Lay summary
Lake Kivu - Learning from the past for managing its future More information at:http://www.eawag.ch/organisation/abteilungen/surf/kivu/index_ENor martin.schmid@eawag.ch; or flavio.anselmetti@eawag.ch; or alfred.wueest@eawag.ch; Wüest Alfred, Eawag, Kastanienbaum, Switzerland(1) Lead: The 485 m deep Lake Kivu (Rwanda, DR Congo) is a unique aquatic ecosystem: It contains ~60 km3 methane and ~300 km3 carbon dioxide and is permanently density-stratified by salty, carbon dioxide-rich water released by sub-aquatic springs. We aim at locating, sampling and quantifying these deep sources. The drastic changes in sedimentation, which occurred ca 1960, will be investigated for geochemical and biological evidence. The goal is to relate those changes to surface and sub-aquatic inputs, such as nutrients from the catchment.(2) Background: Lake Kivu is among the most fascinating lakes on earth. Almost 1/3 of the inflow occurs not by river tributaries, as in most lakes, but via sub-lacustrine springs. Those springs and their chemical composition affect the lake stratification. Especially, lake-internal nutrient upwelling, algae growths and the subsequent methane production in the deep waters depend on the quantity and quality of the spring flows. Over the centuries, methane has accumulated to an amount, which can be economically exploited, but which also poses a risk to the riparian ~ 2 mio people.(3) Goal: The aim of this project is to locate and sample these sub-lacustrine sources in order to quantify the inflow and the geochemical composition. The external nutrient inputs (with large errors) will be measured with better temporal resolution to calibrate a nutrient catchment model. We also want to complement the observed large changes in sedimentation of inorganic carbon and nutrients with biological variables: The recent history of daphnia and diatoms will be analyzed in ~1 m long sediment cores in order to evaluate whether plankton communities changed parallel to geochemical processes. Furthermore, analyzing a longer sediment core will show whether similar changes occurred before the past 1000 to 2000 years.The project also seeks to improve the research capacities in both countries. The partnership, established with ISP Bukavu during the previous project, will be continued. Currently, we support the Government of Rwanda towards a monitoring group, which should independently monitor methane extraction-related changes in physical, chemical and biological parameters. The present project supports such a capacity.(4) Importance: The results are of scientific interest, as Lake Kivu is among the few large lakes, where sub-aquatic springs define the functioning of the lake. It is of great importance to understand the observed sedimentation changes as a function of the anthropogenic activities (such as fish introduction, catchment, etc) and the relation to the methane production in the lake. Results are also of economic interest: To avoid building-up of the risk of a gas eruption, the two governments have decided to use the methane, worth more than $20 billion. Therefore, it is of great interest to quantify the methane production and the related water constituents (especially nutrients). With this research we envisage to integrate field observations to a previously funded SNF/SDC project on the carbon/methane cycling in Lake Kivu.