Isotopic Characterization of Lake Kizaki and Lake Suwa

Abstract

Carbon and nitrogen isotope ratios in sedimentary organics and δ15N of NO3 and dissolved organic nitrogen are reported for two subalpine lakes in order to establish an isotopical and biogeochemical framework characterizing lake ecosystems. A two source model is applicable to evaluate the contribution of land-derived organics to the sedimentary organic matters of Lake Kizaki. A kind of chromatographic sedimetation of the land derived particles with different specific gravity was suggested from the river mouth to the mid-lake. The size distributions of the isotope ratios support this inference. Denitrification and the associated oxidation-reduction reaction seemed to enhance the 15N content of sedimentary organics during epidiagenesis of the sedimentary organic matters. The isotopic compositions of the sedimentary organics and the soluble nitrogenous compounds can be useful indicators to characterize a biogeochemical structure of a lake system. Significant increase in δ13C of sedimentary organics by 3% and in δ15 of NO3- and DON by 2 to 7%o were found under eutrophication caused by human impact, in the highly eutrophic lake, Lake Suwa. External loading of nutrient salts by human activities and its associated heavy water bloom of Microcystis are responsible for these phenomena. © 1988, The Japanese Society of Limnology. All rights reserved.