Horizontal and Vertical Isotopic Model of Lake Biwa Ecosystem

Abstract

Nitrogen and carbon isotope ratios of organisms were measured in order to elucidate and illustrate the food web structure in Lake Biwa. Samplings of particulate organic matter (POM), zooplankton, and fish were performed at the north basin from 1993 to 1994. The food web structure of Lake Biwa (north basin) was divided into five parts judging from carbon and nitrogen isotope ratios. A clear trophic effect of ca. 3‰ was found as δ¹⁵N for the pelagic food chain in the following order : small POM (2.7 to 40 μm) →Eodiaptomus japonicus → Plecoglossus altivelis altivelis, Hypomesus transpacificus → Salmo masou macrostomus, and Silurus biwaensis (euphotic zone food chain - 1). This was considered the major food chain and was manifested in the surface layer. The other food chains in the euphotic zone, hypolimnion, lake bottom and coastal zone were described as follows ; POM with low δ¹⁵N → Daphnia galeata (euphotic zone food chain -2), sinking large POM (40 to 150 μm) and organic matter in mud →Jesogammarus annandalei (hypolimion food chain), organic matter in mud → oligocheata (lake bottom food chain), and attached algae →Sarcocheilichthys variegatus and Rhinogobius sp. (coastal zone food chain). The above mentioned food web structure seems to reflect physical environment at factors such as vertical and horizontal water movements, the sinking of particulate organic matter, and the geographical setting of Lake Biwa.