Stable isotope analyses of the pelagic food web in Lake Baikal

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 delta(13)C levels of animals were close to those of pelagic phytoplankton. The delta(15)N levels of animals showed a clear trend of stepwise enrichment with trophic level according to the following equation: delta(15)N (%) = 3.3 (Trophic Level - 1) - 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. delta(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.