Recent trends in summer surface temperatures of many lakes exceed the corresponding air temperature trends. This disagrees with expectations from lake surface heat budgets, which predict that lake surface temperatures should increase by 75–90% of the increase in air temperatures. Here we investigate the causes for this excess warming for Lower Lake Zurich, a representative deep and stratified Central European lake, by a combined data analysis and modeling approach. Lake temperatures are simulated using a one-dimensional vertical model driven by 33 years of homogenized meteorological data. The model is calibrated and validated using an equally long time series of monthly water temperature profiles. The effects of individual forcing parameters are investigated by scenarios where the trends of single variables are retained while those of all other forcing variables are removed. The results show that ∼60% of the observed warming of spring and summer lake surface temperatures were caused by increased air temperature and ∼40% by increased solar radiation. The effects of the trends of all other forcing variables were small. Following projections of climate models, the increasing trends in solar radiation, and consequently the excess warming of lake surface temperatures, are not likely to continue in the future.

Warm and cold subaquatic groundwater discharge into Lake Kivu forms the large-scale density gradients presently observed in the lake. This structure is pertinent to maintaining the stratification that locks the high volume of gases in the deepwater. Our research presents the first characterisation of these inflows. Temperature and conductivity profiling was conducted from January 2010 to March 2013 to map the locations of groundwater discharge. Water samples were obtained within the lake at the locations of the greatest temperature anomalies observed from the background lake-profile. The isotopic and chemical signatures of the groundwater were applied to assess how these inflows contribute to the overall stratification. It is inferred that Lake Kivu’s deepwater has not been completely recharged by the groundwater inflows since its turnover that is speculated to have occurred within the last ~1000 yrs. Given a recent salinity increase in the lake constrained to within months of seismic activity measured beneath the basin, it is plausible that increased hydrothermal-groundwater inflows into the deep basin are correlated with episodic geologic events. These results invalidate the simple two-component end-member mixing regime that has been postulated up to now, and indicate the importance of monitoring this potentially explosive lake.

Lake Baikal, with a depth of 1637 m, is characterized by deep-water intrusions that bridge the near-surface layer to the hypolimnion. These episodic events transfer heat and oxygen over large vertical scales and maintain the permanent temperature stratified deep-water status of the lake. Here we evaluate a series of intrusion events that reached the bottom of the lake in terms of the stratification and the wind conditions under which they occurred and provide a new insight into the triggering mechanisms. We make use of long-term temperature and current meter data (2000–2013) recorded in the South Basin of the lake combined with wind data produced with a regional downscaling of the global NCEP-RA1 reanalysis product. A total of 13 events were observed during which near-surface cold water reached the bottom of the South Basin at 1350 m depth. We found that the triggering mechanism of the events is related to the time of the year that they take place. We categorized the events in three groups: (1) winter events, observed shortly before the complete ice cover of the lake that are triggered by Ekman coastal downwelling, (2) under-ice events, and (3) spring events, that show no correlation to the wind conditions and are possibly connected to the increased spring outflow of the Selenga River.

We estimate the effects of climatic changes, as predicted by six climate models, on lake surface temperatures on a global scale, using the lake surface equilibrium temperature as a proxy. We evaluate interactions between different forcing variables, the sensitivity of lake surface temperatures to these variables, as well as differences between climate zones. Lake surface equilibrium temperatures are predicted to increase by 70 to 85 % of the increase in air temperatures. On average, air temperature is the main driver for changes in lake surface temperatures, and its effect is reduced by ~10 % by changes in other meteorological variables. However, the contribution of these other variables to the variance is ~40 % of that of air temperature, and their effects can be important at specific locations. The warming increases the importance of longwave radiation and evaporation for the lake surface heat balance compared to shortwave radiation and convective heat fluxes. We discuss the consequences of our findings for the design and evaluation of different types of studies on climate change effects on lakes.

In the heart of Africa, a unique lake has attracted the attention of scientists since the beginning of the 20^th century. At the foot of the Virunga volcano chain, Lake Kivu harbors a vast amount of dissolved carbon dioxide and methane, making it the most dangerous lake on Earth. But the lake also furnishes many goods and services for surrounding populations and may soon become the most important energy supplier in the area. At the beginning of gas exploitation, the time has come to gather the wealth of scientific information acquired during past and present research on Lake Kivu. The eleven chapters cover many aspects of the physics, geochemistry and biology of the lake, with a particular focus on the unique physical and geochemical features of the water column and on the ecological functioning of the surface waters. The impacts of the introduced fish species and the potential impacts of methane exploitation are also summarized. This multi-disciplinary book may also be used as an introduction to the limnology and biogeochemistry of large tropical lakes, as it covers various aspects of the physics, geochemistry, biology and ecology of the African Great Rift lakes.

We studied the methane (CH₄) budget of Lake Baikal, the most voluminous lake in the world and the only freshwater body with known occurrences of methane hydrates in the sediments. CH₄ concentrations were measured in water samples taken during six expeditions between October 2002 and June 2004; these expeditions covered the entire lake volume. A one-dimensional model was applied to (1) estimate the large-scale vertical CH₄ fluxes within the South Basin of Lake Baikal, (2) determine the exchange with the atmosphere, and (3) constrain the CH₄ inputs from seeps and mud volcanoes to the deep water. Fluxes were generally several orders of magnitude below previous estimates. The annual internal source of CH₄ to the pelagic surface mixed layer was roughly estimated to be 40 Mg CH₄. A large part of this input diffuses downwards and is consumed in the water column, with a CH₄ residence time of about 4 yr. The input of CH₄ from deep gas seeps and mud volcanoes is less than a few 10 Mg CH₄ yr^-1, most of which is oxidized before reaching the surface. The net CH4 flux between the atmosphere and the main waterbody distant from shallow areas is negligible and not significantly different from zero. However, occasional high CH₄ concentrations, both in the surface water and in the atmosphere, indicate that the region near the Selenga delta is a local CH₄ source to the atmosphere. CH₄ fluxes in the Central Basin are very similar to those in the South Basin, whereas in the North Basin, the shallow CH₄ sources are weaker.

The deep waters of the East African Rift Lake Kivu contain large amounts of dissolved carbon dioxide and methane. The release of a fraction of these gases, which could be triggered by a magma eruption within the lake, would have catastrophic consequences for the two million people living on its shore. Up to now the safety assessment of the lake was based on the assumption that the gas concentrations in the deep waters are in a steady state with a residence time of 400 years. Turbulent transport was regarded as the main pathway of vertical exchange. Recent measurements and the analysis of the vertical transport processes in the lake radically change this evaluation. The vertical turbulent exchange is negligible, as documented by a spectacular set of several hundred double-diffusive layers. Gases are mainly transported out of the deep zones by a slow upwelling with a residence time of 800-1000 years. Our results indicate that the methane production within the sediment has recently increased, leading to a gas accumulation in the deep waters and consequently decreasing the heat input needed to trigger a devastating gas release. With the estimated current CH₄ production, the gas concentrations could approach saturation within this century.