vol. 59(suppl. 1), 2000

CONTENTS
(click titles to see abstract)

There is a general increase in the awareness of political, business and financial institutions for the necessity of concrete action to allow sustainable development in mountain regions. This increased interest in mountain regions is documented by the resolution of the UN General Assembly that declared 2002 the International Year of Mountains.
The world’s mountains clearly do not lend themselves to a simple definition because of their complexity. For each region, specific characterisation is necessary. The Arctic and Alpine regions of Europe often represent the most remote and least disturbed natural environments; yet they are threatened by acid deposition, toxic air pollutants and by climate change. These environments are fragile, being subject to adverse and harsh climatic conditions (high levels of precipitation, low temperatures, aridity, high solar radiation), natural disasters (avalanches, earthquakes, volcanic eruptions), and poor, shallow soils prone to erosion because of steep slopes. However, the fragility of mountain ecosystems means that they are not only vulnerable to environmental change, but that they are also excellent sensors of change. Their sensitivity and the presence of high quality environmental records preserved in lake sediments can be used to infer the speed, direction and biological impact of changing air quality and climate.
As stated in Chapter 13 (Sustainable Mountain Development) of Agenda adopted at the United Nations Conference on Environment and Development (UNCED) in Rio de Janeiro 1992, one of the main topics is the lack of specific knowledge of mountain ecosystems. It is therefore the aim of this special issue of Journal of Limnology to contribute to a global mountain database, a vital resource to support further programmes that contribute to the sustainable development of mountain ecosystems.
The results presented here were obtained during the EU-funded MOLAR programme, specifically the sub-project dealing with "Climatic variability and ecosystem dynamics at remote mountain lakes". The main aims of this workpackage were to: (i) correlate records of weather pattern over the last 200 y between lowland meteorological station and montane sites; (2) establish long-term variability in ecosystem dynamics from recent paleolimnological records and; (3) compare the reconstructed instrumental weather pattern with the variability of the paleolimnological records. The papers that have been included in this special volume have the aim of tracing ecosystem variability at these remote sites. Whilst in a subsequent issue of the Journal of Paleolimnology which is currently in preparation, the results of the comparison of meteorological patterns with the paleolimnolgical result will be discussed.
We wish to acknowledge and thank all colleagues who have collected and analysed samples or contributed in any other way to the success of the programme. MOLAR is funded by the European Commission Framework Programme IV: Environment and Climate with assistance from INCO (ENV4-CT95-0007/IC20-CT96-0021).

A. Lami, N. Cameron and A. Korhola

1

Radiometric dating of sediment records in European mountain lakes
Peter G. APPLEBY

Sediment cores from seven European mountain lakes collected as part of a study of palaeolimnogical records of climate change (the MOLAR project) were dated radiometrically by ²¹⁰Pb. In spite of the remote locations, only one site recorded more or less uniform sediment accumulation throughout the past 150 years. At three further sites the ²¹⁰Pb record indicated uniform sedimentation up until ca 1950 but significant increases since then. Stratigraphic dates based on records of fallout ¹³⁷Cs and ²⁴¹Am showed that ²¹⁰Pb supply rates to these core sites had nonetheless remained relatively constant and that the sediments could be dated by the CRS model. At the remaining sites there were indications of episodic changes in both sedimentation rates and ²¹⁰Pb supply rates. Since the changes were not in proportion, neither of the simple dating models (CRS or CIC) was applicable. Using the ¹³⁷Cs and ²⁴¹Am stratigraphic dates as reference points it was however possible to construct a realistic chronology for these cores by applying the CRS model piecewise to each time-bounded section.

2

High resolution analysis of fossil pigments, carbon, nitrogen and sulphur in the sediment of eight European Alpine lakes: the MOLAR project
Andrea LAMI, Piero GUILIZZONI and Aldo MARCHETTO

A palaeoenvironmental reconstruction for the past 2-3 centuries of eight remote sites from northern to southern Europe was based on a number of palaeolimnological proxies, especially fossil pigments. Most of the lakes studied are located above the timberline and a great effort centred on the creation and analysis of a data-sets of sedimentary records. A chronology for the last century was based on radiometric techniques (²¹⁰Pb, ²⁴¹Am ¹³⁷Cs). The accumulation rate of recent sediment was found to vary from 0.041 cm y^-1 (Lake Saanajärvi, Finland) to 0.14 cm y^-1 (Jezero v Ledvici, Slovenia). During the time-span represented by the cores were the major changes in organic carbon and nitrogen in Nižné Terianske Pleso (Slovakia), Redó (Spain) and Gossenköllesee (Austria). Constant increase of these nutrients from AD 1900 onwards was shown in lakes Saanajärvi, Nižné Terianske Pleso and Hagelseewli (Switzerland). No common trends in sulphur concentrations was evident. There is evidence of an atmospheric input of sulphur in Hagelseewli. This lake shows the highest concentrations, 10 fold higher at surface than the other lakes (ca 6% d.m.). A decrease of S during very recent times is clearly shown by the cores from Redò and Hagelseewli: this might be related to the reduction in the atmospheric loading (the matching of the atmospheric and sedimentary sulphur trends favours this hypothesis). Concentrations of total pigments and HPLC single carotenoids and chlorophylls showed marked fluctuations throughout the cores of all lakes. High pre-AD 1800 pigment concentrations were detected in Nižné Terianske Pleso, Redó, Hagelseewli and Gossenköllesee. During the last ca 50 years an increase in productivity inferred from fossil pigments is shown by Øvre Neådalsvatn (Norway), Nižné Terianske Pleso, Saanajärvi and Jezero v Ledvici. Except Gossenköllesee (Kamenik et al. 2000, this issue). Significant catchment disturbances are absent in these remote environments, so these increases can be considered to be the result of temperature increase or atmospheric nutrient pollution. Carotenoids belonging to sulphur anaerobic photosynthetic bacteria of the green and red groups (Chlorobiaceae and Chromatiaceae) were found in three lakes, i.e. Jezero v Ledvici, Hagelseewli and Gossenköllesee, implying that these lakes experienced seasonal anoxia in their bottom waters with strong stratification.

3

Changes in the trophic level of an Alpine lake, Jezero v Ledvici (NW Slovenia), induced by earthquakes and climate change
Anton BRANCELJ, Milijan ŠIŠKO, Andrea LAMI, Peter APPLEBY, David M. LIVINGSTONE, Irena REJEC-BRANCELJ and Darko OGRIN

Despite relatively high nutrient levels, the Slovenian Alpine lake Jezero v Ledvici (1824 m a.s.l.; max. depth 15 m) is oligotrophic, with high transparency and low chlorophyll concentrations (<1 µg l^-1). Daily mean air temperatures at the lake are estimated to vary between –15.4 °C and +18.8 °C. Low air temperatures combined with the blocking of incident solar radiation by the local topography result in the lake being ice-covered for over six months of the year, suggesting that the effects of climate on the ecology of the lake may be mediated by the timing of the ice cover. Sediment cores taken in 1996 were dated by ²¹⁰Pb and ¹³⁷Cs and analysed for DW, LOI, cladoceran and diatom remains, pigments, C, N and S. A total of 50 diatom taxa and 4 cladoceran taxa were found. The base of the unsupported ²¹⁰Pb record at 17.4 cm was dated to 1825 AD ± 25 y. An abrupt change in DW and LOI was observed at a depth of 17 cm, coinciding with a change in the cladoceran community and in C and N concentrations. From a depth of 12 cm (83 ± 4 y BP) upward, a sharp change in S and plant pigment concentrations were recorded, followed by an increase in diatom abundance at a depth of about 10 cm (63 ± 4 y BP). The timing of quantitative changes in the physical and chemical properties of the sediment, and in the community structure of diatoms and Cladocera, coincides with the occurrence of three earthquakes in the 19^th century. Changes in the diatom and cladoceran record were compared with a tree-ring width index based on Larix decidua from the shore of the lake over the last 136 years, and with reconstructed air temperatures from 1781 – 1996. With a 4-y lag, a weak positive correlation exists between the tree-ring width index and the air temperatures. Although diatom abundance generally shows no correlation with the tree-ring width index, a negative correlation was found in the case of Fragilaria pinnata and Amphora lybica, interrupted only between 1942 and 1955. A similar pattern to this latter was also observed in the case of the Cladocera. During the last 250 y, anthropogenic influence on the lake catchment area has been confined mainly to limited sheep grazing. The main cause of changes occurring in the lake itself is likely to be eutrophication resulting from the input of allochthonous material from landslides triggered by earthquakes. During the last three decades, however, the main factor influencing biotic change appears to have been the increase in mean air temperature, possibly acting via ice cover.

4

Eight hundred years of environmental changes in a high Alpine lake (Gossenköllesee, Tyrol) inferred from sediment records
Christian KAMENIK, Karin A. KOINIG, Roland SCHMIDT, Peter G. APPLEBY, John A. DEARING, Andrea LAMI, Roy THOMPSON, Roland PSENNER

Documentary and sediment records (diatoms, chrysophyte stomatocysts, plant pigments, carbon and nitrogen, metals and mineral magnetics) were used to reconstruct environmental changes in the high alpine lake Gossenköllesee (Tyrol, Austria) during the last 800 years. The records revealed complex interactions between human impact and climate. Gossenköllesee was predominantly influenced by land-use, which supplied nutrients to the lake. Documentary records report intensive sheep and cattle farming in the area around Gossenköllesee during medieval times. Pigments and chrysophyte stomatocysts indicated high nutrient concentrations prior to ca 1770 AD. First changes in land-use, however, were already detected ca 1670 AD. In 1675 AD the "Schwaighof" near Gossenköllesee, a perennial high altitude settlement, was sold to the Earl of Spaur, and farm management probably changed. After approx. 1770 AD in-lake production was reduced, indicating a decrease in land-use. According to historical records, the perennial settlement near Gossenköllesee was abandoned by at least 1890 AD. Gossenköllesee was also affected by fish stocking. Arctic charr (Salmo trutta morpha fario L.) was introduced into the lake, most probably at the end of the 15^th century. A decline in carbon, nitrogen and the pigments alloxanthin (cryptophytes) and astaxanthin (grazers) indicate a significant removal of grazers by fish. Superimposed on human activity, climate changes have also had a significant impact on Gossenköllesee. High productivity during the 12^th century suggested by the plant pigment records might have been favoured by temperature increases, indicated by pronounced glacier retreats which began during the 10^th/11^th century. The "Schwaighof" near Gossenköllesee was sold to the Earl of Spaur when winter temperatures declined substantially in the 1670s. Changes in C/N ratio, iron, manganese and mineral magnetics indicated increased detrital input from the catchment, starting approx. 1670 AD. Erosion and detrital input into the lake intensified during cold periods (1688 – 1701 AD and 1820 – 1850 AD), as indicated by a high C/N ratio, metals and mineral magnetics.

5

Sedimentological and biostratigraphical analyses of short sediment cores from Hagelseewli (2339 m a.s.l.) in the Swiss Alps
André F. LOTTER, Wolfgang HOFMANN, Christian KAMENIK, Andrea LAMI, Christian OHLENDORF, Michael STURM, Willem O. VAN DER KNAAP and Jacqueline F.N. VAN LEEUWEN

Several short sediment cores of between 35 and 40 cm from Hagelseewli, a small, remote lake in the Swiss Alps at an elevation of 2339 m a.s.l. were correlated according to their organic matter content. The sediments are characterized by organic silts and show in their uppermost part a surprisingly high amount of organic matter (30-35%). Synchronous changes, occurring in pollen from snow-bed vegetation, the alga Pediastrum, chironomids, and grain-size composition, point to a climatic change interpreted as cooler or shorter summers that led to prolonged ice-cover on the lake. According to palynological results the sediments date back to at least the early 15^th century A.D., with the cooling phase encompassing the period between late 16^th and the mid-19^th century thus coinciding with the Little Ice Age. Low concentrations of both chironomid head capsules and cladoceran remains in combination with results from fossil pigment analyses point to longer periods of bottom-water anoxia as a result of long-lasting ice-cover that prevented mixing of the water column. According to our results aquatic biota in Hagelseewli are mainly indirectly influenced by climate change. The duration of ice-cover on the lake controls the mixing of the water column as well as light-availability for phytoplankton blooms.

6

Causes and effects of long periods of ice cover on a remote high Alpine lake
Christian OHLENDORF, Christian BIGLER, Gerrit-Hein GOUDSMIT, Gerry LEMCKE, David M. LIVINGSTONE, André F. LOTTER, Beat MÜLLER and Michael STURM

The response of the physical and chemical limnology of Hagelseewli (2339 m a.s.l.) to local meteorological forcing was investigated from 1996 to 1998 using an automatic weather station, thermistor chains, water samples and sediment traps. On-site meteorological measurements revealed the paramount importance of local topographic shading for the limnology of the lake. A high cliff to the south diminishes incident radiation by 15% to 90%, resulting in a long period of ice cover. Hence, the spring and summer seasons are extremely condensed, allowing only about 2 months per year for mixing, oxygen uptake, nutrient inflow, water exchange and phytoplankton growth. Regular measurements of water temperature, chemistry and diatom composition show that Hagelseewli responds very rapidly to changes in nutrient concentrations and light conditions. This response is restricted mainly to an extremely short productivity pulse, which takes place as soon as the lake is completely free of ice. Ice-free conditions are indicated by the occurrence of planktonic diatoms. In contrast to most low-altitude lakes, maximum productivity occurs in the middle of the water column (6-9 m), where first light, and then soluble reactive phosphorus (SRP), are the limiting factors. During the period of thawing, large amounts of ammonium enter the lake. Nevertheless, allochthonous nutrient input is not important because SRP, the limiting nutrient for algal growth, originates from the sediments. Water chemistry data and data from sediment traps show that, although autochthonous calcite precipitation does occur, the calcite crystals are redissolved completely in the bottom waters during the extended period of ice cover. Thus, the most important factor for changes in the nutrient budget, primary production and preservation of calcite is the bottom water oxygen status, which is governed by the occurrence of an ice-free period. We hypothesise that the duration of the ice-free period is of minor importance for the generation of particles that might be archived in the sedimentary record as proxy climate indicators. Such particles are produced mainly during times of peak primary production, which last only for a few days before production decreases again to very low levels. Therefore, with respect to the type of climatic signal that might be recorded in Hagelseewli, we presume that what is most likely to be archived in the sedimentary record is the mere occurrence, rather than the duration of the ice-free period.

7

Diatom and crustacean zooplankton communities, their seasonal variability and representation in the sediments of subarctic Lake Saanajärvi
Milla RAUTIO, Sanna SORVARI and Atte KORHOLA

Present and past diatom and crustacean zooplankton communities of subarctic Lake Saanajärvi in Finnish Lapland were studied with special emphasis on their representation in the sediment surface. Two years monitoring of the present state of the lake revealed it to be ultraoligotrophic with a biomass peak during autumn overturn. Chrysophytes and diatoms were the most abundant phytoplankton groups, with four species forming the main bulk of the crustacean zooplankton community. The comparison of the present plankton community with the species composition in the sediment surface showed that only some plankton groups are suitable for paleolimnological studies. The value of diatoms for paleolimnological studies was supported by the observation that all of the species recorded from various habitats in the lake were detectable in the sediment. In addition, some species (e.g. Cyclotella glomerata) that limnological sampling had not detected were found in the sediment record. Similarly, the zooplankton class Chydoridae and Daphnia hyalina species were equally or better represented in the sediment than in the water column. On the other hand, the sediment trap and core evidence of the whole crustacean zooplankton community suggested many zooplankton species have little or no significance in paleolimnological studies. For example the two dominant copepod species, Eudiaptomus graciloides and Cyclops abyssorum, were not detectable in sediment trap samples. These findings indicate that only a small fraction of the living biota are preserved in the sediments, and this should be taken into account when drawing conclusions based on the detection of just one such species group.

8

The main features of seasonal variability in the external forcing and dynamics of a deep mountain lake (Redó, Pyrenees)
Marc VENTURA, Lluis CAMARERO, Teresa BUCHACA, Frederic BARTUMEUS, David M. LIVINGSTONE and Jordi CATALAN

Lake Redó, a dimictic oligotrophic mountain lake, was monitored for two complete years from July 1996 to July 1998. The main seasonal variations in the physical, chemical and biological parameters are described, with special emphasis on the comparison of external forcing (weather and atmospheric deposition) with internal lake dynamics. Annual mean air temperature was estimated to be 3.6 °C. The duration of ice cover on the lake was 4.5 months in 1996/97 and 5.8 months in 1997/98. The lake water was very ion-poor (mean annual conductivity 12 µS cm^-1); however, ion concentrations in the lake were higher than in the precipitation, the differences being due mainly to Ca²⁺ and bicarbonates originating in the catchment. NH₄⁺ was the main ion in the precipitation, with an average concentration of 17 µM, while in the lake it was always below 3 µM. However, the concentration of dissolved inorganic nitrogen always exceeded that of soluble reactive phosphorus by two or three orders of magnitude, so the latter is likely to be the limiting nutrient for phytoplankton growth. Four main production episodes were identified, occurring during spring and autumn overturn, in the upper hypolimnion during summer stratification, and under the ice at the beginning of the ice-covered period. The highest chlorophyll-a concentrations (1.2-2.2 µg l^-1) were attained during spring overturn; concentrations of chlorophyll-c were high during both spring and autumn overturn, while chlorophyll-b was comparatively important in the upper hypolimnion during the stratification period. Daphnia pulicaria was the most abundant macrozooplankton species; its abundance was highest during the ice-covered period, when its biomass was comparable to the measured sestonic particulate carbon concentration. The Daphnia maximum was associated with higher concentrations of NH₄⁺ and dissolved organic carbon, suggesting that it may play an important role in the pelagic biogeochemical compartment of the lake under ice. Winter respiration rates for the lake were estimated to be 339 mg O₂ m^-2 d^-1 for 1996/97 and 281 mg O₂ m^-2 d^-1 for 1997/98.

9

Recent warming in a high mountain lake (Laguna Cimera, Central Spain) inferred by means of fossil chironomids
Ignacio GRANADOS and Manuel TORO

Lake Cimera (Lat. 40°15'50'' N; Long. 05°18'15" W, Z_max = 9.4m, A = 4.5 ha, 2140 m a.s.l,) is one of the most remote and unpolluted high mountain lakes on the Sierra de Gredos (Central Spain). Intrannual and interannual variability in maximum water temperature and winter oxygen depletion can be related to climate variability (mainly air temperature), through controlling ice cover length. The extent of the oxygen depletion during ice cover period, which is related to this ice cover length, is a key factor controlling the relative abundance of chironomid (Diptera: Insecta) taxa, especially the low oxygen content adapted Chironomus sp. In this way, we have found a high negative correlation between the relative abundance of Chironomus head capsules in the sediment and the reconstructed air temperature in the last 200 years (n = 20, r = -0.75, p <0.001). The interpretation of such relationship throughout the fossil chironomid assemblage points to a recent warming (since ca mid 1980s) in Lake Cimera. The ecological interpretation of other taxa also supports this view. When applying to fossil chironomids of Lake Cimera the transfer functions developed to reconstruct summer past temperatures in the Alps, it is also well correlated with reconstructed air temperatures (n = 20, r = 0.45, p <0.01), especially when only the most accurate dating levels (top of the core, ca 75 years) are taken into account (n = 13, r = 0.75, p <0.01). However, 1) the linear regressions of both models show significantly different slopes, and 2) chironomid reconstruction underestimates in ca. 3 ºC air reconstruction. The later is probably because the fossil chironomid model has been developed for a different geographical region. Nevertheless, both models provide an independent line of evidence of a recent warming (since ca mid 1980s) in Lake Cimera. Our data also supports the use of chironomids head capsules as an effective tool to infer past temperatures.