Abstract
Lake Fukami-ike is a small eutrophic lake located in central Japan, which stratifies from March to October. This lake has a maximum depth of 7.75 m and an anoxic condition prevails below 4-5 m depths. Vertical distributions of inorganic nitrogen and nitrous oxide and their seasonal and diel changes were studied.
Nitrate supply through precipitation remarkably increased nitrous oxide concentration which accompanied decreases in nitrate and nitrite concentrations in the range between the surface water and the immediately upper layer of oxic and anoxic zones from 21 to 22 June 2003. This phenomenon may have resulted from the denitrification process. Nitrate and nitrous oxide decreased by 15.6 mg at N m -2 (6 h) -1 and 1.09 mg at N m -2 (6 h) -1, respectively. The denitrification seems to have occurred within a very short term (diel change).
Nitrate concentrations throughout the year ranged from 0.571 to 39 mg at N l -3 in the surface water. Increases in nitrite concentration in correspondence with decreasing nitrate concentration were often observed when denitrification in the oxic and anoxic layers took place. Ammonium amounts during the stratification periods were high in the hypolimnion, the maximum value being 696 mg at N m -3 on 10 May 2003.
