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.

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.

SUMMARY 1. To illustrate advances made in biomanipulation research during the last decade, seven main topics that emerged after the first biomanipulation conference in 1989 are discussed in relation to the papers included in this special issue and the general literature. 2. The substantially higher success rates of biomanipulations in shallow as opposed to stratified lakes can be attributed to several positive feedback mechanisms relating mainly to the recovery of submerged macrophytes. 3. The role of both nutrient loading and in‐lake concentrations in predicting the success of biomanipulations is emphasised and supported by empirically defined threshold values. Nutrient recycling by aquatic organisms (such as fish) can contribute to the bottom‐up effects on lake food webs, although the degree can vary greatly among lakes. 4. Ontogenetic niche shifts and size‐structured interactions particularly of fish populations add to the complexity of lake food webs and make scientifically sound predictions of biomanipulation success more difficult than was previously envisaged. 5. Consideration of appropriate temporal and spatial scales in biomanipulation research is crucial to understanding food web effects induced by changes in fish communities. This topic needs to be further developed. 6. An appropriate balance between piscivorous, planktivorous and benthivorous fishes is required for long‐lasting success of biomanipulations. Recommended proportions and absolute densities of piscivorous fish are currently based on data from only a few biomanipulation experiments and need to be corroborated by additional and quantitative assessments of energy flow through lake food webs. 7. Biomanipulation effects in stratified lakes can be sustained in the long term only by continued interventions. Alternate stable states of food web composition probably exist only in shallow lakes, but even here repeated interventions may be needed as long as nutrient inputs remain high. 8. Biomanipulation is increasingly used as a lake restoration technique by considering the needs of all lake users (sustainability approach). The combination of water quality management and fisheries management for piscivores with positive effects for both appears to be particularly promising. 9. Biomanipulation research has contributed substantially to progress in understanding complex lake food webs, which should in turn promote a higher success rate of future whole‐lake biomanipulations.

Evolutionary hypotheses for diel vertical migrations (DVM) of aquatic animals include foraging opportunity, predator avoidance, and bioenergetics efficiency. Here we test which hypothesis predicts DVM in the small planktivorous coregonids vendace, Coregonus albula, and Fontane cisco, Coregonus fontanae, in a deep oligotrophic lake. Densities and population depths of young-of-the-year and larger coregonids were determined by hydroacoustics during day and night over 10 consecutive months. Depth distributions of predator-like fishes and zooplankton resources were recorded as well. Furthermore, Secchi depth, water temperature, oxygen concentrations, and pH values were determined at each sampling month. A DVM of the coregonids was observed in all months. Population depths during the night were significantly correlated to water temperatures, oxygen concentrations, and pH values. In contrast, the vertical distributions of predators or resources were not correlated with the coregonid depth distribution. These results do not correspond to the feeding opportunity or predator avoidance hypotheses of DVM, but support in part the bioenergetics efficiency hypothesis. However, the stable migration pattern of fishes over all months despite substantial changes in biotic and abiotic conditions suggests that diel migrations in the coregonids are a genetically fixed behavioral trait to minimize the anticipated potential predation risk in the illuminated water layers during daytime.

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Dedicated to Prof. Ju¨rgen Benndorf on the occasion of his 65th birthday. AbstractChlorophyll a (chla) concentration was evaluated as a predictor of phytoplankton biomass across a broad trophic gradient of lakes (oligotrophic -highly eutrophic). First, a literature survey was conducted to collect information on the proportion of chla in phytoplankton biomass. As a result of this study (n ¼ 21) a mean value of 0.505%70.197 S.D. chla per unit wet weight of phytoplankton was calculated. Second, analyses were performed on 756 paired measurements from an unpublished database where the specific chla content of phytoplankton biomass was regressed against phytoplankton standing stocks and chla concentration. Within an interval of 0.1-50 g m À3 of phytoplankton wet weight, a substantial decrease in chla proportion from approximately 2.5% to 0.18% was found. Likewise, the proportion in phytoplankton wet weight decreased from 0.7% to 0.15% across a chla concentration interval of 0.001-0.150 g m À3 . These results had a significant impact both on chla-based biomass calculations and the subsequent comparison with phytoplankton biomasses resulting from microscopic counts. Assuming the microscopic method was a measure of the ''true'' phytoplankton standing stocks, then the precision by which phytoplankton biomass might be predicted based on chla measurements is clearly better when using variable proportions as compared to a constant conversion factor. The same holds for temporal coherence between annual records of phytoplankton biomass. The temporal fit was apparently better when relating the results of microscopic counts and biomass estimation based on variable proportions of chla in phytoplankton biomass. Nevertheless, this effect diminished as the tropic status of the lakes increased. Because of their variable specific chla content, separate taxonomic groups of phytoplankton differently affected the proportion of chla in total ARTICLE IN PRESS www.elsevier.de/limno 0075-9511/$ -see front matter (P. Kasprzak). phytoplankton wet weight. Chlorophyceae, Cryptophyceae and cyanobacteria had a high impact, while Bacillariophyceae, Dinophyceae and Chrysophyceae were of lesser importance.

Recent studies have indicated that temporal mismatches between interacting populations may be caused by consequences of global warming, for example rising spring temperatures. However, little is known about the impact of spatial temperature gradients, their vulnerability to global warming, and their importance for interacting populations. Here, we studied the vertical distribution of two planktivorous fish species (Coregonus spp.) and their zooplankton prey in the deep, oligotrophic Lake Stechlin (Germany). The night-time vertical centre of gravity both of the fish populations and of two of their prey groups, daphnids and copepods, were significantly correlated to the seasonally varying water temperature between March and December 2005. During the warmer months, fish and zooplankton occurred closer to the surface of the lake and experienced higher temperatures. The Coregonus populations differed significantly in their centre of gravity; hence, also, the temperature experienced by the populations was different. Likewise, daphnids and copepods occurred in different water depths and hence experienced different temperatures at least during the summer months. We conclude that any changes in the vertical temperature gradient of the lake as a result of potential future global warming may impact the two fish populations differently, and may shape interaction strength and timing between fish and their zooplankton prey.

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