Distribution of greenhouse gases, nitrite, and δ¹³C of dissolved inorganic carbon in Lake Biwa: Implications for hypolimnetic metabolism

Abstract

The vertical distribution of dissolved greenhouse gases (CH₄,ΣCO₂ and N₂O), NO${\text{NO}}_{\text{2}} -$,and δ¹³C of ΣCO₂ in Lake Biwa during a stagnantperiod was precisely determined. ΣCO₂ as well as NO${\text{NO}}_{\text{3}} -$was accumulated in the hypolimnion, whereas NO${\text{NO}}_{\text{2}} -$ and CH₄concen\-trations were generally higher in theepilimnion than in the hypolimnion. In August, NO${\text{NO}}_{\text{2}} -$ andCH₄ were ephemerally accumulated at the thermocline. Theconcentration of CH₄ always exceeded equilibrium with respectto air/water exchange. N₂O was rather uniformly distributed inboth time and space, and remained near equilibrium with respect toair/water exchange. All of these observations are similar to otherstratified, oligotrophic lakes, in which the hypolimnia were welloxygenated. The δ¹³C of ΣCO₂ became morenegative with increasing depth, and showed a strong negativecorrelation with apparent oxygen utilization. From the data, the δ¹³Cvalue of organic matter decomposed into ΣCO₂ inthe hypolimnion was calculated by isotope mass-balance, and found tobe in a similar range to δ¹³C of phytoplankton and benthic algaeand distinctively higher than δ¹³C of both terrestrial andsedimentary organic matters. This suggests that autochthonous organicmatter was the major source of ΣCO₂.