The decrease in summer surf are water temperature with altitude in Swiss Alpine lakes: A comparison with air temperature lapse rates

Using miniature thermistors with integrated data loggers, the decrease in s ummer lake surface water temperature (LSWT) with increasing altitude a.s.l. was investigated in 10 Swiss Alpine lakes located between 613 m a.s.l. and 2339 m a.s.l. The LSWTs exhibit essentially the same short-term structure as regional air temperature, but are about 3 to 5 degrees C higher than the air temperature at the altitude of the lake. LSWTs decrease approximately linearly with increasing altitude at a rate slightly greater than the surfa ce air temperature lapse rate. Diel variations in LSWT are large, implying that single water temperature measurements are unlikely to be representativ e bf the mean. Local factors will affect LSWT more than they affect air tem perature, possibly resulting in severe distortion of the empirical relation ship between the two. Several implications for paleoclimate reconstruction studies result. (1) Paleolimnologically reconstructed LSWTs are likely to b e higher than the air temperatures prevailing at the altitude of the lake. (2) Lakes used for paleoclimate reconstruction should be selected to minimi ze local effects on LSWT. (3) The calibration of organism-specific quantita tive paleotemperature inference models should not be based on single water temperature measurements. (4) Consideration should be given to calibrating such models directly against air temperature rather than water temperature. (5) The primary climate effect on the aquatic biota of high-altitude lakes may be mediated by the timing of the ice cover.