Alpine lake temperatures/ice as climate signals

High Alpine lakes are sensitive to climate impacts. Water temperatures and ice-cover are related to weather/climate. High resolution transfer functions for selected palaeo(bio)indicators and a temperature/ice-cover model (TIM) will be adopted, both based on instrumental (thermistor) water temperature measurements in a larger number of high alpine lakes. These data sets will be applied to long-term series (sediment cores), providing information about climate changes in the past, and predictions of possible impacts of an expected warming on high alpine lakes. In 1998, thermistors were exposed in 45 lakes between altitudes 1500 to 2300 m in the Niedere Tauern (Austrian Central Alps). The loggers provide continous water temperature measurements in 2-hours intervalls. The dataset is used to (1) create calibration data sets for palaeo(bio)indicators (chironomids, chrysophycean cysts, diatoms), by establishing transfer functions for lake water temperature, ice-cover, climate dependent water chemical parameters, and regional air temperature. (2) relate inferred water temperatures and ice-cover to regional air temperature, by consideration of site-specific topographic features (e.g. shading, lake morphometry, slope and exposure of the catchment) At the second step of investigations, (1) the calibration datsets and (2) the TIM-model are applied to well dated long sediment cores from two neighbouring lakes close to timberline (the same watershed area, both lakes are represented in the calibration dataset) to reconstruct Holocene climate change. Cross validation of the inferred climate signals is done by comparison of the multiproxy results of the sediment cores from both lakes investigated, and by comparison the TIM--model with a current Lapse Rates Model (LRP).

Duration of ice-cover and water temperature are important climate variables that determine water chemistry and the distribution of organisms in Alpine lakes. Alpine lake sediments provide climate archives. Diatoms (Bacillariophyceae) and resting stages of golden-algae (Chrysophyceae) are abundant in these sediments. Thermistors that were exposed during 5 years in 45 lakes of the Niedere Tauern (Austrian Central Alps) were used to calibrate these organisms as quantitative climate indicators. Mean monthly water temperatures, and dates of freezing and ice break-up were measured. Multivariate statistics revealed that the date of spring and autumn lake mixing are significant climate-driven environmental variables that determined the distribution of these organisms in the study lakes. From both variables the duration of ice-cover can be calculated. Due to linear correlations with altitude, seasonal air temperatures can be inferred. The quantitative models (transfer functions) that were part of a multi-proxy approach study were applied to two Holocene (last 11,500 years) sediment cores from Alpine lakes in the Niedere Tauern (Austrian Central Alps). First results showed a middle-Holocene warm period that is comparable with the present, preceded by a series of cold climate oscillations between 9,500 and 7,000 before present. They correspond with Alpine glacier advance, with northern Mediterranean pluvial periods, and with the 8,200 cold event detected in Greenland ice-cores. Climate and human impact on Alpine lakes overlap during the last 3500 years. The quantitative models provide appropriate tools to disentangle these interactions.