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 L ake Baikal is simple and consists of five major ecological groups: phytopla nkton (Aulacoseira baicalensis), mesozooplankton (Epischura baicalensis), m acrozooplankton amphipod (Macrohectopus branickii), fish (Coregonus autumna lis migratorius and four species of cottoid fishes), and seal (Phoca sibiri ca). Because of the low diversity and consequently small number of possible diets for each species, we were able to quantitatively estimate the diet c omposition of each animal with stable isotopes. Our carbon isotope data ind icated that pelagic phytoplankton are the primary carbon source of the pela gic food web because delta(13)C levels of animals were close to those of pe lagic phytoplankton. The delta(15)N levels of animals showed a clear trend of stepwise enrichment with trophic level according to the following equati on: delta(15)N (%) = 3.3 (Trophic Level - 1) - 3.8. In addition to interspe cific food web analysis, important pelagic animals, such as M. branickii, t wo species of pelagic sculpin, C. autumnalis migratorius, and P. sibirica, were also examined, with emphasis on ontogenic diet changes. delta(15)N lev els of M. branickii and sculpins increased with body length, suggesting a c hange in feeding habits during growth. We demonstrate that carbon and nitro gen stable isotopes can be successfully applied to elucidate trophic relati onships and conclude that the pelagic food web of Lake Baikal has an ideal, isotopically ordered structure.