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The natural abundance of "$C and "&N was measured in basidiocarps of at least 115 species in 88 genera of ectomycorrhizal, wood-decomposing and litter-decomposing fungi from Japan and Malaysia. The natural abundance of "$C and "&N was also measured in leaves, litter, soil and wood from three different sites. "&N and "$C were enriched in ectomycorrhizal and wood-decomposing fungi, respectively, relative to their substrates. Ectomycorrhizal and wood-decomposing fungi could be distinguished on the basis of their δ"$C and δ"&N signatures. Although there was high variability in the isotopic composition of fungi, the following isotopeenrichment factors (ε, meanpSD) of the fungi relative to substrates were observed : ε ectomycorrhizal fungi/litter l 6.1p0.4= "&N ε ectomycorrhizal fungi/wood l 1.4p0.8= "$C ε wood-decomposing fungi/wood l k0.6p0.7= "&N ε wood-decomposing fungi/wood l 3.5p0.9= "$C The basis of isotope fractionation in C metabolism from wood to wood-decomposing fungus is discussed.

Phytoplankton, zooplankton, benthic invertebrates, and pond smelt were collected in a eutrophic lake (Lake Suwa, Japan) from spring to autumn in 1986 and 1987. Their stable carbon and nitrogen isotope ratios were analyzed together with conventional examination of the gut contents of pond smelt. Temporal variations in isotope rations were similar among the organisms other than benthic animals. The @d15N values of diet for several animals in the pelagic food web were estimated, assuming a stepwise enrichment in 15N with increasing trophic level. A calanoid copepod, Eodiaptomus japonicus, seemed to feed on the predominant phytoplankton, including Microcystis spp., while Microcystis did not seem to be important as an organic source for other zooplankton and pond smelt. The temporal variation of the isotope ratios of pond smelt was well explained by visual gut content analysis. It is suggested that intensive measurements of carbon and nitrogen isotope ratios of each species serve to effectively assess the yearly dynamics of the food web structure in eutrophic lakes when productivity and species composition of plankton change seasonally.

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.

Because of measurement difficulties, only a few studies on natural 15N abundance (δ15N) of inorganic N in forest soil have been pursued despite its importance for interpretations of plant δ15N signatures. To investigate stable N isotope ratios in inorganic N, the δ15N values and concentrations of total N, NH+4‐N, and NO−3‐N of forest mineral soils in four profiles were measured along a slope (altitude of 765–870 m) in a coniferous (Japanese red cedar, Cryptomeria japonica D. Don) forest in Japan. Generally, the δ15N values of total N, NH+4‐N, and NO−3‐N increased with increasing soil depth. The values of δ15N ranged from 1.0 to 6.8‰, 2.5 to 15.6‰, and −14.8 to 5.6‰ for total N, NH+4‐N, and NO−3‐N, respectively. Additionally, the δ15N values were different between NH+4‐N and NO−3‐N for each soil depth. Thus, it was concluded that the assumptions about inorganic N used in interpretation of plant δ15N values were valid. Moreover, on upper slope sites where soil inorganic N was predominantly NH+4‐N, the order of δ15N was generally total N > NH+4‐N > NO−3‐N for each depth, whereas the order of δ15N was NH+4‐N > total N > NO−3‐N on lower slope sites where NH+4‐N was less dominant as soil inorganic N and relatively high net nitrification rates were measured. Our results suggested that nitrification plays an important role in regulating δ15N in forest‐soil N.

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Quantitative and qualitative characterizations of dissolved organic matter (DOM) were carried out at the watershed level in central Japan by measuring dissolved organic carbon (DOC) concentration and the three-dimensional excitation-emission matrix (3-D EEM). DOC concentration was low (mean 37 ± 19 mM C) in the upstream waters, whereas, in general, it increased toward the downstream areas (mean 92 ± 47 mM C). Significant variations in DOC concentration were detected among rivers and channels. DOC concentration in the epilimnion of Lake Biwa increased during the summer period and decreased during the winter period. The lake hypolimnion has lower DOC concentration (mean 87 ± 7 mM C) compared with the epilimnion (107 ± 15 mM C). Fulvic acid (FA)-like substances in the DOM were directly characterized by 3-D EEM. The fluorescence peak for upstream DOM was found in regions with longer wavelengths (excitation/emission 386 ± 6/476 ± 5 nm) compared with downstream and lake DOM (351 ± 12/ 446 ± 15 nm and 341 ± 6/434 ± 6 nm, respectively). The DOC concentration is correlated with fluorescence peak intensity of FA-like substances in DOM in river waters. Such a relationship was not found in lake DOM. A blueshift of the fluorescence peak from upstream to lake DOM was observed. A decrease in fluorescence intensities was also detected during the summer period. These results may suggest that the degradation of FA-like substances in DOM occurs from natural solar irradiation. Protein-like fluorescence was significantly detected in the lake epilimnion during the summer period. A linear relationship between DOC concentration and protein-like fluorescence indicated that an autochthonous input of DOM gave rise to the increase in DOC concentration in the lake epilimnion during the summer. These results may suggest that the 3-D EEM can be used as a tool for the investigation of DOM dynamics at the watershed level with concurrent measurement of DOC concentration and the fluorescence properties of fulvic acidlike and protein-like substances.