The air temperature increase is one of the features of current climate change (IPCC 2013b). Temperatures have been increasing since the beginning of the industrial revolution, and they are expected to increase between 0.3 C and 4.8 C globally, depending on the scenarios of greenhouse gas emissions, for the 2081 to 2100 period (IPCC 2013). The air temperature rise translates into a lake water temperature increase (Livingstone 2003). This warming is expected to be greater in high mountain areas (Beniston 2006). In this sense, the Pyrenees constitute an excellent case study as there are over 3,000 lakes and ponds. Past air temperatures have been reconstructed in this region at some sites using statistical extrapolations for recent centuries (Catalan et al., 2002; Agusti-Panareda & Thompson, 2002) or using lake sediments records (Pla and Catalan 2005; Tarrats et al. 2018). However, lakes in this mountain region and their variation have not been addressed as a whole, even though some research has been done relating the Pyrenean lakes with those of other mountain regions in Europe (Thompson et al. 2009). Here we go further presenting a temperature data series from 2001 to 2014, with nine years of complete summer water temperature data of high-mountain lakes and developing models which help us to understand present climate, as well as future and past climate change.

A precise knowledge of water bodies’ temperature is of great importance to study the biology of aquatic organisms. Temperature is a key environmental variable in lakes, since it accelerates biochemical reactions, and therefore, increases the rates of many biological and ecological processes including photosynthesis and respiration (Allen et al. 2005; Peck et al. 2009; Yvon-Durocher et al. 2012; Gillooly et al. 2017), organic carbon mineralization (Gudasz et al. 2010) and methane emissions (Yvon-Durocher et al. 2014), organism growth (Balcells 1956; Balcells 1957; Angelier and Angelier 1968; Honsey et al. 2018), biomass production (Ernest et al. 2003), organism size (Charnov and Gillooly 2003), and ecological processes as it influences thermal niche and species distribution (Magnuson et al. 1979), biodiversity (Allen et al. 2002; Pinel-Alloul et al. 2013), and trophic cascades (Symons and Shurin 2016).