Climate-driven changes on phytoplankton–zooplankton coupling and nutrient availability in high mountain lakes of Southern Europe RAFAEL MORALES-BAQUERO, PRESENTACIO ´ N CARRILLO, JOSE ´ BAREA-ARCO, CARMEN PE ´ REZ-MARTI ´ NEZ AND MANUEL VILLAR-ARGAIZ Institute of Water Research, University of Granada, Granada, Spain SUMMARY 1. The effect of climate variability on phytoplankton and zooplankton dynamics and nutrient availability was studied in two high mountain fishless lakes (La Caldera and Rı ´o Seco) of contrasting morphology, hydrology and dissolved inorganic nitrogen : soluble reactive phosphate (DIN : SRP) ratios during 1986 and after a 10-year-long drought in 1996 and 1997. 2. Thaw was delayed and water temperatures were lower in both lakes in 1996 than in 1986 and 1997. However, the lake-specific DIN : SRP ratio was maintained in the 3 years studied, reflecting its local control. 3. On other hand, the presumptive limiting nutrient in each lake, P in La Caldera and N in Rı ´o Seco, showed higher concentrations in 1996 versus 1986 and 1997. Significant positive correlations between temperature and chlorophyll a were found in both lakes in 1996 but these relationships were negative or not significant in 1986 and 1997. Zooplankton biomass showed lower values in 1996 than in 1986 or 1997. 4. These findings can be explained by a decoupling of the phytoplankton–zooplankton interaction because of a constraint on zooplankton growth by low temperatures in the coldest year studied. This observation furnishes evidence that regional climatic control on the phytoplankton–zooplankton link can modulate the overall demand for nutrients. Keywords: climate-driven changes, mountain lakes, nutrient availability, phytoplankton–zooplankton coupling, resource limitation Introduction A key question in ecology is how shifts in climate affect aquatic ecosystems. While some ecologists have emphasised the importance of lake-specific factors (Webster et al., 1996), others have shown that ecosys- tems are subject to regional climatic effects (Baines et al., 2000). Interactions between local and regional factors and between abiotic and biotic factors create a complex lake response, making it difficult to draw general conclusions about the impact of climate change on food web structure and functioning. Key climate factors in high mountain lakes include temperature, which influences lake chemistry via abiotic and biotic activity (Sommaruga-Wo ¨ grath et al., 1997) and rainfall and length of ice-free period, which affect trophic dynamics (Park et al., 2004). Beside the effects of regional climate, mountain lakes may be particularly susceptible to local controls. Thus, Baron & Caine (2000) found that individual lake character- istics (i.e. position in the landscape) overwhelmed the regional response to climate in two geologically and morphologically similar nearby mountain basins, especially for non-conservative solutes such as nitrate. Correspondence: Rafael Morales-Baquero, Department of Ecology, Fac. Ciencias, University of Granada, 18071 Granada, Spain. E-mail: rmorales@ugr.es Freshwater Biology (2006) 51, 989–998 doi:10.1111/j.1365-2427.2006.01545.x Ó 2006 The Authors, Journal compilation Ó 2006 Blackwell Publishing Ltd 989