
Joan Goma
`
Received: 20 July 2010 / Accepted: 18 April 2011 / Published online: 7 May 2011
Ó Springer Science+Business Media B.V. 2011
Abstract During the middle and late Holocene, the
Iberian Peninsula underwent large climatic and
hydrologic changes, but the temporal resolution and
regional distribution of available palaeoenvironmen-
tal records is still insufficient for a comprehensive
assessment of the regional variability. The high
sedimentation rate in karstic, meromictic Montcorte
`
s
Lake (Catalan pre-Pyrenees) allows for a detailed
reconstruction of the regional palaeoecology over the
last 5,340 years using diatom analysis, aquatic pol-
len, sedimentological data, and historic documentary
records. Results show marked fluctuations in diatom
species assemblage composition, mainly between
dominant Cyclotella taxa and small Fragilariales.
We suggest that the conspicuous alternation between
Cyclotella comta and C. cyclopuncta reflects changes
in trophic state, while the succession of centric and
pennate species most likely reflects changes in the
hydrology of the lake. The diatom assemblages were
used to identify six main phases: (1) high productivity
and likely lower lake levels before 2350 BC, (2)
lower lake levels and a strong arid phase between
2350 and 1850 BC, (3) lake level increase between
1850 and 850 BC, (4) relatively high lake level with
fluctuating conditions during the Iberian and Roman
Epochs (650 BC–350 AD), (5) lower lake levels,
unfavourable conditions for diatom preservation,
eutrophication and erosion triggered by increased
human activities in the watershed during the Medi-
eval Climate Anomaly (900–1300 AD), and (6)
relatively higher lake levels during the LIA
(1380–1850 AD) and afterwards. The combined
study of diatoms, algae and pollen provides a detailed
reconstruction of past climate, which refines under-
standing of regional environmental variability and
interactions between climate and socio-economic
conditions in the Pyrenees.
Keywords Diatoms Meromixis Multi-proxy
Palaeoenvironmental reconstruction Pollen
Pyrenees
Introduction
With greater awareness of modern global change,
there is growing interest in predicting how, to what
P. Scussolini T. Vegas-Vilarru
´
bia (&) J. Goma
`
Faculty of Biology, Department of Ecology, Universitat
de Barcelona, Barcelona, Spain
e-mail: tvegas@ub.edu
P. Scussolini
Faculty of Earth and Life Sciences, Department of Marine
Biogeology, Vrije Universiteit, Amsterdam,
The Netherlands
V. Rull
Institut Bota
`
nic de Barcelona (CSIC-ICUB),
Barcelona, Spain
J. P. Corella B. Valero-Garce
´
s
Instituto Pirenaico de Ecologı
´
a (CSIC-IPE),
Zaragoza, Spain
123
J Paleolimnol (2011) 46:369–385
DOI 10.1007/s10933-011-9524-y
extent and at what rate ecosystems will change.
Freshwater primary producers can be very sensitive
to climate change, especially to abrupt fluctuations
that alter their habitats and metabolism. Diatom
response to Holocene climatic variability has been
documented across many latitudes, owing to their
direct (Wolfe 2003) and indirect responses to climate
change (Finkelstein and Gajewski 2007; Morello
´
n
et al. 2009a). Diatom community diversity and
productivity respond quickly to changes in conduc-
tivity, nutrient concentration, and habitat availability
that are associated with variations in the hydrologic
balance (precipitation/evaporation) of lakes. Thus,
diatoms are sensitive recorders of climatic fluctua-
tions. Similarly, changes in littoral macrophyte
communities, reflected in their pollen record, record
variations in both water levels and nutrient status of
lakes (Battarbee et al. 2001).
Mediterranean areas, such as most of the Iberian
Peninsula (IP), display negative annual water balance
and have a long history of human occupation. Most
palaeoenvironmental records from the IP indicate a
transition from drier climate in the middle Holocene
to a more humid late Holocene (Cacho et al. 2010).
Studies of the last three millennia show a variable
global climatic pattern, over secular and decadal
scales (Mann and Jones 2003; Valero-Garce
´
s et al.
2006). Among the phases of climate change, the most
pertinent to the IP system are the Iberian Roman
Humid Period (IRHP) (650 BC–350 AD) (Martı
´
n-
Puertas et al. 2009), the Medieval Climate Anomaly
(MCA) (900–1300 AD) and the Little Ice Age (LIA)
(1300–1850 AD) (Mann et al. 2009). The IRHP
includes the most humid interval of the last
4,000 years in southern Spain (Martı
´
n-Puertas et al.
2008). The MCA was relatively warm and arid
(Morello
´
n et al. 2009a; Martı
´
n-Puertas et al. 2008),
while the LIA brought colder and generally wetter,
although fluctuating climate conditions (Pla and
Catalan 2005; Chueca Cı
´
a et al. 2005). Recent
reviews of climate in the IP (Cacho et al. 2010)
show large variability during recent centuries: (1)
increased temperature and precipitation during the
fourteenth century (Saz 2003), (2) an increase in
drought events in the northeastern IP after 1600 AD
(Vicente-Serrano and Cuadrat 2007), (3) an increase
in extreme events during the LIA (Barriendos and
Martı
´
n-Vide 1998), and (4) humidity increase over
the Mediterranean coast near the end of the sixteenth
and eighteenth centuries and during the second half
of the nineteenth century.
Lake Montcorte
`
s, a karstic meromictic lake in the
Pre-Pyrenean Range of NE Spain has a high sedime-
tation rate and a robust chronological model for the last
6,000 years. It was selected for detailed palaeoenvi-
ronmental study using biological proxies because
previous sedimentological (Corella et al.
2010),
limnological (Camps et al. 1976; Modamio et al.
1988), palynological (Rull et al. 2010) and historical
(Marugan and Oliver 2005; Bosch and Santacana
2009;Co
`
ts 2005) analyses of Lake Montcorte
`
s indi-
cated evidence of distinct environmental changes due
to climate and human impact during middle and late
Holocene. Meromictic lakes are known to be very
sensitive to environmental changes (Hakala 2004). In
this study we used diatoms, other algae remains and
pollen from aquatic higher plants to infer past
environmental changes, and we compared our results
to previous regional reconstructions. Our data show
rapid changes in the limnology and hydrology of the
lake during the middle to late Holocene, coherent with
regional and local climate reconstructions.
Study site
Lake Montcorte
`
s (42°19
0
N, 0°59
0
E, 1,027 m altitude)
is a karstic lake situated on the southern slopes of the
Central Pyrenees, in the Pallars region of Catalonia
(Fig. 1). The bedrock of the lake catchment is mainly
Triassic carbonates, claystones and evaporites (Rosell
1994). The basin has a sub-circular shape with very
steep slopes and a surface area of 17 ha. The diameter
is roughly 500 m and the maximum depth is 29 m. The
lake catchment is vegetated and features transitional
elements between Mediterranean and Sub-Mediterra-
nean forests, meadows, and hygrophile littoral vege-
tation around the lake. Land use today is limited to the
cultivation of cereals and to livestock pastures,
however, the area has a relatively long history of
human occupation, with increasing population from
the Neolithic to medieval times, a maximum in the
nineteenth century, and continuous depopulation in
the industrial and post-industrial periods (Rull et al.
2010). Drainage is mostly via groundwater and
secondary ephemeral streams, which seasonally sup-
ply clastic sediments (Corella et al. 2010). An outlet on
the north shore controls maximum water level.
370 J Paleolimnol (2011) 46:369–385
123
Even though Lake Montcorte
`
s is considered mer-
omictic (Camps et al. 1976), there is evidence that it
was holomictic during the winter of 1978–1979
(Modamio et al. 1988). Measurements in August
2007, carried out by the Ebro Hydrographic Survey,
showed an anoxic hypolimnion, alkaline water (pH =
8.4) and an oligotrophic state (total ammonia
\ 0.05 mg l
-1
, nitrates = 0.788 mg l
-1
, nitrites =
0.011 mg l
-1
, total phosphorus \ 0.005 mg l
-1
).
The phytoplankton community is dominated by
Oscillatoria rubescens and Cyclotella comta (Camps
et al. 1976; Modamio et al. 1988).
Materials and methods
Two Kullenberg cores (MON04-1A-1K and MON04-
4A-1K) were retrieved from the deepest part of the
basin to constitute a composite sediment section
(Fig. 1). A short gravity core was taken (MON07-
1A-1U) to obtain the sediment/water interface. In
addition, two surface sediment samples were taken at
maximum depth and near the littoral zone, for use as
modern representatives from the two areas.
Cores were sub-sampled every 2 cm for Total
Organic Carbon (TOC), Total Inorganic Carbon
(TIC) and Total Nitrogen (TN). TOC and TIC were
measured in a LECO SC144 DR furnace, and TN by
a VARIO MAX CN elemental analyzer. Sedimento-
logical and geochemical characteristics of the com-
posite sediment sequence were described by Corella
et al. (2010). Total phosphorus (TP) in the sediment
was analyzed in core MON04-1A-1K by X-Ray
Fluorescence (XRF) using an ITRAX XRF core
scanner from the Large Lakes Observatory (Univer-
sity of Minnesota, Duluth) with 20 mA current, 30 s
count time and 30 kV voltage at 1 mm resolution. TP
content was expressed as element intensities in counts
per second (cps).
For diatom analysis, 52 subsamples were collected
at 10-cm intervals along the composite sediment
record. Diatoms were extracted from 0.1 g of dry
sediment and prepared using the method described by
Abrantes et al. (2005). They were mounted in
Naphrax Ó and analyzed with a Polyvar light
microscope at 1,0009 magnification. A minimum
of 300 valves was counted for each sample whenever
possible. Relative abundance of diatom species is
given in percentages and valve influx was calculated
Fig. 1 Location of Montcorte
`
s Lake. a Geographical location
within the Iberian Peninsula. b Aerial photograph of the area
surrounding Montcorte
`
s Lake. Red line indicates the lake’s
catchment area. c Bathymetric map of Montcorte
`
s Lake and
locations of the three studied Kullenberg cores
J Paleolimnol (2011) 46:369–385 371
123
using microspheres (Battarbee et al. 2001). Taxo-
nomic identifications were made using several refer-
ences (Krammer 1997; Krammer and Lange-Bertalot
1986–2004; Lange-Bertalot 1980). Centric to pennate
diatom ratio (C/P) was calculated and used as an
indicator of the relative abundance of planktonic to
benthic habitat availability, although it is also known
to reflect changes in trophic status (Cooper 1995).
The decrease in diatom preservation was expressed
by the percentage of diatom valves showing dissolu-
tion and/or breakage (dissolution index). Both indi-
cate the impact of diagenesis as well as the
importance of silicification for diatom preservation
(Flower 1993; Ryves et al. 2001). Sometimes differ-
ential preservation among species might not properly
reflect the diagenetic features of the sediment (Reed
et al. 2010). In our case, the central areas of most
Cyclotella taxa remained intact, so they were easily
identified, and hence did not interfere with counting
accuracy. Fragilariales are commonly well preserved,
while other taxa show variable preservation, however
the most abundant taxa could be identified reliably.
Samples of * 3-5 g of wet sediment were collected
every 10 cm for pollen and algal remains. They were
processed with standard palynological methods,
including NaOH, HCl and HF digestions and density
gradient centrifugation, without acetolysis (Bennett
and Willis 2001). Two Lycopodium tablets (batch #
483,216; 18,583 spores/tablet) were added to each
sample before chemical treatment. Residues were
suspended in glycerine and slides were mounted with
glycerine jelly. For more details, see Rull et al.
(2010).
Diatom diagrams were plotted with Psimpoll and
divided into biozones using the Optimal Splitting by
Information Content Method (Bennett 1996). The
identified diatom zones were applied to the palyno-
logical and geochemical data. Only taxa showing
abundances [3% were illustrated. We related the
diatom assemblages to the lake nutrient status using
TP, TIC, TN and TOC content as indicators of trophic
state. To better elucidate this relationship, we applied a
constrained ordination method. To decide whether to
use a linear or a unimodal method, the gradient length
was measured using a detrended canonical correspon-
dence analysis (DCCA), detrending-by-segments and
non-linear re-scaling of axes. Because the gradient
length of the first DCCA axis yielded a result of 3.18
standard deviation units, a linear model was chosen
(Leps
ˇ
and S
ˇ
milauer 2003). The selected redundancy
analysis (RDA) was performed with CANOCO (ter
Braak and S
ˇ
milauer 2002).
The chronology was developed using 11 radiocar-
bon dates on terrestrial plant remains (Corella et al.
2010). Calibrated BC/AD ages are used for discus-
sion. The composite core sequence (538 cm) spans the
last 5,340 years. Varve counts yielded a more accu-
rate chronology for the last 1,500 years. Sedimenta-
tion rate between 538 cm (3390 BC) and 400 cm (124
AD) was about 0.04 cm year
-1
, resulting in a diatom
sample resolution of 316 ± 80 years. Between
400 cm and the top of the sequence, sedimentation
rate increased to 0.26 cm year
-1
, yielding a diatom
sampling resolution of 36 ± 18 years.
Results
Diatom stratigraphy
According to the changes in diatom assemblage
composition, five diatom zones were differentiated.
Valve influx rates display large fluctuations. Between
356 cm and 124 cm, and in three other samples (538,
43, and 13 cm), diatoms are either too scarce for
accurate counting or are lacking completely (Fig. 2).
The most striking pattern throughout the record is
that Cyclotella comta and Cyclotella cyclopuncta
vary inversely, and only appear together at a single
depth in zone III (Fig. 2). Botryococcus is always
present, while pronounced and isolated peaks of
Tetraedron occur at 310 cm and between 178 and
168 cm (Fig. 5). Cyperaceae pollen dominates the
aquatic pollen throughout the entire record, except for
the uppermost levels, where Typha is dominant or co-
dominant (Fig. 4).
Diatom zone I: 538–483 cm
In this zone, centric diatoms (Cyclotella taxa) dom-
inate and influx varies between 8.02 9 10
4
and
3.97 9 10
6
valves cm
-2
year
-1
(Fig. 2), while the
dissolution index varies between 39 and 85%.
C. comta displays three peaks. Its first minimum
(518 cm) coincides with high abundances of Pseu-
dostaurosira brevistriata, Staurosirella pinnata,
Fragilaria tenera and Eunotia meisteri. After
the third peak of C. comta, it is replaced by
372 J Paleolimnol (2011) 46:369–385
123
