ABSTRACT

Seasonal variations in dissolved nitrous oxide concentrations in Lake Kasumigaura were measured to examine a eutrophic shallow lake without anaerobic layer as one of natural nitrous oxide emission sources. Samples were monthly taken using a Go-Flo sampler at 6 sites including lake center during April, 1991 to March, 1992. Dissolved nitrous oxide in the lake water samples was measured by an automatic analyzer consisting of purge and trap system and GC-ECD detector. The concentrations in surface water at lake center ranged from 9 nM to 21 nM and there was no large vertical difference. On the other hand, those at the inner-most part of Takahamairi Bay, where heavy blooming of cyanobacteria occurs every summer, ranged from 13 nM in summer to 63 nM in autumn. The variation pattern that nitrous oxide concentrations in Takahamairi Bay were high in autumn to winter and low in summer was similar to that of nitrate-nitrogen concentration. The present results show that nitrous oxide concentrations in surface lake waters at all sites in this lake were supersaturated or at least equilibrated, and suggest that nitrous oxide may be being produced in many eutrophic shallow lakes, even though anaerobic layer is not formed in summer, and that they can be one of the natural nitrous oxide emission sources.