Fluctuations of Nitrogen Isotope Ratio of Gobiid Fish (Isaza) Specimens and Sediments in Lake Biwa, Japan, during the 20th Century

Nitrogen isotope ratios (δ15N of sediment and formalin-fixed fish specimens (Isaza fish, Chaenogobius isaza, Tanaka), which had been collected and preserved over 40 yr, were used to provide a new methodology to reconstruct recent eutrophication and historical changes in the nitrogen cycle in Lake Biwa. (δ15N records of Isaza fish collected in the north basin of Lake Biwa, a lake that had been rapidly eutrophicated in the last several decades, showed a rapid increase from the early 1960s to 1980s with an overall amplitude of$>3$permil. The same trend was simultaneously observed from that of sedimentary nitrogen. We evaluated several factors controlling δ15N of Isaza fish and sediments during the 20th century and concluded that an increase of δ15N was either associated with an increase in loading nitrate with high S5N values from the watershed or enhanced denitrification in the lake.