Similarities and discrepancies between chironomid- and diatom-inferred temperature reconstructions through the Holocene at Lake 850, northern Sweden

ARTICLE IN PRESS

Quaternary International 122 (2004) 109–121

Similarities and discrepancies between chironomid- and diatom-inferred temperature reconstructions through the Holocene at Lake 850, northern Sweden I. Larocquea,b,*, C. Biglera,c a

Climate Impacts Research Centre, Box 62, SE-981 07 Abisko, Sweden b PAGES, Baerenplatz 2, CH 3011 Bern, Switzerland c NCCR Climate, Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland

Abstract A quantitative temperature reconstruction using chironomids and diatoms has been attempted from a high elevation lake in northern Sweden (Lake 850). Since 7000 cal. years BP, both chironomids and diatoms recorded similar temperatures (in the range of present-day estimates) but the correspondence between chironomid and diatom-inferred temperatures was highest in the recent Holocene (2500 cal. years BP to the present). Between ca. 9000 and 7000 cal. years BP, inferred temperatures from chironomids were warmer than today (ca. 1–2
C), in accord with other climate reconstruction using pollen, plant macrofossils and oxygen isotope analysis in lakes of northern Scandinavia. In contrast, diatom analysis did not infer warmer temperatures during this period. The insensitivity of diatoms to temperature in Lake 850 between 9000 and 7000 cal. years BP could be attributed to other environmental factors affecting the diatom assemblages through time, especially lake-water pH. Diatom-inferred pH showed a gradual decrease (0.5 pH units) between 9000 and 7000 cal. years BP while it remained more or less constant since 7000 cal. years BP. Changes in lakewater pH acting on diatoms seem to mask the effect of climate, leading to temperature reconstructions that are inaccurate. Ways of disentangling climate and other environmental factors when attempting climate reconstruction should be further investigated. r 2004 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Chironomids (non-biting midges) are dependent on temperature for pupation, emergence, growth, feeding and hatching of the larva, and they appear to be sensitive palaeolimnological indicators of past climate changes (Walker and Mathewes, 1989; Smol et al., 1991). They have been described as one of the most promising quantitative biological indicators of both air and water temperature (Battarbee, 2000). Some taxa are often identified as specific cold- or warm-water taxa, being found only in particular habitats (Cranston et al., 1983). In order to reconstruct quantitatively past temperature patterns, many training sets have been developed in Europe and North America, identifying temperature as one of the most important factor influencing the distribution of chironomid taxa (Walker et al., 1991; Lotter et al., 1997; Olander et al., 1999). The temperature reconstructions for the Late-glacial period have been *Corresponding author. INRS-ETE, Universit!e du Qu!ebec, Carrefour Molson, 2800 rue Einstein, Qu!ebec, Canada G1V 4C7.

proven accurate (Brooks and Birks, 2000, 2001) and chironomid analysis could replace coleopteran analysis for temperature reconstruction (Levesque et al., 1993). However, the accuracy of temperature reconstruction using chironomids still needs to be tested for the Holocene period (Battarbee, 2000) because the amplitude of temperature changes during this period is smaller and might not be recorded by biological indicators due to errors of predictions generally between 1.1
C and 1.9
C (Larocque et al., 2001; Olander et al., 1999). A comparison of chironomid-inferred temperatures with instrumental data showed that inferences were accurate estimates of measured mean July air temperature (Larocque and Hall, 2003) but this result does not imply that the good relationship between estimates and measured air temperature remains through the Holocene. Diatoms can also be used for quantitative temperature estimates (Pienitz et al., 1995; Lotter et al., 1997) although their power of adequately reconstructing temperature has been questioned (Anderson, 2000). As for chironomids, diatom-based temperature training sets . et al., are numerous (e.g. Lotter et al., 1997; Weckstrom

1040-6182/$ - see front matter r 2004 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2004.01.033

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1997; Rose! n et al., 2000; Bigler and Hall, 2002) but, up to date, few temperature reconstructions have been made using diatoms (Korhola et al., 2000a; Rose! n et al., 2001; Bigler et al., 2002, 2003). A comparison of diatominferred temperatures with instrumental data proved that estimates were accurate, particularly when other environmental factors (such as pH) remained stable (Bigler and Hall, 2003). Only by using multi-proxy analysis can we determine the importance of various environmental factors on the biological indicators (Huntley et al., 1997; Ammann and Oldfield, 2000). As a consequence, multi-proxy reconstructions are becoming common (Guilizzoni and Oldfield, 1996; Barber et al., 1999; Birks et al., 1999; Rose! n et al., 2001; Bigler et al., 2002, 2003) and comparisons between the responses of different biological organisms to external forcing can be obtained (Birks et al., 1999). These comparisons are useful for discriminating between proxies that represent climate from those that represent lake responses (Battarbee, 2000). The sensitivity to external forcing (especially climate) of high elevation lakes has been well established (Beniston et al., 1997; Battarbee et al., 2002) and global warming is mainly amplified at high latitudes where the temperature over the last 30 years has increased more than elsewhere (Chapman and Walsh, 1993; Serreze et al., 2000). Many of the high-elevation lakes in the northern Swedish training sets have chironomid and diatom assemblages that differ from the lower altitude lakes (Larocque et al., 2001; Bigler and Hall, 2002). These communities are adapted to extreme conditions and can be expected to vary greatly with a sharp change in climate. Air and water temperature are highly correlated (Livingstone and Lotter, 1998; Livingstone et al., 1999) and they can both affect aquatic organisms. With multivariate quantitative models, changes in past climate can be reconstructed using these organisms. In northern Sweden, previous palaeoclimatic reconstructions have been based on records of pollen and plant macrofossils (Berglund et al., 1996; Barnekow, 1999), tree rings (Briffa et al., 1992), glacier movements (Karle! n, 1988; Karle! n and Kuylenstierna, 1996), and plant megafossils (Kullman, 1999). Results obtained from these studies are partly contradictory. Pollen can provide an accurate reconstruction of past vegetation changes but longdistance transport can contribute to longer time lags between temperature change and the observed vegetation response (Birks, 1981; Davis and Botkin, 1985). Tree rings can be used to infer short-term temperature changes but are limited to time scales rarely longer than 2000 years (Cook et al., 1995). Reconstructions from glaciers are compromised in northern Sweden due to problems in dating past glacier positions (Matthews, 1997). As a consequence, climate reconstruction using aquatic organisms, which are assumed to be in equilibrium with climate, might solve some of these problems.

Multi-proxy analyses using chironomids and diatoms have been attempted near Abisko in northern Sweden (Bigler et al., 2002, 2003) and both proxies have recorded a decrease of temperature through time in the range of 1.5–2.4
C. Similar results were obtained in an area to the south of the Abisko valley (Rose! n et al., 2001). Interpretation from a tree-megafossil study (Kullman, 1999) led to an estimated decrease of temperature higher than reconstructed by aquatic organisms, but this estimation was based on very few samples. Shemesh et al. (2001), using oxygen isotopes extracted from diatoms, also showed that the decrease of temperature was in the order of 4
C. The variations in the magnitude of the temperature decrease reconstructed by various proxies might be due to the influence of other environmental factors affecting the variation of aquatic organisms through time. If one factor other than temperature becomes more important in influencing the aquatic organisms, the reconstructed temperature might be affected. Here, an attempt is made to reconstruct Holocene temperature changes at one site and better understand the relationship between climate and the response of aquatic organisms. Other environmental factors affecting aquatic organisms will also be addressed.

2. Site description The study site (Lake 850, 68
180 N; 19
070 E) is located 13 km from Abisko, in northern Sweden (Fig. 1). The lake is part of the Tornetr.ask area in the Scandinavian mountain range. Peaks, some above 1000 m a.s.l., surrounding the area. The major activities in this area are mining in Kiruna and tourism. The studied lake and its catchment have, however, remained mainly untouched by human activities. In the Tornetr.ask area, mountain birch (Betula pubescens spp. tortuosa) forms the altitudinal tree-line at 600–700 m a.s.l. Continuous distribution of Scots pine (Pinus sylvestris) is found in the eastern part of the Tornetr.ask area. Pine occurs sporadically below 450 m a.s.l. at the most favourable sites (dry soils with thin snow cover in winter). Deciduous trees such as aspen (Populus tremula), grey alder (Alnus incana) and willows (Salix spp.) can be found. The understory is typically dominated by shrubs and dwarf shrubs (Empetrum hermaphroditum, Vaccinium vitis-idaea, Vaccinium myrtillus, Betula nana and Juniperus communis). The study site is situated at an elevation of 850 m a.s.l. and the local vegetation is dominated by grass heath, with some dwarf birch (B. nana), lichens and mosses. The lake is located 200 m above the mountain birch treeline, and 400 m above the pine tree-line. The lake is fed by surface runoff water, primarily during the snow melting season. A summary of all measured parameters is given in Table 1.

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Fig. 1. Map of the study area. Lake 850 is indicated, with two other lakes where reconstructions have also been made using chironomids and diatoms. (Figure by K. Aune) Table 1 Lake characteristics of Lake 850 Elevation (m a.s.l.) Mean July air temperature (
C) Mean January air temperature (
C) Maximum lake depth (m) Secchi depth (m) Water temperature (
C) Loss on ignition surface sediment (LOI%) Lake water pH (pH units) Conductivity (mS/cm) Ca (meq/L) Mg (meq/L) Na (meq/L) K (meq/L) SO4 (meq/L) Cl (meq/L) TOC (mg/L) DOC (mg/L) Si (mg/L)

850 9.1 15.3 8.2 7.0 8.2 9.3 6.8 12.5 0.047 0.024 0.026 0.006 0.017 0.017 2.4 2.3 0.920

Mean air temperatures are 30-year means of temperature records (1961–1990) interpolated from 18 nearby climate stations and applying a regional lapse rate of 0.57
C per 100 m elevation (Laaksonen, 1976). All other parameters were measured once at the time of sampling (August 20th 1998).

The amount of precipitation at Lake 850 is relatively low, as indicated by the closest climate station at the Abisko Scientific Research Station with a mean annual precipitation of 300 mm (Alexandersson et al., 1991). However, local differences in precipitation are high in the Tornetr.ask area, due to orographic effects and changing influences of Atlantic and Arctic air

masses. Ice covers the lakes generally from October to June. Deglaciation of this site occurred at around 10,000 cal. years BP but stagnant ice may have remained locally until 9100 cal. years BP (Lundqvist, 1998; Barnekow, 1999). Pollen and plant macrofossil records in lakes located at 625 and 999 m a.s.l. in the Abisko area suggested that after deglaciation, the vegetation consisted of a subalpine birch woodland tundra (Betula pubescens, Salix polaris, Salix herbacea and other dwarf shrubs). While pollen and plant macrofossils suggested that a subalpine birch tundra was present at the highest elevation site between 6500 and 4500 cal. years BP (Barnekow, 1999), a megafossil analysis suggested that pine, birch and alder were present at the site at that time (Kullman, 1999). These different studies result in contradictory climate reconstruction for that time. At 4500 cal. years BP, the vegetation in the catchment of the lower site reverted to the subalpine birch woodland tundra while vegetation at the high elevation site was replaced by alpine tundra (Barnekow, 1999).

3. Methods Sediment cores were taken at the deepest part of the lake using a 5 cm-diameter modified Livingstone piston corer. The longest core, measuring 125 cm, was chosen for analyses. The topmost 2.5 cm were relatively loose gyttja including a moss layer on the top. Between 2.5

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and 119.5 cm, the sediments were composed of relatively solid gyttja with a layer of loose gyttja between 48 and 50 cm. Clay was present between 119.5 and 120.5 cm and the lowest 5.5 cm were fine sand and coarse silt. The core was sliced at every centimetre in the laboratory. Each sample was placed in a plastic bag and kept frozen until used for the analyses. Estimates of organic matter content as the percent weight loss-on-ignition (LOI) were obtained after heating dried sediment at 550
C for 1 h (Dean, 1974). 3.1. Dating Each centimetre of the core was sieved for plant macrofossils to obtain material for 14C dating. Only seeds and leaves from terrestrial plants were used for dating and all woody parts were discarded. Five levels provided sufficient material for AMS dating and these ( . laboratory at samples were submitted to the Angstr om the Uppsala University. The 14C AMS dates were calibrated in years before present (cal. years BP) using the program OxCal version 3.5 (Ramsey, 2000) and a linear regression including all dates was used for calculating the depth–age relationship. 3.2. Chironomid analysis Every centimetre of sediment was used for chironomid analysis. Approximately three cubic centimetres were used for each sample and this volume was measured by water displacement. The subsamples were deflocculated using a KOH 10% solution (overnight). Samples were then sieved using a 90 mm mesh. Head capsules retained in the mesh were picked under a stereomicroscope. The solution was placed in a counting petri dish separated into four chambers. According to the number of head capsules found in one of the chambers, a subsample or the whole sample was picked. Head capsules were mounted on a slide with Euparals. At least 50 chironomids were mounted but 150 head capsules was the aim in most samples. This number of head capsules has been shown to be representative of the whole assemblage found in the three cubic centimetres (Larocque, 2001), although as few as 50 head capsules should provide accurate temperature estimates (Heiri and Lotter, 2001; Larocque, 2001; Quinlan and Smol, 2001). Identification of chironomids was made at 400X and 1000X and was mainly based on Wiederholm (1983). Fifty-six taxa were identified to the genus or species level. Specialised keys were used to identify the Tanytarsini tribe (Brooks et al., 1997; Brooks, unpublished). Corynocera oliveri and Tanytarsus lugens were separated only when mandibles were present, otherwise they were identified as Tanytarsus sp. If head capsules were split, two halves were taken to represent one unit.

3.3. Diatom analysis Diatoms were prepared using standard methods, applying a technique for large sets of samples (Battarbee, 1986; Renberg, 1990). The samples (between 50 and 100 mg wet weight per sample) were treated for 7 h at ca and 70
C with 5 ml H2O2 (30%) and a few drops of HCl (10%). The diatoms were permanently fixed on slides using Naphraxs. At least 400 valves were counted on each slide using a phase contrast lens at 1000x magnification. Diatom taxonomy followed largely Krammer and Lange-Bertalot (1986–1991) and the guideline for the SWAP project. For Aulacoseira species, the key of Camburn and Kingston (1986) was consulted. 3.4. Numerical analysis For reconstruction of pH and temperature, previously published regional training sets were used. Lake depth, mean July air temperature, LOI at 550
C (% organic matter) and mean January air temperature were the four factors explaining 18.4% of the distribution of chironomids in the modern training set using canonical correspondence analysis (CCA). A prediction model for mean July air temperature was developed using WAPLS (ter Braak and Juggins, 1993). The coefficient of determination ðr2 Þ based on leave-one-out cross-validation was 0.65, root mean squared error of prediction (RMSEP) was 1.12
C and the maximum bias was 1.96
C (Larocque et al., 2001). For diatoms, the environmental variables pH, LOI and mean July air temperature were the most powerful variables explaining the species distribution. The three factors explained 16.3% of the distribution of diatoms in the modern training set. A model for mean July air temperature was developed also using WA-PLS, the r2 (based on leaveone-out cross-validation) was 0.75, RMSEP was 0.96
C and maximum bias was 1.38
C (Bigler and Hall, 2002). The model for pH yielded summary statistics of r2 (based on leave-one-out cross-validation)=0.77, RMSEP=0.19 and maximum bias of 0.31 using 99 calibration lakes. Of the 56 chironomid taxa identified in the Holocene sediment core from Lake 850, 48 were present in the previously developed chironomid training set (Larocque et al., 2001) and thus used to reconstruct temperature. The taxa that were not present in the training set were of very low abundance in Lake 850 and appeared only sporadically. Their exclusion from the analysis should have a negligible effect on the temperature reconstruction. The fossil assemblages in Lake 850 were composed of more than 90% of taxa found in the training set. For diatoms, 140 species were recorded in the core of Lake 850, 112 of these species were present in the training set. Between 88.3% and 100% of the fossil assemblages were represented in the modern training set.

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The CALIBRATE program (version 0.82, ter Braak and Juggins, 1993) was used for the temperature reconstruction with diatoms and chironomids and the pH reconstruction using diatoms. Sample-specific error estimates (data not shown) were calculated using the computer program WA-PLS version 1.0 (Juggins, unpublished). A detrended CCA (DCCA) analysis was used to assess the influence of environmental factors that may regulate patterns of chironomid and diatom community change over Holocene timescales. Chironomid and diatom samples from the Holocene cores were plotted as passive samples within the DCCA ordination space defined by surface sediment assemblages from the 100lake calibration set. The resulting trajectories, smoothed with a five-point running mean, illustrate long-term patterns of Holocene development of aquatic assemblages in the context of modern taxa–environment relationships. All ordinations were performed using CANOCO version 4 (ter Braak and Smilauer, 1998) with square root transformed species data and rare species down-weighted. The zones in the chironomid and diatom diagrams were identified by optimal partitioning using sum of square criteria (Birks and Gordon, 1985). The number of significant zones was determined with reference to the broken stick model (Bennett, 1996).

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inferences was weak (Fig. 3). Although estimates lay within the sample-specific errors, the pattern was mainly opposite, and values were far apart. During this period, chironomid analysis inferred the highest mean July air temperatures recorded throughout the entire Holocene with values of 1–2
C above present-day temperatures. In contrast, diatom-inferred temperatures are similar to present-day values (Fig. 3). From ca. 7000–2500 cal. years BP (92–33 cm sediment depth) the reconstructed temperatures overlapped within the sample-specific errors, indicating that diatoms and chironomids provided similar quantitative reconstructions, although the patterns were sometimes opposite. Since ca. 2500 cal. years BP (33 cm), the chironomid and diatom-based temperature estimates were very close and similar patterns were observed (Fig. 4). In the surface sample, both chironomid and diatom inferences slightly overestimated the present-day mean July air temperature at Lake 850 (9.1
C): chironomids by 0.3
C and diatoms by 0.6
C, respectively (Fig. 3). However, the differences between estimated values based on the palaeoindicators and measured values based on the records of the nearest Climate Stations

4. Results 4.1. Dating Table 2 shows the AMS dates obtained from terrestrial macrofossils. Sedimentation rates determined from the age–depth model were on the order of 0.13 mm/year (Fig. 2). Shemesh et al. (2001) obtained a similar sedimentation rate based on a depth–age model using both dates from bulk sediment and aquatic mosses. 4.2. Temperature and pH reconstructions During the early Holocene (ca. 9500–7000 cal. years BP), the agreement between chironomid and diatom-based

Fig. 2. AMS radiocarbon dates of plant macrofossils from Lake 850 and the depth–age relationship of the investigated sediment core.

Table 2 AMS dates Sample depth (cm)

Laboratory no.

d13C % PDB

14

32–33 49–50 72–73 96–97 106–107

Ua-15154 Ua-15155 Ua-15156 Ua-15157 Ua-16005

27.2 28.8 28.9 29.7 30.5

2590 3605 4685 7000 6960

C years BP 7 7 7 7 7

70 70 70 85 115

Cal. years BP (95.4% probability) 2850 4100 5600 7970 7780

    

2360 3690 5290 7660 7580

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Fig. 3. Temperature reconstructions using chironomids and diatoms, and pH reconstruction using diatoms. The lines are 5-point running means.

Fig. 4. Comparison of temperature reconstructions using chironomids and diatoms over the last 3000 cal. years.

(1961–1990) using spatial interpolation were within the prediction ability of the transfer functions. Diatoms were used to reconstruct pH (Fig. 3). After deglaciation, lake-water pH levels were up to 0.5 pH units higher than today. The decrease occurred mainly between 9000 and 7000 cal. years BP, when present-day values were reached.

Fig. 5. DCCA analysis of chironomids and diatoms assemblages in a 100-lake training set in northern Sweden (Larocque et al., 2001; Bigler and Hall, 2002). Assemblages typical of alpine lakes are represented by triangles, those typical of birch forest are circles and those of coniferous forest are squares. Arrows represent the most significant factors explaining the distribution of aquatic organisms in the 100-lake training sets. For graphical purposes, the less significant variables were not represented and the arrows of the environmental variables represented were increased by a factor of 3. Assemblages of each level of the core 850 were passively added to the DCCA analysis and changes through time are represented by a line.

4.3. Changes of communities through time The DCCA analysis indicates the variation of communities through the Holocene (Fig. 5). Chironomid

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assemblages remained relatively constant through time, as indicated by the lack of directional change. All assemblages remained similar to assemblages found today in lakes surrounded by alpine vegetation. Although the DCCA analysis of the chironomid assemblages showed very little variation through time (Fig. 5), five zones were established in the chironomid stratigraphy (Fig. 6). In the first zone (850-I), the assemblages were dominated by cold-water taxa such as Heterotrissocladius brundini, C. oliveri and Micropsectra insignilobus although warm-water taxa were also present (Microtendipes, T. lugens, Tanytarsus spp. B). Dominance of undifferentiated Heterotrissocladius and Tanytarsina were also noted in lakes in Canada after the ice had left the area (Levesque et al., 1996). In the British Isles, the early assemblages were also dominated by a mixture of cold and intermediate taxa (Lowe et al., 1999; Brooks et al., 1997). The second zone (850-II) was marked by an increase in Zalutschia zalutschicola, a cold-water indicator. However, this increase was associated with decreases in other cold indicators such as C. oliveri, M. insignilobus and H. brundini. In zone 3 (850III), the most remarkable change involved the increase and maintenance of H. brundini, and some Heterotrissocladius marcidus. Heterotrissocladius groups (especially H. brundini and H. marcidus) are typical of ultra to strongly oligotrophic lakes (Cranston et al., 1983) but also cold and deep lakes (Korhola et al., 2000b). In zone 4 (850-IV), the sharp decrease in H. brundini and increase in T. lugens could be, in part, indicative of lake level changes as well as a change from cold to warmer taxa. Heterotrissocladius is composed of deep water taxa while the Tanytarsus group has littoral taxa. The last zone was characterised by sharp increases in Psectrocladius sordidellus group and Sergentia while Z. zalutschicola decreased sharply. Sergentia is a taxon associated with meso-eutrophic status, as well is Tanytarsus (Meril.ainen and Hamina, 1993; Itkonen et al., 1999; Quinlan, 2000). Diatoms show larger changes through time, but only between 9000 and 7000 cal. years BP (Fig. 5). Just after deglaciation, assemblages were similar to those found today in lakes around the treeline. Afterwards, all assemblages had similar DCCA scores as those found in lakes above treeline. The directional change observed mainly followed the pH vector up to 7000 cal. years BP years. Then, assemblages remained more constant up to the present. The rapid changes observed in the early Holocene in the diatom assemblages were also shown by the zones created in the diatom stratigraphy (Fig. 7). Six statistical significant zones were identified. The first four zones occurred in rapid succession illustrating the instability of the diatom assemblages after deglaciation. The last two zones covered thousands of years, indicating that assemblages remained stable. In zones 1 and 2 (850-I, 850-II) very low concentrations of

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diatoms were recorded, and the percent values were based in some cases on 10 diatom valves only. Here, the organic content of the sediment was the lowest, as indicated by the %LOI. The recorded species belong mainly to the genus Fragilaria, and in addition some Aulacoseira species were found. Zone 3 (850-III) is characterised by a prevailing abundance of Fragilaria construens var. venter, accounting for more than 90% of the assemblage. Dominance of small Fragilaria species as F. construens var. venter was typical during early Holocene in Fennoscandia and was recorded as well at other places (Korhola et al., 2000a; Rose! n et al., 2001). In zone 4 (850-IV), the diatom assemblage stabilised to its present day state. The diatom assemblage was dominated by Eunotia praerupta, a species often recorded in association with mosses and habitats with changing moisture conditions. F. construens var. venter was replaced by other small Fragilaria species as F. pseudoconstruens and F. brevistriata. Towards the end of the zone, E. praerupta and Fragilaria species almost disappeared and the relative abundance of both Aulacoseira (e.g., A. distans var. alpigena) and Navicula (e.g. N. minima) increased. Zone 5 (850-V) covers the time period from ca. 8000 to 3000 cal. years BP. The prevailing diatom species in this zone belong to the planktonic genus Aulacoseira (A. distans var. alpigena, A. distans var. nivalis, A. lirata), and occurred in combination with Navicula minima, Navicula digitulus and Pinnularia biceps. The only Fragilaria species constantly present at low percentages was Fragilaria virescens var. exigua. Since 3000 cal. years BP (zone 850VI), the mostly planktonic Aulacoseira species decreased slightly and were partly replaced by benthic species as Navicula seminulum var. intermedia, N. digitulus, Nitzschia dissipata var. dissipata and Achnanthes species such as A. curtissima, A. nodosa and A. pusilla.

5. Discussion 5.1. Temperature reconstructions One of the benefits of using multi-proxy analysis is to separate the climatic from the lake inherent factors affecting the aquatic organisms (Battarbee, 2000). At this site, two possible explanations arise for the variation in the correlation through time between diatom and chironomid-based reconstructions. In the late Holocene, climate might has been the most important factor affecting both type of organisms. Earlier in the Holocene, other factors such as catchment development, changes in lake-water pH or a different nutrient regime might have affected diatoms and chironomids differently, leading to different inferred-temperature patterns. Another explanation for these differences might be of a statistical nature:

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Fig. 6. Chironomid stratigraphy. The x-axis is the percentage of each taxon.

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Fig. 7. Diatom stratigraphy. The x-axis is the percentage of each taxon.

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reconstructions are based on the assumption that links between modern assemblages and temperature have not changed through time (e.g. Birks, 1995) and this assumption may be false. During the past 100 years, both chironomid and diatom-inferred temperatures were well correlated with instrumental data (Bigler and Hall, 2003; Larocque and Hall, 2003). However, direct comparisons between proxy-inferred temperatures and meteorological records are restricted to the period covered by instrumental climate data, and we can only assume that the relationships between biological proxy indicators and environmental conditions remained comparable through time. As the temperature decline recorded by chironomids in Lake 850 is similar to other reconstructions in northern Sweden based on lake sediments (Rose! n et al., 2001; Bigler et al., 2002, 2003), we suggest that the first hypothesis provides the most plausible explanation for the differences between the reconstructions before 7000 cal. years BP. After 2500 cal. years BP, the good correspondence between chironomid- and diatom-inferences suggests that temperature was the most important factor to explain the distribution of both aquatic organisms. The absence of a pronounced long-term trend in the diatom-based temperature inference at Lake 850 is in contradiction with other diatom-based reconstructions in Lake Vuoskkuja! vri located a few km north of Lake 850 (Bigler et al., 2002), in Lake Njulla located 10 km from lake 850 (Bigler et al., 2003) and from reconstructions made in the Sarek area, ca. 150 km south of the Abisko area (Rose! n et al., 2001). Many other palaeoarchives and proxies have been used in the area, such as vegetation remains and pollen (Sonesson, 1974; Berglund et al., 1996; Barnekow, 1999), tree-rings (Briffa et al., 1992), tree megafossils (Kullman, 1999) and oxygen isotope records (Berglund et al., 1996; Shemesh et al., 2001; Hammarlund et al., 2002) and they all indicated variations of temperature (decreases of 2–4
C since 7000 cal. years BP) through the Holocene. Although diatoms have been shown to be sensitive to climate, they failed to track a similar temperature decrease in Lake 850. 5.2. Diatoms and environmental factors The DCCA analysis showed that from deglaciation to 7000 cal. years BP, diatoms were more influenced by pH than by temperature. This higher dependence of diatom assemblages on pH rather than climate might explain why diatoms cannot record accurately the changes of temperature recorded by chironomids at that site, and by diatoms and other proxies at other sites. Similar results were obtained in recent sediments: when comparing diatom-inferred temperatures with 100-year instrumental data, inferences were not as accurate when pH reconstructions showed strong variations (Bigler and

Hall, 2003). In Lake Vuoskkuja! vri and Lake Njulla, diatoms showed a decrease of temperature consistent with other proxies (Bigler et al., 2002, 2003) although early Holocene diatom assemblage changes were also mainly driven by pH changes (Bigler et al., 2002). The pH reconstructions at both sites indicated a decreasing pH trend of ca. 0.5–0.6 pH units. Present day pH was reached at about 5000 cal. years BP in Lake Vuoskkuja! vri (Bigler et al., 2002) while in Lake Njulla there was a continuous trend up to the present (Bigler et al., 2003). The pH reconstruction in Lake 850 showed a decrease of 0.5 pH units up to 7000 cal. years BP. Variations from one lake to the other indicate that site-specific processes control the lake development in northern Sweden and in the case of Lake 850, these inherent factors seem to be more important than climate in the early Holocene so that diatoms do not necessarily provide temperature reconstructions similar to the ones obtained in other lakes and with other proxies. 5.3. Chironomid, temperature and ecological responses Although the DCCA analysis showed small variations of chironomids through time, a decrease of temperature was still reconstructed. This result suggests that even small variations in chironomid assemblages can be associated to temperature changes. While the reconstructed decrease of temperature is consistent with other studies in the area using chironomids or in multi-proxy studies using pollen, chironomids and diatoms (Bigler et al., 2002), the magnitude of the recorded temperature decrease is of much lesser importance (2
C) than recorded by some other proxies (tree megafossils, Kullman, 1999; isotopes, Shemesh et al., 2001). One limitation in using chironomid for climate reconstruction is that little is still known about their ecological needs, despite the development of training sets. Although temperature has been identified as one of the most important factors influencing the distribution of chironomids (Walker et al., 1997; Olander et al., 1999; Brooks and Birks, 2000; Larocque et al., 2001) other ecological variables might also be important locally. In the chironomid stratigraphy, assemblages were often dominated by both cold- and warm-water taxa together (e.g. zone I) or the increase of one coldwater taxon was associated with decreases of other coldwater taxa (e.g. zone II). These results indicate that temperature is not the only factor influencing the changes in chironomid assemblages and the influence of other factors simultaneously with climate might lead to less accurate inferences. LOI%, lake depth and January temperature were also important factors explaining the distribution of chironomids in the 100-lake training set (Larocque et al., 2001). After 8500 cal. years BP, LOI% remained more or less constant through time and can thus be ruled out as an

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important factor affecting the chironomid assemblages in Lake 850. Heterotrissocladius groups have been described as cold-water indicators but were also associated with lake depth (Korhola et al., 2000b). Variations of these taxa were marked in our core and could be, in part, indicative of lake-level changes, as well as temperature. This high interaction between temperature and other factors to explain chironomid assemblages through time might lead to less accurate temperature inferences, or at least creates the lower magnitude of the decrease observed in Lake 850 compared with the decrease recorded by isotopes in the same lake. This enhances the need of knowing more about the ecology of biological indicators to better interpret the temperature reconstructions obtained. Also, this stresses the need to find ways of disentangling climate from other factors affecting the variation of assemblages through time.

6. Conclusions Temperature reconstructions using chironomids and diatoms showed similar patterns during the last 7000 cal. years. Before this period, discrepancies occurred: chironomids inferred a decrease of temperature through time while diatoms recorded no temperature changes in the past 9000 cal. years. These discrepancies might be due to factors other than temperature affecting the biological indicators through time. After deglaciation and until 7000 cal. years BP, pH was an important factor driving the diatom assemblages and probably masking the effect of climate. After 2500 cal. years BP, both chironomids and diatoms seem to have been affected similarly by climate and thus provided highly correlated temperature inferences. This study enhances the importance of finding ways to disentangle the effect of climate from other factors on biological indicators.

Acknowledgements The project was supported by the Climate Impacts Research Centre (CIRC) and the Space and Environment Research Institute (MRI) via EU structural funds and Swedish national and regional funds. Isabelle Larocque was supported by an NFR postdoctoral fellowship grant Dnr G-GU 12468-300. We thank Mats Eriksson, Ola Fredin, Katarina Jonsson, Miri Rietti-Shati, Bengt Wanhatalo, Ninis Rosqvist, Lena Rubensdotter and Aldo Shemesh for the collaboration during fieldwork and core processing. We thank the staff at the Abisko Naturvetenskapliga Station for technical support.

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