Palaeolimnological approaches to climate change, with special regard to the biological record

Lakes systems respond physically, chemically and biologically to changes in climate and these responses are registered in many various ways in lake sediment records. This paper focuses principally on the biological record and describes recent research that aims to reconstruct (i) past variability in effective moisture, and (ii) past variability in temperature. Changes in effective moisture are manifested especially by changes in lake water level. In closed :lake basins water level change can cause significant changes in water salinity that is registered in sediments by changes an the composition of fossil assemblages, especially diatoms. In open (freshwater) lake basins changes in water level are best reconstructed from the aquatic macrofossil record and potentially from the ratio of open water to littoral species of Cladocera, ostracods and diatoms. The most promising biological method for reconstructing past temperature is chironomid analysis, and a range of new chironomid-temperature transfer functions are being developed. However, biological responses to changing temperature are usually indirect as a result of temperature effects on water-column stratification, nutrient cycling and alkalinity generation, and in most cases a correct interpretation of the climatic significance of changes in the fossil record requires an understanding of the processes that are operating in the water column. A multi-proxy, whole-lake approach is advocated as the best way forward, not only as a means of reconstructing past climate but also as a means of assessing the impact of climate change on lake systems (C) 1999 Elsevier Science Ltd. All rights reserved.