Late Holocene thermokarst variability inferred from diatoms in a lake sediment record from the Lena Delta, Siberian Arctic

Thermokarst lakes in the Siberian Arctic contain sediment archives that can be used for paleoenvironmental inference. Until now, however, there has been no study from the inner Lena River Delta with a focus on diatoms. The objective of this study was to investigate how the diatom community in a thermokarst lake responded to past limnogeological changes and what specific factors drove variations in the diatom assemblage. We analysed fossil diatom species, organic content, grain-size distribution and elemental composition in a sediment core retrieved in 2009 from a shallow thermokarst lake in the Arga Complex, western Lena River Delta. The core contains a 3,000-year record of sediment accumulation. Shifts in the predominantly benthic and epiphytic diatom species composition parallel changes in sediment characteristics. Paleoenvironmental and limnogeological development, inferred from multiple biological and sedimentological variables, are discussed in the context of four diatom zones, and indicate a strong relation between changes in the diatom assemblage and thermokarst processes. We conclude that limnogeological and thermokarst processes such as lake drainage, rather than direct climate forcing, were the main factors that altered the aquatic ecosystem by influencing, for example, habitat availability, hydrochemistry, and water level.     

Diatom taphonomy and silica cycling in two freshwater lakes and their implications for inferring past lake productivity

Diatom preservation can be a major taphonomic issue in many lakes but is often unrecognised and its impacts on qualitative and quantitative inferences (such as productivity and biodiversity estimates) from sedimentary archives are seldom explored. Here two palaeolimnological case studies of 20th-century anthropogenic eutrophication of freshwater lakes in Northern Ireland (Lough Neagh and Lough Augher) are re-visited and new data presented on diatom preservation. Assessing problems of taphonomy challenges previous interpretations of silica dynamics and diatom productivity at these sites. Diatom preservation was assessed in both sediment trap material and sediment cores from Lough Neagh, and in sediment cores from Lough Augher. Preservation data, combined with geochemical analysis (Si, Fe), provide an insight into silica cycling and diatom accumulation over a range of temporal scales from these lakes. Diatom preservation was generally good for the Lough Neagh material, although differential (better) preservation of the smaller Aulacoseira subarctica compared to the larger Stephanodiscus neoastraea sensu lato valves was clear, especially in sediments. Porewater silica showed a complex seasonal pattern in the upper sediment, against expectations of steady-state. The Lough Augher material was generally poorly preserved, although preservation (dissolution) was significantly (and positively) correlated to bulk sedimentation rate, and was found to be a major control on (net) diatom accumulation rate across the basin. Past seasonal and severe anoxia at Lough Augher did not improve diatom preservation, contrary to some previous studies, which may be due to extreme changes in sedimentary redox conditions. Finally, using published experimental relationships between dissolution and diatom valve loss, correction factors were applied to previously published profiles of diatom accumulation over the last ~150 years (biovolume from Lough Neagh and frustule accumulation rate from Lough Augher), which suggest that diatom productivity estimates from sedimentary records are underestimated by a factor of 2–4 due to dissolution effects alone. The results clearly have implications for the reliability and accuracy of diatom-based inferences made from sediment records, both qualitative and quantitative, especially for those that employ diatoms as direct measures of productivity or biodiversity.     

Seasonal Diatom Variability and Paleolimnological Inferences – A Case Study

The seasonality of physical, chemical, and biological water variables is a major characteristic of temperate, dimictic lakes. Yet, few investigations have considered the potential information that is encoded in seasonal dynamics with respect to the paleolimnological record. We used a one-year sequence of diatoms obtained from sediment traps and water samples, as well as the sedimentary diatom record covering the past ca. 1000 years in Bates Pond, Connecticut (USA), to investigate which variables influence the seasonal distribution of diatoms and how this can be used for the interpretation of the fossil record. The seasonal patterns in diatom assemblages were related to stratification and, to a lesser extent, to nitrate, silica, and phosphorus. During mixing periods in spring and autumn, both planktonic and benthic species were collected in the traps, while few lightly silicified, spindle-shaped planktonic diatoms dominated during thermal stratification in summer. Changes in fossil diatom assemblages reflected human activity in the watershed after European settlement and subsequent recovery in the 20th century. A long-term trend in diatom assemblage change initiated before European settlement was probably related to increased length of mixing periods during the Little Ice Age, indicated by the increase of taxa that presently grow during mixing periods and by application of a preliminary seasonal temperature model. We argue that the analysis of seasonal diatom dynamics in temperate lakes may provide important information for the refinement of paleolimnological interpretations. However, investigations of several lakes and years would be desirable in order to establish a more robust seasonal data set for the enhancement of paleolimnological interpretations.     

Lake Jezero v Ledvici (NW Solvenia) – changes in sediment records over the last two centuries

Lake Jezero v Ledvici (NW Slovenia) is a 14 m deep mountain lake at an elevation of 1860 m, situated on limestone bedrock. It is an oligotrophic, alkaline and hard-water lake with a transparency of about 14 m and has suffered several times over recent centuries from strong earthquakes. In 1996 five sediment cores, between 35 and 45 cm long, were collected from the deepest part of the lake and analysed to reconstruct environmental changes over the last few centuries. The data indicate changes induced by pollution and climate change during the last two centuries similar to those in other European mountain and remote lakes. However, at this site earthquakes have also affected the lake and partly obscure the interpretation of the sediment record. From 1780 to 1890, sediment records show low abundance of diatoms and relatively high abundance of Cladocera. After 1890, the number of diatoms started to increase coinciding with a strong earthquake. From the beginning of the 20th century, concentrations of spheroidal carbonaceous particles (SCP), diatom valves and head capsules of chironomids gradually increased whilst in Cladocera the main difference observed was a change in the proportion of benthic taxa. After 1960, in parallel with a rise in air temperature, a further increase in accumulation rate of diatoms started, but there was a decrease in Cladocera.     

Tracking eutrophication in Taihu Lake using the diatom record: potential and problems

Taihu Lake is the third largest freshwater lake in China and has been experiencing eutrophication problems for several decades. Diatoms in short sediment cores from three bays in northern Taihu Lake were studied in addition to 1-year of seasonal phytoplankton samples in order to evaluate the rate and magnitude of nutrient enrichment. The dominant species found in the phytoplankton samples appeared in high percentages in the surface sediment samples, suggesting that the latter faithfully record the modern diatom flora. The diatom preservation status varied among the three cores, while in all cores the preservation deteriorated with sediment depth. Due to the superior diatom preservation in the core from Mashan Bay, the fossil diatom record of this core and an established diatom total phosphorus (TP) transfer function were used to reconstruct the nutrient history of Taihu Lake. Diatom assemblages changed from Aulacoseira-dominated to other eutrophic planktonic species, such as Stephanodiscus minutulus, Cyclostephanos tholiformis, Cyclotella atomus, C. meneghiniana and S. hantzschii in ca. 1980. Diatom-inferred TP concentrations exhibited little change prior to 1980, with values around 50 μg/l. However, after 1980 TP concentrations increased significantly and remained in excess of 100 μg/l, reflecting eutrophication of Taihu Lake. Comparison with TP measurements in the water column from 1988 to 2004, as well as the analogue analysis among fossil and modern samples, demonstrates that the diatom-TP inference model can reliably hindcast past TP concentrations. Therefore, the baseline TP value of about 50 μg/l, can be used as a restoration target for Taihu Lake. However, due to the complexity of this very large, shallow aquatic ecosystem, caution should be exercised when employing the diatom record to track eutrophication. Further studies on the mechanism of diatom distribution, evolution and preservation are recommended for Taihu Lake.     

Paleolimnology and recent environmental change in Lake Baikal: an introduction and overview of interrelated concurrent studies

Recent environmental change research in Lake Baikal is introduced together with an overview of several interrelated papers published concurrently in this issue of Journal of Paleolimnology. Five themes are tackled by analysis of recent Baikal sediment cores, dating, geochemistry, particulate pollutants, magnetism and diatoms. The concurrent papers focus on the first four themes in some detail and summary results of diatom analysis (from Mackay et al., 1998) are given here. Taken together these studies provide a time-space framework for recent environmental change in Lake Baikal not previously available.There are significant shifts in species composition of the endemic planktonic diatom assemblages in uppermost sediments collected from throughout the lake. However, these changes usually precede the sediment record of low level but widespread contamination by industrial products. The most clear sign of industrial contamination is the presence of particles from fossil fuel combustion in sediment post dating the 1930s.Although evidence for widespread biostratigraphic changes by pollution is lacking, radionuclide, diatom, lithostratigraphic and magnetic stratigraphies indicate two main features, (i) it is possible to make stratigraphic correlations within and between basins using recent sediment cores, (ii) that turbidite deposits, from several to tens of cm thick, are frequently encountered in recent sediments.Turbidite deposits occur in ²¹⁰Pb dated and pre-²¹⁰Pb sediment core sections and are undoubtedly a major macro-disturbance feature in many deep water locations in Lake Baikal. If profiles are to be used as direct proxy records of climate variability, then screening of cores for turbidites is a pre-requisite for quality assurance in future paleoenvironmental studies.On-going international research including Swiss, Russian and British joint paleoenvironmental studies on the distribution and biological formation of recent sediments will hopefully lead to better interpretation of Holocene and pre-Holocene sediment records in Lake Baikal.     

Variability in diatom and chrysophyte assemblages and inferred pH: paleolimnological studies of Big Moose Lake,New York,USA

We measured variability in the composition of diatom and chrysophyte assemblages, and the pH inferred from these assemblages, in sediment samples from Big Moose Lake, in the Adirondack Mountains of New York. Replicate samples were analyzed from (1) a single sediment core interval, (2) 12 different intervals from each of 3 separate cores, and (3) 10 widely spaced surface sediment samples (0–1 cm). The variability associated with sample preparation (subsampling, processing, and counting) was relatively small compared to between-core and within-lake variability. The relative abundances of the dominant diatom taxa varied to a greater extent than those of the chrysophyte scale assemblages. Standard deviations of pH inferences for multiple counts from the same sediment interval from diatom, chrysophyte, and diatom plus chrysophyte inference equations were 0.04 (n=8), 0.06 (n=32), and 0.06 (n=8) of a pH unit, respectively. Stratigraphic analysis of diatoms and chrysophytes from three widely spaced pelagic sediment cores provided a similar record of lake acidification trends, although with slight differences in temporal rates of change. Average standard deviations of pH inferences from diatom, chrysophyte and diatom plus chrysophyte inference equations for eight sediment intervals representing similar time periods but in different cores were 0.10, 0.20, and 0.09 pH unit, respectively. Our data support the assumption that a single sediment core can provide an accurate representation of historical change in a lake. The major sources of diatom variability in the surface sediments (i.e., top 1.0 cm) were (1) differences in diatom assemblage contributions from benthic and littoral sources, and (2) the rapid change in assemblage composition with sediment depth, which is characteristic of recently acidified lakes. Because scaled chrysophytes are exclusively planktonic, their spatial distribution in lake sediments is less variable than the diatom assemblages. Standard deviations of pH inferences for 10 widely spaced surface sediment samples from diatom, chrysophyte and diatom plus chrysophyte inference equations were 0.21, 0.09, and 0.16 of a pH unit, respectively.     

A weighted-averaging regression and calibration model for inferring lakewater salinity from fossil diatom assemblages in saline lakes of the Vestfold Hills: a new tool for interpreting Holocene lake histories in Antarctica

The relationship between surface sediment diatom assemblages and measured limnological variables in thirty-three coastal Antarctic lakes from the Vestfold Hills was examined by constructing a diatom-water chemistry dataset. Previous analysis of this dataset by canonical correspondence analysis revealed that salinity accounted for a significant amount of the variation in the distribution of the diatom assemblages. Weighted-averaging regression and calibration of this diatom-salinity relationship was used to establish a transfer function for the reconstruction of past lakewater salinity from fossil diatom assemblages. Weighted-averaging regression and calibration with classical deshrinking provided the best model for salinity reconstructions and this was applied to the fossil diatom assemblages from one of the saline lakes in the Vestfold Hills in order to assess its potential for palaeosalinity and palaeoclimate reconstruction.     

A diatom-conductivity transfer function for Spanish salt lakes

Diatom-salinity transfer functions for interpretation of palaeosalinity and palaeoclimate change have been developed successfully for parts of North America and North and East Africa, but there is a need for data-sets in other saline lake regions of the world. A data-set of 74 modern diatom samples and associated water chemistry data is described from Spain. The influence of conductivity and other environmental variables on diatom distribution is explored using canonical correspondence analysis (CCA) and partial CCAs. A transfer function is derived for conductivity (70 samples) whose apparent predictive ability is high (apparent r² = 0.91). Performance under jackknifing is poor due to the heterogeneous nature of the data-set and poor coverage of the freshwater end of the salinity gradient. There is a lack of suitable low-salinity sites in Spain, and the accuracy of estimated salinity optima and tolerance ranges may be improved by merging this data-set with those of other regions. The Spanish transfer function has strong affinities with the African data-set and contributes important ecological data for diatom taxa which are absent or poorly represented in the modern flora of African lakes, and for which, in fossil material, there were previously no good modern analogues.     

The ostracode record from Harris Lake, southwestern Saskatchewan: 9200 years of local environmental change

Holocene paleoenvironments of Harris Lake, southwestern Saskatchewan, are reconstructed from the ostracode stratigraphy of a 10.4 m sediment core. Twenty three taxa, representing nine genera, were identified and counted from 113 samples. At each depth, a theoretical faunal assemblage was derived from the raw counts. The mean and variance of chemical, climatic and physical variables were inferred from modern analogues of the fossil assemblages, using existing autecological data from 6720 sites, mostly in western Canada. These data suggest four paleoenvironments: an early-Holocene (9240–6400 years BP) variable climate supporting aspen parkland vegetation; the warm dry hypsithermal (6400–4500 years BP); a short transitional period of ameliorating climate and expanding subboreal forest (4500–3600 years BP); and the present environment since 3600 years BP. A change in regional climate with the draining of Glacial Lake Agassiz (ca. 8500 years BP) and landsliding in the watershed (ca. 4000 years BP) caused relatively rapid environmental change. The ostracode record generally corroborates the interpretations of other proxy data previously published for Harris Lake. Most of the discrepancy involves the timing and severity of maximum Holocene warmth and aridity. Peak aridity interpreted from the pollen data is earlier than in the other proxy records. Both the diatoms and ostracodes indicate highest paleosalinity between ca. 6500 and 5000 years BP, but maximum salinity in the diatom record occurs between ca. 6000–5700 years BP, whereas the ostracode-inferred salinity is relatively low at this time and peaks later at ca. 5000 years. Neither of these reconstructions suggests the short episodes of hypersalinity interpreted from the mineralogy.     

Modelling diatom responses to climate induced fluctuations in the moisture balance in continental Antarctic lakes

The water chemistry of lake systems on the edge of the Antarctic continent responds quickly to changes in the moisture balance. This is expressed as increasing salinity and decreasing lake water level during dry periods, and the opposite during wet periods. The diatom composition of the lakes also changes with these fluctuations in salinity and lake water depth. This is important, as their siliceous remains become incorporated into lake sediments and can provide long-term records of past salinity using transfer functions. In order to develop transfer functions, diatoms and water chemistry data were inter-calibrated from five different East Antarctic oases, namely the Larsemann Hills, the Bølingen Islands, the Vestfold Hills, the Rauer Islands and the Windmill Islands. Results indicate that salinity is the most important environmental variable explaining the variance in the diatom flora in East Antarctic lakes. In oligo- saline lakes the variance is mainly explained by lake water depth. This dataset was used to construct a weighted averaging transfer function for salinity in order to infer historical changes in the moisture balance. This model has a jack-knifed r² of 0.83 and a RMSEP of 0.31. The disadvantage of this transfer function is that salinity changes in oligo-saline lakes are reconstructed inaccurately due to the edge effect and due to the low species turnover along the salinity gradient at its lower end. In order to infer changes in the moisture balance in these lakes, a second transfer function using weighted averaging partial least squares (with two components) for depth was constructed. This model has a jack-knifed r² of 0.76 and a RMSEP of 0.22. Both transfer functions can be used to infer climate driven changes in the moisture balance in lake sediment cores from oligo-, hypo-, meso- and hyper-saline lakes in East Antarctic oases between 102–75°E. The transfer function for lake water depth is promising to track trends in the moisture balance of small freshwater lakes, where changes in shallow and deep-water sediments are readily reflected in changing diatom composition.     

Distribution of diatoms among intermittent ponds on the Atlantic Coastal Plain: development of a model to preduct drought periodicity from surface-sediment assemblages

Inferences of past climate from the fossil record in lakes rely on the accurate quantification of a relationship of fossilizing organisms to their environment. Whereas the relationship of diatoms to water chemistry parameters has been modeled in many systems, few studies adequately address the relationship of diatoms to physical properties, such as water depth or hydrology, that may be more directly tied to climate. We examined the composition of modern diatoms in surface sediments of 75 isolated ponds (mostly Carolina bays) of the Atlantic Coastal Plain to: (1) assess the influence of physical and chemical variables on the distribution of diatoms among ponds of the region, and (2) develop a model that predicts hydroperiod (a measure of pond permanence) from diatom assemblages. We constructed two hydroperiod calibration models: the first infers hydroperiod from the weighted-average optima and tolerances of taxa along the hydroperiod gradient, the second bases inferences on the hydroperiod estimates of compositionally similar samples. Both approaches incorporate a-priori and post-hoc tests of assumptions often inherent in the construction of transfer functions. Diatom assemblage composition had strong, approximately linear relationships to hydroperiod, water depth, and calcium concentration in non-metric multidimensional ordination space; effects of other variables, including pH, were non-linear or ambiguous. Overall, the assemblages reflected the dilute, acidic chemical characteristics of bays. The assemblages contained differing abundances of euterrestrial, benthic and planktonic taxa, depending on a pond's susceptibility to drying. A weighted-averaging regression model based on taxon-specific hydroperiod optima generated adequate, unbiased hydroperiod inferences from diatom species composition (r² = 0.81). This model may be used to infer past drought episodes from fossil diatom assemblages at appropriate sites on the Atlantic Coastal Plain.     

Nutrients as a link between ionic concentration/composition and diatom distributions in saline lakes

Diatom profiles in closed-basin lake sediments are commonly used to reconstruct climate change based on the observed correlations between salinity (ionic concentration) and modern diatom assemblages. Diatom assemblages are strongly correlated not only with salinity but also anion composition, with certain taxa characteristic of carbonate systems and others sulfate-dominated waters. Although strong correlations exist, the actual mechanisms behind these correlations are unknown. Here we briefly review the influence of salinity and ionic composition on nutrient dynamics in saline lakes and suggest that these interactions may drive shifts in diatom species composition along gradients of ionic concentration/composition. We discuss the influence of salinity and anion composition on nutrient availability, as well as on nutrient requirements and uptake by diatoms.     

Interpretation of Holocene lake-level change from diatom assemblages in Lake Sidi Ali,Middle Atlas,Morocco

While palaeohydrological changes in non-outlet lakes provide a key proxy indicator of past climatic fluctuations, for lake systems which have been chemically insensitive, it is necessary to use indicators of water depth rather than salinity to reconstruct their hydro- climatic histories. A study of diatoms in the modern sediments of Sidi Ali, a non-outlet lake in the Middle Atlas of Morocco, has shown a statistically significant correlation between water depth and the ratio of planktonic to littoral diatoms. This relationship is used to calibrate fossil diatom assemblages from a lake sediment core from the same lake to provide a quantitative index of water levels over the pastc. 6500 years. Palaeoecological evidence suggests that climatically induced hydrological variations have dominated the bulk of the mid-late Holocene lake sediment record, with significant human-induced catchment disturbance only occurring during the twentieth century. The pattern of water depth fluctuations suggests that the response time of the regional groundwater system to climatic forcing is <100 years.This is the third in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.     

A high resolution multi-proxy record of pronounced recent environmental change at Baker Lake,Nunavut

Arctic aquatic systems are considered to be especially sensitive to anthropogenic disturbance, which can have cascading effects on biological communities as aquatic food-web structure is altered. Bio-indicators that respond to major limnological changes can be used to detect and infer major environmental change, such as climate warming, with the use of paleolimnological techniques. A multi-proxy approach was used to quantify recent environmental changes at Baker Lake, Nunavut, Arctic Canada. Analyses of fossilized remains of chironomids and diatoms were conducted on a sediment core of 20 cm in length sampled at 0.5-cm intervals. A new surface sediment training set of subfossil chironomid assemblages from 65 lakes across the eastern Canadian Arctic generated a robust (r _jack² = 0.79) surface water paleotemperature transfer function. The transfer function was applied to stratigraphic intervals from the Baker Lake sediment core to generate a paleotemperature reconstruction of sub-decadal resolution. The surface water temperature reconstruction inferred a 2°C increase in mid-summer surface water temperature for Baker Lake over the last 60 years, which was corroborated by the local instrumental record spanning the period of 1950–2007 AD. The chironomid record shows a recent decline of several cold-water taxa and appearance of warm-water indicators. This shift in community structure began circa 1906 AD, and intensified after 1940 AD. The corresponding fossil diatom record showed an increase in small planktonic Cyclotella taxa over the past 60 years, intensifying in the last 5 years, which also suggests a warmer climate and longer ice-free periods. The shifts in the diatom assemblages began later than the shifts in the chironomid assemblages, and were of lower magnitude, reflecting differences in the mechanisms in which these two indicators respond to environmental change.     

Biogeochemistry of silica in Devils Lake: implications for diatom preservation

Diatom-salinity records from sediment cores have been used to construct climate records of saline-lake basins. In many cases, this has been done without thorough understanding of the preservation potential of the diatoms in the sediments through time. The purpose of this study was to determine the biogeochemistry of silica in Devils Lake and evaluate the potential effects of silica cycling on diatom preservation. During the period of record, 1867–1999, lake levels have fluctuated from 427 m above sea level in 1940 to 441.1 m above sea level in 1999. The biogeochemistry of silica in Devils Lake is dominated by internal cycling. During the early 1990s when lake levels were relatively high, about 94% of the biogenic silica (BSi) produced in Devils Lake was recycled in the water column before burial. About 42% of the BSi that was incorporated in bottom sediments was dissolved and diffused back into the lake, and the remaining 58% was buried. Therefore, the BSi accumulation rate was about 3% of the BSi assimilation rate. Generally, the results obtained from this study are similar to those obtained from studies of the biogeochemistry of silica in large oligotrophic lakes and the open ocean where most of the BSi produced is recycled in surface water. During the mid 1960s when lake levels were relatively low, BSi assimilation and water-column dissolution rates were much higher than when lake levels were high. The BSi assimilation rate was as much as three times higher during low lake levels. Even with the much higher BSi assimilation rate, the BSi accumulation rate was about three times lower because the BSi water-column dissolution rate was more than 99% of the BSi assimilation rate compared to 94% during high lake levels. Variations in the biogeochemistry of silica with lake level have important implications for paleolimnologic studies. Increased BSi water-column dissolution during decreasing lake levels may alter the diatom-salinity record by selectively removing the less resistant diatoms. Also, BSi accumulation may be proportional to the amount of silica input from tributary sources. Therefore, BSi accumulation chronologies from sediment cores may be effective records of tributary inflow.     

Environmental Changes at the Desert Margin: An Assessment of Recent Paleolimnological Records in Lake Qarun, Middle Egypt

Lake Qarun has been profoundly affected by a combination of human activities and climatic changes during the past 5000 years. Instrumental records available for the 20th century show that during most of this period both lake water level and salinity increased and that by the late 1980s lake water salinity was approximately that of seawater. Sediment cores (c. 1 m long) were collected from this shallow (Z_max 8.4 m) saline lake in 1998 and the master core (QARU1) was used to examine the potential of paleolimnology for reconstructing the recent environmental history of the site. According to ¹³⁷Cs and ²¹⁰Pb radio-assay, the recent sediment accumulation rate in QARU1 was around 5 mm year⁻¹ during the latter half of the 20th century but radionuclide levels were low. Spheroidal carbonaceous particles (SCPs) were present in the upper c. 30 cm of QARU1 and indicates contamination by low level particulate pollution, probably beginning around 1950. The record of exotic pollen (Casuarina) indicated that sediment at 51–52 cm depth dated to around 1930. Otherwise the pollen spectra indicated a strongly disturbed landscape with high ruderals and increased tree planting particularly since c. 1950. Diatom records were strongly affected by taphonomic processes including reworking and differential preservation but typical marine diatoms increased after the 1920s. Instrumental records show that the lake became more saline at this time. Freshwater taxa were present at approximately similar abundances throughout the core. This distribution probably reflected a combination of processes. Reworking of ancient freshwater diatomites is one likely source for freshwater diatoms in QARU1 but some taxa must also be contributed via the freshwater inflows. Overall, the diatom stratigraphy indicated increasingly salinity since the 1920s but provided no firm evidence of lake eutrophication. Diatom inferred salinity reconstructions were in only partial agreement with instrumental records but inferred for the lower section of the core (pre 20th century to the 1960s) accord with measured water salinity values. Surficial sediments of Lake Qarun contain environmental change records for the 20th century period but high sediment accumulation rate and pollen reflect the high degree of human disturbance in the region. Because of poor preservation and evidence of reworking, the relationships between diatom records and past water quality changes require careful interpretation, especially in the upper section of the core. Nevertheless, early to mid 20th century measurements of increasing lake water salinity are well supported by sediment records, a change that is probably linked to ingress of saline ground water     

Climate changes in East Siberia (Russia) in the Holocene based on diatom,chironomid and pollen records from the sediments of Lake Kotokel

We analysed a 620-cm-long sediment record from Lake Kotokel located in East Siberia (Russia) for subfossil diatoms, chironomids and pollen to provide a reconstruction of the climate history of the area for the last 12.2 kyr. The subfossil records show differing time lags in their responses to climate change; diatoms and chironomids were more sensitive to climate change than the pollen record. Changes in the biogenic proxies seem related with changes in insolation, the temperature of the North Atlantic and solar activity. The chironomids Chironomus plumosus-type and Einfeldia carbonaria-type and the diatom Aulacoseira granulata were interpreted as markers of warm climate condition. The proxy records were divided into four periods (A, B, C and D) suggesting differing climate in East Siberia during the Holocene. Period D (12.2–9.5 kyr BP) at the beginning of the Holocene, according to chironomid and diatom records, was characterized by warm climate with summer temperatures close to modern. However, forest vegetation had not become fully established yet. During Period C (9.5–5.8 kyr BP), the climate seemed to gradually become colder and wetter from the beginning of Period C to 7 kyr BP. From 7 to 5.8 kyr BP, the climate seemed to remain cold, but aridity increased. Period B (5.8–1.7 kyr BP) was characterised by frequent and sharp alternations between warm and cold conditions. Unstable conditions during this time are also registered in records from Lakes Baikal, Khubsugul and various other shallow lakes of the region. Optimal warm and wet conditions seemed to occur ca. 4 kyr BP. During Period A (the last 1.5 kyr) the diatom and chironomid records show evidence of cold conditions at 1.5–1 kyr BP, but the forest vegetation did not change significantly.     

Midge- and diatom-based palaeosalinity reconstructions for Mahoney Lake, Okanagan Valley, British Columbia, Canada

Salinity fluctuations in lakes of semi-arid regions have long been recognised as indicators of palaeoclimatic change, and have provided a valuable line of evidence in palaeo-climatic reconstruction. In the present study, fossil remains of diatoms and midges were used to reconstruct salinity changes at Mahoney Lake from the early postglacial, through the early, mid and late Holocene. A transition from midges typical of a freshwater community (Protanypus, Sergentia, Heterotrissocladius, Cladopelma, Dicrotendipes) during the early postglacial, to those indicative of saline environments (Cricotopus/Orthocladius, Tanypus) occurred in the early Holocene. The midge-inferred salinity values reflected the shift from freshwater (0.031 g/L) immediately after deglaciation, to saline water (2.4 to 55.2 g/L) in subsequent periods. A less saline period was found to have occurred after 1000 yr BP, suggesting a cooler or wetter period. The diatom record indicates similar trends, with freshwater taxa (e.g.,Cyclotella bodanica var. aff.lemanica) dominating near the bottom of the core. Diatom-inferred salinities indicate that saline conditions (about 30 g/L) prevailed throughout subsequent Holocene time, although relatively fresh conditions are indicated following deposition of the Mazama Ash, and from about 1500 yr BP until the present day. Midge and diatom-inferred salinity reconstructions for Mahoney Lake compare favorably with each other, and with climate trends inferred from earlier palynological evidence. The palaeosalinity record thus contributes new data relevant to past climatic conditions, in a region where little data have previously been collected.     

Climate variability and ecosystem dynamics of remote alpine and arctic lakes: the MOLAR project

This paper introduces the results of an integrated project designed to compare high resolution analysis of proxy records of climate change in the sediments of seven mountain lakes across Europe with reconstructed instrumental records of climate change over the last 200 years. Palaeolimnological methods used include radiometric dating (²¹⁰Pb, ¹³⁷Cs), mineral magnetics, dry weight, loss-on-ignition, carbon, nitrogen, sulphur, pigments, diatoms, chrysophyte cysts, cladocera and chironomids. Changes in fossil assemblages were summarised using principal components analysis. The stratigraphic data were compared with the instrumental record using linear regression techniques. The dated sediment records for each proxy from each site were treated as the response variables and the various attributes of the instrumental climate record as the predictor variables. The predictor variables were generated for each site for the period 1781 to 1997 using temperature reconstructions based on meteorological records. To harmonise the climatic predictors and the response variables, the climatic variables were smoothed along time with a LOESS regression. The results of the various analyses at the seven sites are presented in the following papers. A synthesis of the project and the relative performance of the different proxy methods are discussed in the final paper.