Financial support was provided by MMA project 087/2007, CGL and FPU grant to L. Jiménez. SAMPLING AND DATING Sediment core was collected from the deepest part of the lake in September 2009 with a percussion corer. Core (16 cm) was sectioned in the field (0.5 cm) and dated by radionuclide 137 Cs and 210 Pb. CLADOCERA & DIATOMS Cladoceran analysis were performed following Szeroczyńska and Sarmaja-Korjonen (2007) methodology. For diatom analysis the methodology proposed by Battarbee et al. (2001) and PEARL ( was followed. Río Seco (37º 03’N, 3º20’W) is a small (0.4 ha), oligotrophic, and shallow (Z max =2.90 m) lake of glacial origin located at 3,020 m.a.s.l. in the Sierra Nevada Mountains (southern Spain) on siliceous bedrock basin, composed mainly by mica-schists, and above the tree-line. This fish-free lake is generally ice and snow-covered from October-November until June-July. During the ice-free period, the Secchi disk visibility exceeds water depth and it is not thermally stratified. The lake has not a clearly differentiated littoral zone but its shoreline is dominated by the bryophyte Drepanocladus fluitans and its catchment area is partially covered (~15%) by alpine meadows There are not long-term instrumental climatic data from Sierra Nevada Mountains. We used a extrapolated climatic series ( ) provided by the “Sierra Nevada Global Change Observatory” for Río Seco Lake. Long-term climatic series are available from different stations located at the South and Centre of Spain. Pearson correlations on detrended variables were used to select the climatic series with the best adjustment to our short series of Río Seco Lake. The temperature series from Madrid (AEMET 3195), available since 1920, and the precipitation series from San Fernando (Naval base), available since 1839, showed the best results. ( The North Atlantic Oscillation index used in this work was based on the principal component time series of the leading empirical orthogonal function of seasonal (December through March) sea level pressure anomalies over the Atlantic sector from 1899 to For each core interval, the average air temperature, precipitation and winter NAO index during the time of its accumulation was calculated. CLIMATIC DATA Pronounced changes are observed during the 20 th century in both cladoceran and diatom assemblages. Gradually increase PCA axis-1 scores from the mid-19 th century indicate changes in diatom species composition, with the most pronounced floristic shift occurring around the 60s. The most marked shift in cladoceran assemblages is observed in the late 80s and secondary shifts are observed in the 20s and in the 70s. The species with highest contribution to these changes were the diatoms A. alpigena, S. pinnata, S. exiguiformis and N. perminuta and the cladoceran D. pulex, A. quadrangularis and C. sphaericus. The diatom and cladoceran PCA axis-1 and all target species, except S. exiguiformis, showed highly significant correlations with temperature and lower with precipitation but only D. pulex showed a significant correlation with NAO index values. According to forward stepwise multiple regression analyses, with PCA axes-1 and each of the target species as dependent variables and temperature, precipitation and NAO index as predictor variables, temperature was the main predictor of these species changes (% variance explained >50% for all analyses) except for A. alpigena and S. pinnata which were predicted by both temperature and precipitation and for D. pulex and cladoceran PCA axis-1 which were predicted by both temperature and the NAO index (r 2 = 0.49 for PCA-axis, r 2 = 0.56 for Daphnia). S. exiguiformis was only predicted by NAO index but with r 2 = Both diatom and cladoceran assemblages and species could be notably affected by the temperature increase observed since the second half of the 20 th century. However D. pulex is the only taxon related to the NAO index. Changes in D. pulicaria relative abundance in Río Seco Lake could be partially explained by the evolution of Saharan Ca deposition, which is governed by the NAO. 1. Sierra Nevada Mountains are notably affected by Saharan dust intrusions, mainly during spring and summer. These dust intrusions are rich in Ca while Sierra Nevada lakes are low Ca and low alkalinity lakes. Most of the Ca of Río Seco Lake is explained by Saharan dust deposition (Pulido-Villena et al. 2006). 2. A high NAO index is associated to drier conditions over southern Europe and yields dust transport from Sahara desert over the western Mediterranean basin during summer (Moulin et al. 1997). 3. D. pulicaria could be limited by Ca in Río Seco Lake, whose Ca level (approx. 1.5 mgL -1 ) is in the limit for D. pulicaria reproduction (Cairns 2010). A positive correlation between Daphnia and the NAO index is also found for 10 years of Daphnia sampling in Río Seco Lake ( and ). No relationships were found between D. pulicaria abundance and temperature or precipitation.