Diatoms as quantitative indicators of July temperature: a validation attempt at century-scale with meteorological data from northern Sweden

Abstract

This study evaluates the ability of diatoms to estimate past changes in temperature by directly comparing diatom-based inferences of mean July air temperature (July T) with measured values of century-long meteorological records. The comparison includes sediment cores from three lakes in northern Sweden (Alanen Laanijärvi, 365 m above sea level (a.s.l.), Lake 850, 850 m a.s.l. and Lake Njulla, 999 m a.s.l.) dated by radioisotopic methods ( 210Pb, 137Cs) and records from meteorological stations in Kiruna and Abisko. In the sediment cores of all three study lakes, benthic species dominate the diatom assemblages with percent abundance ranging from 60 to 80%. Over the past century, the diatom-based quantitative July T inferences (weighted averaging partial least squares regression and calibration) based on a regional calibration set correspond in general closely with the meteorological records. In addition, the proportion of planktonic diatoms in the stratigraphy reflects the meteorological record as well. For example, in Alanen Laanijärvi, the proportion of planktonic diatoms (e.g. Cyclotella comensis) has increased markedly since 1985, coincident with a marked increase of measured July T (>1°C) at the meteorological station in nearby Kiruna. In contrast, July T in Abisko has remained relatively constant since 1985 and relatively little changes are observed in diatom assemblages in the two nearby lakes, Lake 850 and Lake Njulla. Interestingly, periods of relatively weak correspondence between measured and inferred July T (at Lake 850 from 1910 to 1940, at Lake Njulla from 1955 to 1990) correspond with periods when diatom-inferred lake-water pH shows distinct trends of lake-water pH change. Because lake-water pH is a stronger factor influencing diatom community composition than July T, the accuracy of diatom-inferred July T appears to be reduced during periods when lake-water pH fluctuates. Overall, we conclude that diatoms can provide reliable estimates of July T over the past century from sediment samples in northern Sweden and the reliability of the inferences further increases when lake-water pH remains constant. One palaeoecological implication based on these findings is that the early Holocene is likely a problematic period for estimating July T from diatoms, because natural processes such as soil development, catchment stabilisation and weathering of bedrock resulted in a marked natural decline of lake-water pH. Similarly, short-term stochastic events, such as sediment in-wash due to erosional events, forest clearance and forest fires, also may lead to inaccurate diatom–temperature inferences through changes in lake-water pH. In order to identify periods that may affect the accuracy of diatom-based July T inferences, we suggest that diatom analyses should be combined with other palaeoecological proxy indicators (e.g. pollen, chironomids).