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Four biotic proxies (plant macrofossils, pollen, chironomids and diatoms) are employed to quantitatively reconstruct variations in mean July air temperatures ( Tjul) at Lake Loitsana (northern Finland) during the Holocene. The aim is to evaluate the robustness and biases in these temperature reconstructions and to compare the timing of highest Tjul in the individual reconstructions. The reconstructed Tjul values are evaluated in relation to local-scale/site-specific processes associated with the Holocene lake development at Loitsana as these factors have been shown to significantly influence the fossil assemblages found in the Lake Loitsana sediments. While pollen-based temperatures follow the classical trend of gradually increasing early-Holocene Tjul with a mid-Holocene maximum, the aquatic/wetland assemblages reconstruct higher-than-present Tjul already during the early Holocene, that is, at the peak of summer insolation. The relatively low early-Holocene July temperatures recorded by the pollen are the result of site-specific factors possibly combined with a delayed response of the terrestrial ecosystem compared with the aquatic ecosystem. Our study shows that all reconstructions are influenced at least to some extent by local factors. This finding stresses the need to evaluate quantitatively reconstructed climate values against local lake development and highlights the benefit of using multi-proxy data in Holocene climate reconstructions.

A lake sediment record from the north-eastern European Russian Arctic was examined using palaeolimnological methods, including subfossil chironomid and diatom analysis. The objective of this study is to disentangle environmental history of the lake and climate variability during the past 2000 years. The sediment profile was divided into two main sections following changes in the lithology, separating the limno-telmatic phase between ~2000 and 1200 cal. yr BP and the lacustrine phase between ~1200 cal. yr BP and the present. Owing to the large proportion of semi-terrestrial chironomids and poor modern analogues, a reliable chironomid-based temperature reconstruction for the limno-telmatic phase was not possible. However, the lacustrine phase showed gradually cooling climate conditions from ~1200 cal. yr BP until ~700 cal. yr BP. The increase in stream chironomids within this sediment section indicates that this period may also have had increased precipitation that caused the adjacent river to overflow, subsequently transporting chironomids to the lacustrine basin. After a short-lived warm phase at ~700 cal. yr BP, the climate again cooled, and a progressive climate warming trend was evident from the most recent sediment samples, where the biological assemblages seem to have experienced an eutrophication-like response to climate warming. The temperature reconstruction showed more similarities with the climate development in the Siberian side of the Urals than with northern Europe. This study provides a characteristic archive of arctic lake ontogeny and a valuable temperature record from a remote climate-sensitive area of northern Russia.

We used sedimentary records of Cladocera and chydorid (Chydoridae) ephippia to reconstruct Holocene climate trends from Lake Arapisto in southern Finland. The quantitative temperature record ( TJulyCla) inferred from subfossil Cladocera was compared with a previously published pollen-based mean annual temperature reconstruction ( TAnn) from the same lake. Furthermore, proportions of total chydorid ephippia (TCE), indicating proportions of asexual and sexual reproduction, were examined from the sediment core to provide supplementary data. The TJulyCla record showed a consistent trend of slowly rising July temperatures (from 12–13°C to ~15°C) during the early Holocene until ~9500 cal. BP and indicated slightly higher temperatures than prior (~16°C) during the Holocene thermal maximum (HTM) at ~7000–4000 cal. BP. The TCE record was strongly similar to the TJulyCla reconstruction during the Holocene. It suggested that chydorids would have reproduced predominantly sexually (higher TCE) under environmentally unfavorable periods (early Holocene) and some other occasions, and used mainly asexual reproduction mode under periods of stable environmental conditions (HTM). The monotony of the Cladocera-based reconstruction, when compared with the previously available pollen-based TAnn record and other reference material from the adjacent region, suggests that it was partly hampered by other forcing factors more important than temperature, such as major food-web changes or adaptation ability of cladoceran species.

This study aimed to distinguish natural and anthropogenic environmental changes, mainly in climate and nutrient conditions, during the past c. 700 years in Lake Hampträsk, southern Finland. We used sedimentary assemblages of aquatic organisms and physical properties of the sediment as proxies for the past environmental conditions. The results of diatom-inferred phosphorus reconstruction indicated that the lake was already meso-eutrophic at c. AD 1400, possibly because the preceding cultivation of the area had increased the lake’s nutrient condition. Chironomid-inferred temperatures indicated favourable climatic conditions at the end of the ‘Medieval Warm Period’, but the temperatures steadily decreased until a significant drop in the values occurred at c. AD 1700, representing the coldest period of the ‘Little Ice Age’ (LIA) in southern Finland. This cold period was illustrated by major changes in the lake’s ecosystem and physical environment. After the LIA, the chironomid-inferred temperatures increased, as expected in the light of modern observations. However, the diatom-inferred phosphorus showed a decreasing trend, which is in contrast to the measured phosphorus values that imply a currently eutrophic condition. The reason for the underestimation may be the predominance of periphytic taxa that are assigned low TP optima in the inference model.