NITROGEN ELIMINATION IN 2 DEEP EUTROPHIC LAKES

Nitrogen elimination was investigated in two eutrophic Swiss lakes wit h different hypolimnetic oxygen conditions, Nitrogen burial was estima ted from sediment-trap and sediment-con studies. Denitrification and N O3--ammonification rates were quantified with (NO3-)-N-15 and acetylen e incubation experiments and whole-lake mass balances. The study confi rmed earlier reports that the acetylene-block technique yields denitri fication rates that Ne systematically too low. Denitrification rates o btained from isotope tracer experiments were compatible with nitrogen consumption rates observed in flux chamber experiments and whole-lake mass balances. The NO3--ammonification contributed <5% to the NO3- con sumption rate in Lake Baldegg. Coupled nitrification-denitrification s eemed to be insignificant at the deepest station of Lake Baldegg. The comparison of in situ denitrification rates measured at the deepest si te (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 enhance d denitrification may be present in shallower sediments with a better supply of O-2. Mass transfer coefficients for NO3- were similar in bot h lakes (21.7 and 21.4 m yr(-1)). The NO3- concentration seems to be a key parameter in determining denitrification rates.