A high-resolution plant macrofossil analysis was applied to investigate wetness dynamics in a southern Finnish boreal bog, Kontolanrahka, during the last 5000 years. The chronological control and the age—depth model were based on 40 AMS radiocarbon dates. Pollen analysis provided information on regional-scale vegetation changes. Macrofossil analysis revealed prominent changes in vegetation assemblages during the late Holocene, indicating fluctuations in water-table. The reconstruction suggests that at the coring point, which nowadays is a relatively wet lawn, habitat type has repeatedly varied between transient communities similar to those currently represented in dry hummocks, very wet lawns and even hollows. In order to quantify historical changes in water-table, Generalized Additive Models (GAM) were used to investigate the current relationships between surface plant species and water-table depth. Modern water-table measurements and a survey of associated plant communities along moisture gradients provided data for GAM-analyses. The plant species showed unimodal distributions with apparent optima and narrow tolerances along the water-table gradient. A transfer function for water-table reconstruction was created by calibrating plant macrofossil records against the modern vegetation/water-table relationship using the weighted averaging partial least squares (WA-PLS) regression method. The quantitative water-table reconstruction for the late Holocene showed that the water-table depth had varied between 38 and 2.5 cm, the root mean square error of prediction being 3 cm. The detected historical wet and dry shifts were compared with other similar data from Finland, Sweden and Estonia, and from Western Europe. Despite some similarities, especially during the last c. 1000 years, noticeable differences in timing and duration occur, suggesting they may not have been driven only by climate, but also by local factors, including surface fires.

The late-glacial Bølling period was first identified by Johs. Iversen on the basis of pollen results from Lake Bølling Sø in Denmark. Because there were no radiocarbon dates from the sequence the Bølling Chronozone (12 000-13 000 14 C yr BP) was later established on the basis of dates from other sites. A new project is reinvestigating the sediments from the Bølling Sø sequence with AMS radiocarbon dating and multiproxy analyses. Here we present results of AMS radiocarbon dating, macrofossil analyses, cladoceran analyses (Cladocera concentrations and chydorid ephippia) and Pediastrum analyses (concentrations). The AMS dates on land plant remains show that the lower part of the sequence is around 12 500 14 C yr BP, and thus clearly pre-dates the Allerød chronozone. However, construction of a chronology for the sequence was problematic, partly because of reworking of macroscopic plant remains. The climate ameliorated after glacial conditions to such an extent that growth of plants could begin at ca. 12 500 14 C yr BP, but the results of multiproxy analyses show little evidence for a further warming period during the pre-Allerød part of the sequence. Lake productivity was low, and tree birch rare or maybe absent. This may reflect widespread occurrence of dead ice, unstable soils, heavy in-wash of minerogenic matter to the lake, resulting in turbid water and rapid sedimentation. The early pioneer vegetation was characterised by Salix polaris and Dryas octopetala, and by herbs. The Allerød Chronozone, and especially its initial part, appears to have been relatively warm but reduced cladoceran concentrations and increased proportion of chydorid ephippia suggest that climate cooled in the middle Allerød and that the late Allerød was colder than the early part. The early Younger Dryas was probably colder than the late Younger Dryas. Clear warming is apparent at the beginning of the Holocene, where the first macrofossil evidence of trees (Betula pubescens, Populus tremula) is found.

This study presents a new proxy method for studying past environmental change. The method is based on the relative abundance of ephippia of chydorid Cladocera (Chydoridae). Ephippia are a modification of the carapace that protects the resting eggs against, for example, freezing and desiccation. Holocene records of chydorid ephippia and other gamogenetic remains (male headshields and postabdomens, and headshields of ephippial females) from two lakes in southern Finland (Rutikka and Kaksoislammi) are presented. Results of pollen, diatom and cladoceran analyses from Rutikka Lake are also described. Results from this study suggest that chydorid ephippia are valuable indicators of the environmental stresses occurring in lakes. In particular, ephippia were abundant at both sites during the late Pleistocene/early Holocene transition when the climate was still severe and the open-water season was short. In the later-Holocene record, the relative abundance of the ephippia also reflected the environmental stress during profound changes in predator-prey relationships in Kaksoislammi Lake. In contrast, a change in the relative abundance of ephippia of only one species (Alona affinis) occurred in the late Holocene in Rutikka Lake when prehistoric cultivation took place and affected the water chemistry.

A 8400-7700 yr cal. BP section of the sediments of a small lake in southern Finland was studied by high-resolution pollen and cladoceran analyses to examine the response of terrestrial and aquatic ecosystems to the 8200 cal. yr BP cold event. Furthermore, a pollen-based T ann reconstruction and proportions of chydorid ephippia were used to determine changes in temperature and in the length of openwater season during the event. The pollen data, with abrupt decreases of the early-flowering Alnus, Corylus and Ulmus at c. 8350 cal. yr BP, may partly reflect vegetation dynamics but possibly also decreased pollen productivity and reproduction caused by cold winters and springs. The reconstructed T ann indicated a rapid temperature fall and a subsequent recovery. Proportions of chydorid ephippia suggested a considerable reduction in the length of open-water season at the beginning of the event, followed by periodically shorter open-water seasons. In the aquatic ecosystem only one species, the dominant planktonic cladoceran Bosmina longirostris, responded by decreasing dramatically and almost became extinct during the coldest period. However, we were unable to reconstruct the causative link between the inferred climate change and its decrease. The response of both aquatic and terrestrial proxies at c. 8350 cal. yr BP was consistent, indicating an abrupt start of the event and also an abrupt end at c. 8100 cal. yr BP.

Aim Spatial and temporal variations in the composition of freshwater invertebrate assemblages may be useful indicators of past climate change and help us to predict species responses to future climate change. The objective of this study was to understand the relationship between climate variables and fossil Cladocera assemblages in lake sediments along a contemporary climate gradient from hemiboreal to subarctic ecoregions in Finland and throughout the Holocene climate succession.Location Seventy-three small shallow lakes across a climate gradient from hemiboreal (60°N) to subarctic (70°N) Finland and a case study lake, Lake Arapisto, in south boreal Finland.Methods Fossil cladoceran assemblages were analysed from the surface sediments of 73 lakes to investigate their association with climate, using ordination techniques (detrended correspondence analysis and redundancy analysis) and generalized linear modelling (GLM). The long-term variation of the abundance of specific climate-sensitive cladoceran species was then analysed in a sediment core from a boreal lake and compared with a chironomid-based Holocene temperature reconstruction from an adjacent lake.Results Mean July air temperature was identified as a significant variable in explaining species distribution across the spatial climate gradient. This result, together with GLMs that recognized several climate-sensitive species, indicates that certain cladoceran taxa may be useful indicators of climate. For example, the planktonic Bosmina longirostris and the phytophilous Graptoleberis testudinaria were more abundant in the warm southern lakes, in contrast to the benthic Alona affinis and Alona intermedia, which were typical for the cold northern lakes. The surface-sediment inferences were supported by the core analysis from Lake Arapisto, because species associated with cold lakes increased in abundance during the cold early Holocene, and species associated with warm lakes thrived during the Holocene Thermal Maximum and Anthropocene. Main conclusionsThe spatio-temporal distribution of Cladocera is regulated by regional climate, and therefore the species-climate relationships in space and time can be used to interpret past, present and future distributional patterns under a changing climate.