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We studied the methane (CH 4 ) 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. CH 4 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 CH 4 fluxes within the South Basin of Lake Baikal, (2) determine the exchange with the atmosphere, and (3) constrain the CH 4 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 CH 4 to the pelagic surface mixed layer was roughly estimated to be 40 Mg CH 4 . A large part of this input diffuses downwards and is consumed in the water column, with a CH 4 residence time of about 4 yr. The input of CH 4 from deep gas seeps and mud volcanoes is less than a few 10 Mg CH 4 yr 21 , most of which is oxidized before reaching the surface. The net CH 4 flux between the atmosphere and the main waterbody distant from shallow areas is negligible and not significantly different from zero. However, occasional high CH 4 concentrations, both in the surface water and in the atmosphere, indicate that the region near the Selenga delta is a local CH 4 source to the atmosphere. CH 4 fluxes in the Central Basin are very similar to those in the South Basin, whereas in the North Basin, the shallow CH 4 sources are weaker.1 Corresponding author (martin.schmid@eawag.ch). AcknowledgmentsWe thank the captains and crews of RV Vereshchagin and RV Titov for their hard work during so many surveys; A. Ageev, V. Blinov, V. Druzhinin, R. Gnatovsky, A. Salyuk, M. Schurter, and O. Vereshchagina for their invaluable support in the field work; and I. Tyryshkin and Y. Ponomarev for their assistance in the design of the methane gas analyzer. Finally, the constructive input from two reviewers helped to improve the clarity of the manuscript.This project was supported by the INTAS grant 2001-2309 (multidisciplinary study of natural gas seeps in Lake Baikal-MULTISGAS). The water temperature measurements were supported by the EU-project CONTINENT (High-resolution CONTINENTal paleoclimate record in Lake Baikal: A key-site for Eurasian teleconnections to the North Atlantic Ocean and monsoonal system; EVK2-CT-2000-00057).

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