Stable isotope ratios of various organisms were analyzed to elucidate food web structure in the pelagic zone of Lake Baikal. The pelagic food web of Lake Baikal is simple and consists of five major ecological groups: phytoplankton (Aulacoseira baicalensis), mesozooplankton (Epischura baicalensis), macrozooplankton amphipod (Macrohectopus branickii), fish (Coregonus autumnalis migratorius and four species of cottoid fishes), and seal (Phoca sibirica). Because of the low diversity and consequently small number of possible diets for each species, we were able to quantitatively estimate the diet composition of each animal with stable isotopes. Our carbon isotope data indicated that pelagic phytoplankton are the primary carbon source of the pelagic food web because ␦ 13 C levels of animals were close to those of pelagic phytoplankton. The ␦ 15 N levels of animals showed a clear trend of stepwise enrichment with trophic level according to the following equation:8. In addition to interspecific food web analysis, important pelagic animals, such as M. branickii, two species of pelagic sculpin, C. autumnalis migratorius, and P. sibirica, were also examined, with emphasis on ontogenic diet changes. ␦ 15 N levels of M. branickii and sculpins increased with body length, suggesting a change in feeding habits during growth. We demonstrate that carbon and nitrogen stable isotopes can be successfully applied to elucidate trophic relationships and conclude that the pelagic food web of Lake Baikal has an ideal, isotopically ordered structure. Lake Baikal is located in the central part of southern Siberia (52-56ЊN, 104-110ЊE) at an altitude of 455.6 m above sea level. The lake is 635 km long and 80 km across at its widest point, covering an area of 31,500 km 2 . Lake Baikal is endowed with many unique characteristics. First, the lake is the oldest in the world (20-25 million yr). Second, it is the deepest lake in the world, with a maximum depth of 1,637 m (Stewart 1990) and more than 80% of its area exceeding 250 m (Kozhov 1963). Lake Baikal contains as much as 20% of the world's freshwater (23,000 km 3 ). As a result, Afanasyev (1960) reported that the residence time of water and major ions in the lake is about 330 yr. Lake Baikal is dimictic, with the surface water column turning over twice a year (Votintsev 1985). The surface layer, at least above 400 m, is known to be well mixed in May and October. Based on the vertical distribution of chlorofluorocarbons, AcknowledgmentsThe authors greatly thank Michael A. Grachev, Director of the Limnological Institute of the Russian Academy of Science, for his invitation to Lake Baikal. The authors also thank Jim Elser for reviewing the paper; Y. Yamada for various help, including collaboration in the research expedition; and N. Miyazaki for use of his equipment to sample the seals. We are grateful to all staff members and students of the Center for Ecological Research, Kyoto University, and the Limnological Institute, Siberian Division, Russian Academy of Sciences.

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The under-ice development of the meroplanktonic diatom Aulacoseira baicalensis was studied, using underwater video recording. This is an ice alga, which not only succeeds in vegetating under the ice, but also reproduces intensively in interstitial water-filled spaces inside the ice. This report yields first results of the underwater filming done during the under-ice period of 2000. Our studies have shown that the A. baicalensis aggregations on the ice bottom can be easily destroyed by water currents and drift away as large flakes up to tens of centimetres in size. These macro-aggregations are capable of free-floating in the water-column for some time, gradually sinking to the bottom. Cells in the aggregations continue to divide, and they cover objects on the bottom with peculiar mucous coats. These coats act as a major food resource for bottom invertebrates.