Hagelseewli: a fascinating high mountain lake — suitable for palæoclimate studies?

Abstract

Summary and conclusions The Swiss high mountain lake Hagelseewli is cur-rently being sampled within the framework of theinternational MOLAR project to investigate its suit-ability as a source of proxy climate data. Preliminaryresults from 1996–1997 suggest that the water bodyis essentially decoupled from climatic forcing duringmuch of the year by ice cover, and that nearly allimportant physical, chemical, and biological pro-cesses in the lake are affected more by freezing andthawing processes than by direct climatic forcing.The undoubted sensitivity of the lake to climaticforcing is therefore indirect, being concentrated onthe periods during which freezing, and especiallythawing, take place. Due to the presence of the highcliff face to the south, the lake is shaded to a largeextent from direct solar radiation, so that the thaw-ing of the ice is slow and heterogeneous, with a longtransitional period of partial ice cover which keepslake surface temperatures low regardless of the pre-vailing air temperature. The productivity and eco-logical dynamics of the lake appear to be dominatedby the length of the ice-free season, and not – or onlyto a minor degree – by thermal stratification. Thereis evidence to suggest that the lake does not thaw atall in some years. The dominance of ice cover mayappear to call into question the usefulness of Hagel-seewli as a repository of climatic information as far asa direct association with air temperature is con-cerned. However, Hagelseewli demonstrates well thecomplexity of processes that must be taken intoaccount when attempting to link sediment data tosynoptic climate in high mountain lakes, and theimportance of understanding the role played by localtopography and by the dynamics of the freezing andthawing of the ice cover. Any climatic signalsarchived in the sediments of Hagelseewli can beexpected to reflect the length of the ice-free seasonrather than the prevailing air temperature directly.Hagelseewli was never entirely mixed during theperiod of observation, although partial mixingoccurred during the short ice-free period, when oxy-gen-rich water mixed down to about the 10-mdepth. The dynamics of the lake are at their mostenergetic during the time of ice break-up, when theinput of large amounts of ammonium, but not ofphosphate, was observed, followed by an increase inoxygen concentrations and in pH. The ammoniuminput to the water column might take place viaatmospheric deposition and snowmelt, possiblyincluding a component from the soil underlying thesnow. Dissolved phosphate in the lake water origi-nates mainly from redissolution from the sediments.During the ice-free period, algal uptake reduces dis-solved phosphate concentrations to below the limitof detection. Preliminary sediment trap results sug-gest the hypothesis that the ratio of planktonic toperiphytic diatoms may indicate periods of changingice-cover duration. Future results of sediment analy-ses will reveal whether this hypothesis is valid andwhether the ratio of planktonic to periphytic diatomspecies may represent a key proxy for use in palaeocli-mate reconstructions in lakes of this kind.