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1. As future climate change is expected to have a major impact on freshwater lake ecosystems, it is important to assess the extent to which changes taking place in freshwater lakes can be attributed to the degree of climate change that has already taken place. 2. To address this issue, it is necessary to examine evidence spanning many decades by combining long-term observational data sets and palaeolimnological records. 3. Here, we introduce a series of case studies of seven European lakes for which both long-term data sets and sediment records are available. Most of the sites have been affected by eutrophication and are now in recovery. 4. The studies attempt to disentangle the effects of climate change from those of nutrient pollution and conclude that nutrient pollution is still the dominant factor controlling the trophic state of lakes. 5. At most sites, however, there is also evidence of climate influence related in some cases to natural variability in the climate system, and in others to the trend to higher temperatures over recent decades attributed to anthropogenic warming. 6. More generally and despite some problems, the studies indicate the value of combining limnological and palaeolimnological records in reconstructing lake history and in disentangling the changing role of different pressures on lake ecosystems. [ABSTRACT FROM AUTHOR]

Aim Recent papers have used large palaeolimnological datasets to reveal the biodiversity patterns of aquatic microorganisms. However, scant attention has been paid to the influence of time on these patterns. Where lake surficial sediment samples are used as integrals of diversity, the time interval of each sample varies according to differences in sediment accumulation rates. This paper aims to test the reliability of using lake surface sediments to measure and to compare microbial diversity when the potential influences of the species-time relationships are taken into account. Location Alpine lakes in Europe. Methods We analysed microorganism (siliceous microalgae) assemblages in three European Alpine lakes using short sediment cores (210Pb-dated) and annual sediment trap samples from 12 UK lakes. The same number of individuals was pooled for each sample 500 times to avoid sampling effort effects and to standardize species diversity estimation. The influence of time on the diversity score was assessed by simulating an increase of time span for surface sediment samples by cumulatively adding in successive sediment core samples (from the most recent to the oldest). We used species richness ( S) and the exponential of the bias-corrected Shannon entropy index (exp( Hb-c)) to estimate diversity. Results Increasing the time interval represented by a surficial sediment sample did not affect the diversity results. The estimation of diversity was similar for cumulative and non-cumulative samples. Diversity estimation was only altered in lakes experiencing high community turnover due to strong environmental forcing during the time period spanned by the cumulative sample. Main conclusions The use of surface lake sediments is suitable for estimating the average site diversity of free-living microorganisms. Diversity is integrated in a single sample and species assemblage composition is derived from microbial communities living in distinct lake microhabitats. Species remains, accumulated in a single sample over several years of environmental variability, represent a diversity integral that captures a spatio-temporal component equivalent to the γ-diversity measure. [ABSTRACT FROM AUTHOR]

knowledge of pre-disturbance conditions is important for setting realistic restoration targets for lakes. For European waters this is now a requirement of the European Council Water Framework Directive where ecological status must be assessed based on the degree to which present day conditions deviate from reference conditions. Here, we employ palaeolimnological techniques, principally inferences of total phosphorus from diatom assemblages (DI-TP) and classification of diatom composition data from the time slice in sediment cores dated to ~1850 AD, to define chemical and ecological reference conditions, respectively, for a range of UK lake types. The DI-TP results from 169 sites indicate that reference TP values for low alkalinity lakes are typically <10 μg L and in many cases <5 μg L, whilst those for medium and high alkalinity lakes are in the range 10-30 and 20-40 μg L, respectively. Within the latter two alkalinity types, the deeper waters (>3 m mean depth) generally had lower reference TP concentrations than the shallow sites. A small group of shallow marl lakes had concentrations of ~30 μg L. Cluster analysis of diatom composition data from 106 lakes where the key pressure of interest was eutrophication identified three clusters, each associated with particular lake types, suggesting that the typology has ecological relevance, although poor cross matching of the diatom groups and the lake typology at type boundaries highlights the value of a site-specific approach to defining reference conditions. Finally the floristic difference between the reference and present day (surface sample) diatom assemblages of each site was estimated using the squared chord distance dissimilarity coefficient. Only 25 of the 106 lakes experienced insignificant change and the findings indicate that eutrophication has impacted all lake types with >50% of sites exhibiting significant floristic change. The study illustrates the role of the sediment record in determining both chemical and ecological reference conditions, and assessing deviation from the latter. Whilst restoration targets may require modification in the future to account for climate induced alterations, the long temporal perspective offered by palaeolimnology ensures that such changes are assessed against a sound baseline. [ABSTRACT FROM AUTHOR]

We describe a palaeolimnological meta-database ('LakeCores') that includes information for lakes in Europe that have been cored for studies of recent environmental change. Only cores with intact mud-water interfaces and good chronologies for the last ~100-200 years are currently included. The information provided in the database includes the general characteristics of the lakes cored, the methods used for dating and analysis and the bibliographic source of the information selected. At present the database contains information on 1847 studies from 975 sites. Here we use the database to identify cores for which diatom-based pH and total phosphorus (TP) reconstructions have been derived and, from an inspection of the relevant papers, we compile summary data for the timing and extent of acidification and eutrophication of lakes across Europe. We show that there were few cases of acidification and eutrophication in Europe prior to about 1850, and therefore that conditions prior to 1850 can be used for reference with respect to current attempts to restore surface waters to 'good ecological status'. We also show the extent to which pH and TP levels have changed across the continent both for all lakes, and, in the case of TP, for lakes of different types covering a range of alkalinities, altitudes, water depths and surface areas. [ABSTRACT FROM AUTHOR]

1. The variable ecological response of lakes to reduced nutrient loading (oligotrophication) at sites in Europe and North America was discussed at a workshop held in Silkeborg (Denmark) in January 2003. Studies of lake oligotrophication were presented based on both long-term monitoring and data generated by palaeolimnological methods. 2. This introduction to the special issue provides short summaries of a series of the papers presented and their limnological context. Results show that the majority of lakes had approached a new equilibrium of phosphorus (P) and nitrogen (N) concentrations 10–15 years (P) and 0–5 years (N) after a major reduction in loading, irrespective of hydraulic retention time. Phytoplankton biomass decreased and a shift towards meso-oligotrophic species dominance occurred. The fish responded surprisingly fast to the loading reduction in most lakes. As a result, the percentage of piscivores increased and total fish biomass declined markedly, which may explain an increase in the body size of cladocerans and an increase in the zooplankton to phytoplankton biomass ratio seen in many of the lakes. 3. Monitoring has in general been initiated after the effects of eutrophication became apparent. In this context palaeolimnological techniques become very useful because they allow limnologists to extend time scales of coverage and to define restoration targets and baseline conditions. Moreover, lake sediments pre-dating anthropogenic disturbance can be used to examine ecological response to, for instance, climate variability, allowing problems associated with multiple stressors to be addressed. 4. It is concluded that there is a great need for a synthetic, holistic approach to studying lake oligotrophication, combining multiple techniques of palaeolimnological sediment analysis with detailed but temporally limited long-term monitoring of chemical and biological variables. This is important, not least to assess future responses to nutrient loading reductions, as global warming will interact with a range of external stressors and ultimately affect lake management strategies to deal with the resultant feedbacks. [ABSTRACT FROM AUTHOR]

Baseline and historical environmental data are sparse in high-altitude regions. Diatom assemblages preserved in lake sediment records can, however, provide proxy data of past environmental and biological conditions. Sediment cores were retrieved from 209 high altitude lakes from 11 countries in Europe. Diatoms were extracted and counted from surface-sediment samples and sediments representing pre-industrial conditions. Regional changes in diatom assemblages covering at least the last c. 150 years are discussed and quantified using ordination techniques and dissimilarity indices. Distinct changes in diatom composition are identified and regional patterns highlighted, with two lake regions showing particularly large changes in diatom composition (Central Swiss Alps and the Pyrenees). Several driving mechanisms that might have caused the changes in the diatom community are discussed. Pre-industrial and present-day lake-water pH are inferred from the diatom assemblages and their prediction accuracy compared to contemporary water-chemistry measurements for each lake. Regional pH changes are minimal with no lake region exceeding 0.27 pH units of change in the period covering the last c. 150 years. The majority of lakes studied show an increase in planktonic diatom species over the period covering the past c. 150 years. We hypothesise that changes in the ratio of planktonic to non-planktonic diatoms within the study lake-regions are related to changes in climate and to associated changes in nutrients, ice-cover and erosion caused by climate warming. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The seed-like larval cases of hydroptilid caddis flies may be mistaken for plant remains in subfossil studies. However, being correctly identified, they may be important for the interpretation of former freshwater environments. Two such subfossil caddis fly remains from Holocene lake sediments and river deposits are described morphologically and their palaeoecological significance interpreted. [ABSTRACT FROM AUTHOR]