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 NO3−‐ammonification 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 NO3−‐ammonification 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 mmolm−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− ). The NO3− concentration seems to be a key parameter in determining denitrification rates.