Inter-and intra-storm oxygen-18 variations in rain, throughfall, and stemflow were measured to estimate accurate "new" water inputs to the watershed within a 0.84-ha watershed planted with 80-year-old Cryptomeria japonica and Chamaecyparis obutusa artificial forest at the Hitachi Ohta experimental watershed in Japan. In addition two-component hydrograph separation using oxygen-18 concentrations was conducted with four types of "new" water concentration: (1) incremental volume-weighted mean concentrations of rainfall; (2) constant concentrations of rainfall; (3) incremental volume-weighted mean concentrations of throughfall; and (4) constant concentrations of throughfall. Six storms from June to October 2000 were investigated. In the 26 July, 24 September, and 29 October storms, old-water percentages at the hydrograph peak were more than 56.4%, 66.6%, and 80.4%, respectively. In the 24 September and 29 October storms, the old-water contribution estimated by incremental volume-weighted mean concentrations of throughfall was the greatest. On the other hand, this was the smallest after the 26 July storm. Thus, by estimating the "new" water inputs more accurately, the "old" water contribution could be either large or small. In addition, there was a relative difference of about 5-10% between old water percentages calculated from the four types of "new" water. The uncertainty associated with the analysis of oxygen-18 concentrations was up to 4.8%. Thus the difference in estimates of "new" water affected the "old" water percentage more than the uncertainty of the two-component model itself.

Rainwater, throughfall, stream water during a non-storm runoff period, and soil water were sampled about every 10 days at six nested basins within the Hitachi Ohta Experimental Watershed in Japan from 1998 to 2001. Basins HA and HB are covered with an 80-year-old artificial forest of Japanese cypress and Japanese cedar. HC and HV are covered with 12-year-old forest of the same species. HO and HX are covered with various aged stands of the same species. The samples were analyzed for the isotopic compositions of 18 O and deuterium (D). The weighted means of δ 18 O (δD) in HA and HB were Ϫ7.35 (Ϫ46.7) and Ϫ7.38 (Ϫ46.8)‰, while they were Ϫ7.51 (Ϫ48.1) and Ϫ7.57 (n/a)‰ in HC and HV, respectively. They were Ϫ7.50 (Ϫ47.6) and Ϫ7.41 (Ϫ47.1)‰ in HO and HX, respectively. There was a relative difference of 0.2 (1.4)‰ in δ 18 O (δD) between 80-year-old and 12-year-old forest. The stream water during a non-storm runoff period is considered to reflect the effect of evaporation from the forest floor. The evaporation rates from the forest floor were estimated using δ values in throughfall and stream water using the Rayleigh distillation equation under equilibrium conditions. They were estimated to be 5.5 (9.1), 5.2 (9.0), and 4.9 (8.7)% of annual throughfall in HA, HB, and HX (mature forests), respectively, using δ 18 O (δD), and 4.0 (7.6), 4.1 (8.1), and 3.5 (n/a)% of annual throughfall in HC, HO, and HV (young forests), respectively.

Although most carabids are primarily carnivorous, some carabid species are omnivorous, with mainly granivorous feeding habits during the larval and/or adult stages (granivorous carabids). This feeding habit has been established based on laboratory and field experiments; however, our knowledge of the feeding ecology of these beetles in the field is limited owing to the lack of an appropriate methodology. In this study, we tested the utility of stable isotope analysis in investigations of the feeding ecology of granivorous carabids in the field, using two closely related syntopic species, Amara chalcites and Amara congrua. We addressed two issues concerning the feeding ecology of granivorous carabids: food niche differentiation between related syntopic species during the larval stage and the effect on adult body size of supplementing seeds with an animal diet during the larval stage. To investigate larval feeding habits, we analysed newly emerged adults, most somatic tissues of which are considered of larval origin. In the two populations examined, both δ15N and δ13C were significantly higher in A. chalcites than A. congrua, suggesting that the two species differentiate food niches, with A. chalcites larvae being more carnivorous than A. congrua larvae. The two isotope ratios of A. chalcites samples from one locality were positively correlated with body size, suggesting that more carnivorous larvae become larger adults. However, this relationship was not detected in other species/locality groups. Thus, our results were inconclusive on the issue of diet supplementation. Nevertheless, overall, these results are comparable with those of previous laboratory‐rearing experiments and demonstrate the potential utility of stable isotope analysis in field studies on the feeding ecology of granivorous carabids.

No abstract

Abstract:The Great Lake (Tonle Sap), located in mid-western Cambodia, is the largest lake in southeast Asia and joins the Mekong through the Tonle Sap River. During the annual monsoon peak from about the end of May, the flow direction in the Tonle Sap reverses and river water from the Mekong flows to the Great Lake. When the water level of the Mekong decreases sufficiently, usually in October, the Tonle Sap again flows in its normal direction from the Great Lake to the Mekong. This study quantitatively investigated interactions between the Tonle Sap River and the Mekong River using the stable isotope ratios of oxygen (υ 18 O) in waters from both rivers, and estimated the contribution rate of Tonle Sap River water to Mekong River water during the peak flow period of the Tonle Sap River. During this period, the Tonle Sap River contributed approximately 70-97% along the right bank and 0-5% in the middle and left banks of the Mekong. We also observed υ 18 O of the Mekong River along a longitudinal section on the right bank side, where the contribution rate of the Tonle Sap River water remained high (80%) even 21 km downstream from the confluence. This study clearly demonstrates that we can determine the mixing rate of these rivers at each point after they converge by using stable oxygen isotopes as tracers, when the Tonle Sap flows in its normal direction.