Nitrogen elimination in two deep eutrophic lakes

Nitrogen elimination was investigated in two eutrophic Swiss lakes with different hypolimnetic oxygen conditions, Nitrogen burial was estimated from sediment-trap and sediment-con studies. Denitrification and NO(3)(-)-ammonification rates were quantified with (15)NO(3)(-) and acetylene incubation experiments and whole-lake mass balances. The study confirmed earlier reports that the acetylene-block technique yields denitrification rates that Ne systematically too low. Denitrification rates obtained from isotope tracer experiments were compatible with nitrogen consumption rates observed in flux chamber experiments and whole-lake mass balances. The NO(3)(-)-ammonification contributed <5% to the NO(3)(-) consumption rate in Lake Baldegg. Coupled nitrification-denitrification seemed to be insignificant at the deepest station of Lake Baldegg. The comparison of in situ denitrification rates measured at the deepest site (4.3 mmol m(-2) d(-1)) with the denitrification rate obtained from whole-lake mass balances (6.1 mmol m(-2) d(-1)) indicates that enhanced denitrification may be present in shallower sediments with a better supply of O(2). Mass transfer coefficients for NO(3)(-) were similar in both lakes (21.7 and 21.4 m yr(-1)). The NO(3)(-) concentration seems to be a key parameter in determining denitrification rates.