25
Journal of Paleolimnology 28: 25–46, 2002.
© 2002
K
l
uwer Academic Publishers. Printed in the Netherlands.
Seasonal ecosystem variability in remote mountain lakes:
implications for detecting climatic signals in sediment
records
J. Catalan
1,
*, M. Ventura
1
, A. Brancelj
2
, I. Granados
3
, H. Thies
4
, U. Nickus
5
, A. Korhola
6
, A. F. Lotter
7
,
A. Barbieri
8
, E. Stuchlík
9
, L. Lien
10
, P. Bitušík
11
, T. Buchaca
1
, L. Camarero
1
, G.H. Goudsmit
12
, J. Kopá‹ek
13
,
G. Lemcke
12
, D.M. Livingstone
12
, B. Müller
12
, M. Rautio
6
, M. Šiško
2
, S. Sorvari
6
, F. Šporka
14
, O. Strunecký
9
&
M. Toro
3
1
Department of Ecology, University of Barcelona, Diagonal 645, E-08028 Barcelona, Spain
(E-mail: catalan@bio.ub.es)
2
National Institute of Biology, Vecna pot 111, SI 1000 Ljubliana, Slovenia
3
Departamento de Ecología, Facultad de Ciencias, Universidad Autónoma de Madrid, E-28049 Cantoblanco,
Madrid, Spain
4
Institut für Zoologie und Limnologie, Leopold Franzens Universität Innsbruck, Technikerstr. 25, A-6020 Innsbruck,
Austria
5
Institute of Meteorology and Geophysics, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria
6
Department of Ecology and Systematics, Division of Hydrology, University of Helsinki, P.O. Box 17, Helsinki,
FIN-00014, Finland
7
Laboratory of Palaeobotany and Palynology, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
8
Dipartimento del Territorio, SPAA Laboratorio di Studi Ambientali, Riva Paradiso 15, 6900 Lugano-Paradiso,
Switzerland
9
Department of Hydrobiology, Charles University Vini
..
..
.
ná 7, CZ 128 44 Prague 2, Czech Republic
10
NIVA, Post Box 173, Kjelsås, 0411 Oslo, Norway
11
Department of General Ecology, Technical University, Masarykova 24, Zvolen, Slovakia
12
Swiss Federal Institute of Environmental Science and Technology, EAWAG, Überlandstrasse 133, CH-8600,
Düdendorf, Switzerland
13
Hydrological Institute, AS CR, Na Sádkách 7, 370 05
.
eské Bud
˜
jovice, Czech Republic
14
Department of Hydrobiology, Institute of Zoology, Slovak Academy of Sciences, Dubravska cesta 9, SK-842 06
Bratislava, Slovakia
*Present address: CEAB-CSIC, Accés Cala St. Francesc 14, 17300 Blanes, Spain
Received 12 February 2000; accepted 9 January 2002
Key words: alpine lakes, thermal regime, chlorophyll, nutrients, major chemicals, oxygen
Abstract
Weather variation and climate fluctuations are the main sources of ecosystem variability in remote mountain lakes.
Here we describe the main patterns of seasonal variability in the ecosystems of nine lakes in Europe, and discuss
the implications for recording climatic features in their sediments. Despite the diversity in latitude and size, the
lakes showed a number of common features. They were ice-covered between 5–9 months, and all but one were
dimictic. This particular lake was long and shallow, and wind action episodically mixed the water column throughout
the ice-free period. All lakes showed characteristic oxygen depletion during the ice-covered-period, which was
greater in the most productive lakes. Two types of lakes were distinguished according to the number of production
peaks during the ice-free season. Lakes with longer summer stratification tended to have two productive periods:
This is the third of 11 papers published in this special issue on the palaeolimnology of remote mountain lakes in Europe resulting from the
MOLAR project funded by the European Union. The guest editor was Richard W. Battarbee.
Mountain Lake Research
MOLAR
26
one at the onset of stratification, and the other during the autumn overturn. Lakes with shorter stratification had a
single peak during the ice-free period. All lakes presented deep chlorophyll maxima during summer stratification,
and subsurface chlorophyll maxima beneath the ice. Phosphorus limitation was common to all lakes, since nitro-
gen compounds were significantly more abundant than the requirements for the primary production observed. The
major chemical components present in the lakes showed a short but extreme dilution during thawing. Certain lake
features may favour the recording of particular climatic fluctuations, for instance: lakes with two distinct produc-
tive periods, climatic fluctuations in spring or autumn (e.g., through chrysophycean cysts); lakes with higher oxy-
gen consumption, climatic factors affecting the duration of the ice-cover (e.g., through low-oxygen tolerant
chironomids); lakes with higher water retention time; changes in atmospheric deposition (e.g., through carbon or
pigment burial); lakes with longer stratification, air temperature changes during summer and autumn (e.g., through
all epilimnetic species).
Introduction
Remote mountain lakes above the treeline in sparsely
vegetated catchments are suitable for studying the
impact of weather and seasonal and long-term changes
in natural ecosystems (Battarbee et al., 2002, this is-
sue). The study of the meteorological forcing in these
lakes is more direct than in lowland lakes, since they
are not as affected by complex soil and vegetation re-
sponses and by human activities in their catchment, all
of which might modify the external loading of carbon,
nutrients, major ions and suspended sediments.
The physical conditions of mountain lakes undergo
major seasonal changes, which affect the chemical and
biological dynamics of the lake. The extent to which
the various components of the lake ecosystem are af-
fected may vary significantly depending on when the
changes occur. The morphological and hydrological
characteristics of the lake determine whether the re-
sponses of the ecosystem are recorded in sediments.
The ecosystem response is based on the differential
growth of distinct components in the food web, which
eventually modify fluxes of matter or, at least, assem-
blage composition. Differences can arise from changes
in the length of the growing season of organisms, in
their intensity of growth within a given period, or both.
Changes in intensity might be related to variations in
resource availability (nutrient loading, food availabil-
ity), temperature, and environmental conditions (e.g.,
pH and oxygen), and may therefore favour the growth
of certain species. Furthermore, some organisms may
be more sensitive because of their habitat within the
lake (e.g., epilimnion vs. hypolimnion, plankton vs.
benthos) or because of their way of living (e.g., au-
totrophic vs. heterotrophic). The signal preserved has
greater information for organisms that produce iden-
tifiable microfossils (e.g., diatoms, chrysophytes, clad-
ocerans, and chironomids). Other sedimentary proxies
only record signals for a whole group (e.g., pigments
for chlorophytes, dinoflagellates, cryptophytes) with
the risk that responses from different species with con-
trasting behaviour may mask the signal. Finally, other
organisms will lack any direct signal, and their fluctua-
tions will only be recorded in a subsidiary way if they
affect the bulk fluxes of organic matter to sediments.
This paper discusses the implications of seasonal
ecosystem variability in remote mountain lakes as re-
gards their capacity to record weather and climatic sig-
nals in sediments, particularly by means of biotic
proxies. We describe the main patterns of seasonal
ecosystem variability in nine remote mountain lakes at
various locations throughout Europe, and discuss the
features they share and the range of variability that can
be expected in their behaviour. Finally, we discuss how
some seasonal features and lake morphology affect the
recording of climatic signals in the sediments of remote
mountain lakes.
Site description and methods
The lakes studied cover a large latitudinal gradient
within Europe (40–69° N) and most major mountain
ranges were included (Figure 1). All lakes were situ-
ated above the treeline in similar alpine environments
of small, steep, and sparsely vegetated catchments. The
more extreme climatic conditions at the latitudes of
Fennoscandia compensated for lower altitudes (Table 1).
The lakes covered typical depth (9.4–73 m) and surface
area (1.7–70 ha) ranges for alpine and subarctic lakes.
However, the Fennoscandian lakes had much larger area/
maximum depth ratios than those at lower latitudes.
A regular survey of a number of key descriptors of
lake dynamics was carried out from July 1996 to Au-
gust 1998. Temperature, oxygen, pH and chlorophyll-a
were selected for the synoptic description of the seasonal
27
Table 1. Lake location and morphology, catchment features and sampling features at each site. Lakes are ordered from the shallowest to the deepest
28
variability of physical, chemical and biological prop-
erties of the lake. Nutrients, major ions and organisms
were also measured in most of the lakes but at lower
spatial and temporal resolutions. These complementary
data were extensively reported in Straškrabová et al.
(1999a). The sampling effort was adapted to the sam-
pling facilities available for each lake, but common
minimum requirements were established as follows:
during the ice-free season the lakes were sampled at
least monthly while during the ice-covered period a
minimum of three times. For lakes shallower than 20 m,
a minimum of five regularly spaced sampling depths
were required, which covered the entire lake depth,
while ten depths were established for deeper lakes.
Temperature and oxygen were instrumentally measured
every metre at the deepest part of the lake. Chlorophyll-
a was extracted using acetone (90%) and evaluated
spectrophotometrically using wavelengths and equa-
tions following Jeffrey and Humphrey (1975). Alter-
natively, for NiÓné Terianske Pleso, chlorophyll-a was
determined fluorometrically after extraction in a mix-
ture of acetone and methanol (Fott et al., 1999). At the
beginning of the survey both methods were applied
simultaneously and no differences in chlorophyll-a
estimation were observed (Stuchlík, personal commu-
nication). Details on sampling, chemical and biologi-
cal analyses and quality control are described in Wathne
and Hansen (1997), The MOLAR Water Chemistry
Group (1999), Straškrabová et al. (1999b).
Results
Thermal and mixing patterns
The lakes showed a distinctive seasonal thermal pat-
tern, with a long ice-covered period, followed by rapid
warming after melting, a short period of high heat con-
tent and a long cooling period until freezing (Figure 2).
The peaks of the apparent heat fluxes depended on the
sampling frequency; long sampling intervals tended
to smooth out the values. However, the sampling fre-
quency was sufficient to show that the deeper the lake,
the larger the heat fluxes and the higher the seasonal
Figure 1. Map indicating the location of the lakes.
29
Figure 2. Seasonal changes in heat content and apparent heat flux in the lakes (inflow-outflow exchanges were not considered) . The thick
solid line indicates the ice-covered periods.
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né Tné T
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erianske Plesoerianske Pleso
erianske Plesoerianske Pleso
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