Limnol. Oceanogr., 44(3), 1999, 502–511

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 d13C levels of animals were close to those of pelagic phytoplankton. The d15N levels of animals showed a clear trend of stepwise enrichment with trophic level according to the following equation: d15N (‰) 5 3.3 (Trophic Level 2 1) 2 3.8. 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. d15N 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 (52–568N, 104–1108E) 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 km2. 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 km3). 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,