Nitrogen elimination in two deep eutrophic lakes

Abstract

Nitrogen elimination was investigated in two eutrophic Swiss lakes with different hypolimnetic oxygen conditions. Nitrogen burial was estimated from sediment-trap and sediment-core studies. Denitrification and NO3ammonification rates were quantified with 15NO3- and acetylene incubation experiments and whole-lake mass balances. The study confirmed earlier reports that the acetylene-block technique yields denitrification rates that are 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 NO3ammonification contributed <5% to the NO3- 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 O2. Mass transfer coefficients for NO3- were similar in both lakes (21.7 and 21.4 m yr-1). The NO3- concentration seems to be a key parameter in determining denitrification rates.