Recent warming in a high mountain lake (Laguna Cimera, Central Spain) inferred by means of fossil chironomids

Lake Cimera (Lat. 40°15'50'' N; Long. 05°18'15" W, Zmax = 9.4m, A = 4.5 ha, 2140 m a.s.l,) is one of the most remote and unpolluted high mountain lakes on the Sierra de Gredos (Central Spain). Intrannual and interannual variability in maximum water temperature and winter oxygen depletion can be related to climate variability (mainly air temperature), through controlling ice cover length. The extent of the oxygen depletion during ice cover period, which is related to this ice cover length, is a key factor controlling the relative abundance of chironomid (Diptera: Insecta) taxa, especially the low oxygen content adapted Chironomus sp. In this way, we have found a high negative correlation between the relative abundance of Chironomus head capsules in the sediment and the reconstructed air temperature in the last 200 years (n = 20, r = -0.75, p <0.001). The interpretation of such relationship throughout the fossil chironomid assemblage points to a recent warming (since ca mid 1980s) in Lake Cimera. The ecological interpretation of other taxa also supports this view. When applying to fossil chironomids of Lake Cimera the transfer functions developed to reconstruct summer past temperatures in the Alps, it is also well correlated with reconstructed air temperatures (n = 20, r = 0.45, p <0.01), especially when only the most accurate dating levels (top of the core, ca 75 years) are taken into account (n = 13, r = 0.75, p <0.01). However, 1) the linear regressions of both models show significantly different slopes, and 2) chironomid reconstruction underestimates in ca. 3 ºC air reconstruction. The later is probably because the fossil chironomid model has been developed for a different geographical region. Nevertheless, both models provide an independent line of evidence of a recent warming (since ca mid 1980s) in Lake Cimera. Our data also supports the use of chironomids head capsules as an effective tool to infer past temperatures.