In this first worldwide synthesis of in situ and satellite‐derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice‐covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice‐free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

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Recent studies have highlighted the impact of the winter North Atlantic Oscillation (NAO) on water temperature, ice conditions, and spring plankton phenology in specific lakes and regions in Europe. Here, we use meta-analysis techniques to test whether 18 lakes in northern, western, and central Europe respond coherently to winter climate forcing, and to assess the persistence of the winter climate signal in physical, chemical, and biological variables during the year. A meta-analysis approach was chosen because we wished to emphasize the overall coherence pattern rather than individual lake responses. A particular strength of our approach is that time-series from each of the 18 lakes were subjected to the same robust statistical analysis covering the same 23-year period. Although the strongest overall coherence in response to the winter NAO was exhibited by lake water temperatures, a strong, coherent response was also exhibited by concentrations of soluble reactive phosphorus and soluble reactive silicate, most likely as a result of the coherent response exhibited by the spring phytoplankton bloom. Lake nitrate concentrations showed significant coherence in winter. With the exception of the cyanobacterial biomass in summer, phytoplankton biomass in all seasons was unrelated to the winter NAO. A strong coherence in the abundance of daphnids during spring can most likely be attributed to coherence in daphnid phenology. A strong coherence in the summer abundance of the cyclopoid copepods may have been related to a coherent change in their emergence from resting stages. We discuss the complex nature of the potential mechanisms that drive the observed changes.

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

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Twenty to fifty years of annual mean deepwater (hypolimnetic) temperature data from twelve deep lakes spaced across Europe (2u959W to 14u09E, 46u279 to 59u009N) show a high degree of coherence among lakes, particularly within geographic regions. Hypolimnetic temperatures vary between years but increased consistently in all lakes by about 0.1-0.2uC per decade. The observed increase was related to the weather generated by largescale climatic processes over the Atlantic. To be effective, the climatic signal from the North Atlantic Oscillation (NAO) must affect deep lakes in spring before the onset of thermal stratification. The most consistent predictor of hypolimnetic temperature is the mean NAO index for January-May (NAO J-M ), which explains 22-63% of the interannual variation in deepwater temperature in 10 of the 12 lakes. The two exceptions are remote, less windexposed alpine valley lakes. In four of the deepest lakes, the climate signal fades with depth. The projected hypolimnetic temperature increase of approximately 1uC in 100 yr, obtained using a conservative approach, seems small. Effects on mixing conditions, thermal stability, or the replenishment of oxygen to deep waters result in accumulation of nutrients, which in turn will affect the trophic status and the food web.

This is the first study using flow cytometry to characterize the population dynamics of freshwater autotrophic picoplankton (APP) over a full seasonal cycle, the goal of which was to accurately quantify and qualify the natural APP populations in relation to major environmental parameters. In particular, we wanted to test current assumptions about the seasonal succession of prokaryotic and eukaryotic picoplankton cells, including the relationship between solitary picocyanobacteria and microcolonies. Using flow cytometry, we were able to efficiently characterize the abundances of 4 lake APP assemblages in Lake Mondsee, including that of a solitary picocyanobacterial population exhibiting high 'side scatter' values. Such cells were not readily enumerated by epifluorescence microscopy. Unlike Lakes Constance and Maggiore, we found no evidence of a spring peak in solitary picocyanobacteria -we propose that the lack of a spring peak in Lake Mondsee was due to weak stratification in March-April and relatively deep vertical mixing. Since summer declines in the abundance of solitary picocyanobacteria were associated with extended periods of reduced light availability, it is likely that such declines were in part due to low relative growth rates. Finally, we argue that the formation of microcolonies by picocyanobacteria is unlikely to be a strategy for more efficient nutrient recycling (e.g. Stockner & Antia 1986). Rather, we suggest that microcolonies reach high concentrations in surface and near-surface waters due to the production of a photosynthate-rich mucilage resulting from active photosynthesis during periods of severe nutrient deficiency.