129 Journal of Paleolimnology 28: 129–145, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. Lake Redó ecosystem response to an increasing warming in the Pyrenees during the twentieth century J. Catalan 1,* , S. Pla 1 , M. Rieradevall 1 , M. Felip 1 , M. Ventura 1 , T. Buchaca 1 , L. Camarero 1 , A. Brancelj 2 , P.G. Appleby 3 , A. Lami 4 , J.A. Grytnes 5 , A. Agustí-Panareda 6 & R. Thompson 6 1 Department of Ecology, University of Barcelona, Diagonal 645, 08028 Barcelona, Spain (E-mail: catalan@bio.ub.es) 2 Laboratory for Freshwater and Terrestrial Ecosystems Research, National Institute of Biology. Vecna pot 111. Ljubljana 1000, Slovenia 3 Department of Mathematical Sciences, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK 4 CNR – Ist. Ital. Idrobiol, Largo Tonolli, 50, Verbania-Pallanza, Italy 5 Botanical Institute, University of Bergen, Allégaten 41, N-5007 Bergen, Norway 6 Department of Geology and Geophysics, University of Edinburgh, West Main Road, Edinburgh EH9 3JW, Scotland, UK *Present address: CEAB-CSIC, Accés Cala St. Francesc 14, 17300 Blanes, Spain Received 6 December 1999; accepted 9 January 2002 Key words: climate change, alpine lake, paleolimnology, lake sediments, fossil pigments, diatoms, cladocera, chironomids, chrysophytes, long-term lake dynamics Abstract The ecosystem response of Lake Redó (Central Pyrenees) to fluctuations in seasonal air temperature during the last two centuries was investigated by comparison of reconstructed air temperatures with the sediment record. Fine slicing allowed a resolution of 3–6 years according to the 210 Pb dating, although it was still difficult to easily inves- tigate the response to air temperature forcing, since extreme fluctuations in temperature occur on interannual time- scales. However, the resolution was sufficient to show responses on decadal and century scales. An overall tendency to warming in mean annual temperature in the Central Pyrenees has been caused by summer and in particular by autumn increases. Many of the measured sediment variables apparently responded to these long term trends, but the significance of the relationships was highly conditioned by the structure of the data. The variables responding most on the finer time scales were the microfossils. For diatoms, chironomids and chrysophytes the main variabil- ity correlated to summer and to autumn temperatures. For two planktonic species, Fragilaria nanana and Cyclotella pseudostelligera, we found a link of their variability with temperature fluctuations in their growing months (Sep- tember and October, respectively). This relationship appeared at a certain point during a general warming trend, indicating a threshold in the response. On the other hand, no significant changes in the dominant species could be linked to temperature, nor in any significant subgroup of the 180 diatom species present in the core. In contrast, for most chironomids (particularly Paratanytarsus austriacus, Heterotrissocladius marcidus and Micropsectra radialis) a negative relationship with summer temperature extended throughout the studied period. This response of the whole group gives chironomids a more robust role as indicators for recording temperature changes on long time-scales (e.g., through the Holocene) and for lake signal inter-comparison. Finally, our results indicated that, in all cases, there was a significant resilience to high frequency changes and hysteresis despite extreme fluctuations. Although we were dealing with organisms with one or many generations per year, their populations seemed to follow the decadal trends in air temperature. Mountain Lake Research MOLAR This is the ninth of 11 papers published in this special issue on the palaeolimnology of remote mountain lakes in Europe resulting from the MOLAR project funded by the European Union. The guest editor was Richard W. Battarbee.