In summer 2003 central Europe suffered an unusually severe heat wave, with air temperatures similar to those predicted for an average summer during the late 21st century. We use a unique set of over half a century of lake data from two lakes in Switzerland to determine the effect of the 2003 heat wave on water temperature and oxygen conditions in order to assess how temperate lakes will react when exposed to the increased ambient summer air temperatures that will be encountered in a generally warmer world and to test the predictions of relevant simulation models. In both lakes, surface temperature and thermal stability in summer 2003 were the highest ever recorded, exceeding the long-term mean by more than 2.5 standard deviations. The extremely high degree of thermal stability resulted in extraordinarily strong hypolimnetic oxygen depletion. These results are consistent with the predictions of the simulation models. Additionally, the results indicate that climatic warming will increase the risk of occurrence of deep-water anoxia, thus counteracting long-term efforts that have been undertaken to ameliorate the effects of anthropogenic eutrophication.

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.

We quantified the areal hypolimnetic mineralization rate (AHM; total areal hypolimnetic oxygen depletion including the formation of reduced substances) in two formerly eutrophic lakes based on 20 yr of water-column data collected during oligotrophication. The upward diffusion of reduced substances originating from the decomposition of organic matter in the sediment was determined from pore-water profiles and related to the time of deposition. More than 80% of AHM was due to degradation of organic matter in the water column (including sediment surface) and diffusion of reduced substances from sediment layers younger than 10 yr. Sediments older than 10 yr, including the eutrophic past, accounted for , 15% of AHM. This ''old'' contribution corresponds to a 20-43% fraction of the total sediment-based AHM. The contribution from old sediment layers to AHM is expected to be even lower in lakes with deeper hypolimnia (. 12 m). In summary, oxygen consumption in stratified hypolimnia is controlled mainly by the present lake productivity. As a result, technical lake management measures, such as oxygenation, artificial mixing, or sediment dredging, cannot efficiently decrease the flux of reduced substances from the sediment.In deep stratified lakes there is generally a clear correlation between lake productivity and areal hypolimnetic oxygen depletion rate (AHOD; g O 2 m 22 d 21 ), as first shown by Hutchinson (1938). Consequently, eutrophication leads to an increase in AHOD (Rast and Lee 1978; Chapra and Canale 1991) and potentially low dissolved oxygen (DO) levels in the hypolimnion during summer stratification. Low hypolimnetic DO can in turn have a serious effect on biological processes, either via direct toxicity on fish and bottom organisms (Kalff 2002) or indirectly via toxic by-products of anaerobic mineralization (Wang and Chapman 1999). Moreover, it has been shown that hypolimnetic anoxia can enhance the sediment release of soluble reactive phosphorus (P; Larsen et al. 1981;Hupfer and Lewandowski 2008), although the DO concentration at the sediment-water interface is not the only controlling factor (Moosmann et al. 2006).As a result, reaching or maintaining a sufficient hypolimnetic DO level is often a focus of lake management. Commonly used approaches focus on (1) the reduction of external P loading (Gä chter and Wehrli 1998), (2) increased DO supply to the hypolimnion through oxygenation or aeration (Singleton and Little 2006) or through artificial mixing in winter (Mü ller and Stadelmann 2004), or (3) the removal of accumulated deposits of organic matter through sediment dredging (Annadotter et al. 1999). However, in many lakes AHOD decreased only slowly following the reduction of external P sources and the subsequent decrease in lake productivity. Such a resilience of AHOD during oligotrophication was observed in Lake Shagawa (Larsen et al. 1981) and Lake Eire (Charlton et al. 1993). A similar effect was described by Matthews and Effler (2006) for Lake Onondaga, where an abrupt decrease in anthropogeni...

The European winter of 2006–2007 was unusually mild, with record high mean winter air temperatures comparable with those predicted to become the norm by the end of the current century as a result of climate warming. In Lake Zurich and Greifensee, two neighboring Swiss perialpine lakes with several decades of data, mean lake temperatures for this winter were the highest ever recorded, as was thermal stability. Associated with the high thermal stability, mean winter oxygen concentrations in Lake Zurich were unusually high in the epilimnion and metalimnion, but normal in the hypolimnion. In Greifensee, however, which is much shallower, mean winter oxygen concentrations did not deviate substantially from the norm anywhere in the water column. From 17–19 January 2007, an unusually severe cyclonic storm, “Kyrill”, traversed Europe. Monthly oxygen profiles suggest that the stabilizing effect of the mild winter on the two lakes was greatest before the occurrence of the storm, and that wind mixing resulted in a deepening of the mixed layer in both lakes. The mixing was able to encompass the entire water column of Greifensee, but not of Lake Zurich. These results, supported by more limited data from two other neighboring lakes, suggest that climate warming will likely inhibit complete mixing of some deep, temperate, normally monomictic lakes in winter even when extremely intense cyclonic storms occur. In shallower lakes, however, complete mixing is unlikely to be inhibited.

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