We studied microbial N(2) production via anammox and denitrification in the anoxic water column of a restored mining pit lake in Germany over an annual cycle. We obtained high-resolution hydrochemical profiles using a continuous pumping sampler. Lake Rassnitzer is permanently stratified at ca. 29m depth, entraining anoxic water below a saline density gradient. Mixed-layer nitrate concentrations averaged ca. 200 micromol L(-1), but decreased to zero in the anoxic bottom waters. In contrast, ammonium was <5 micromol L(-1) in the mixed layer but increased in the anoxic waters to ca. 600 micromol L(-1) near the sediments. In January and October, (15)N tracer measurements detected anammox activity (maximum 504 nmol N(2)L(-1)d(-1) in (15)NH(4)(+)-amended incubations), but no denitrification. In contrast, in May, N(2) production was dominated by denitrification (maximum 74 nmol N(2)L(-1)d(-1)). Anammox activity in May was significantly lower than in October, as characterized by anammox rates (maximum 6 vs. 16 nmol N(2)L(-1)d(-1) in incubations with (15)NO(3)(-)), as well as relative and absolute anammox bacterial cell abundances (0.56% vs. 0.98% of all bacteria, and 2.7x10(4) vs. 5.2x10(4)anammox cells mL(-1), respectively) (quantified by catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) with anammox bacteria-specific probes). Anammox bacterial diversity was investigated with anammox bacteria-specific 16S rRNA gene clone libraries. The majority of anammox bacterial sequences were related to the widespread Candidatus Scalindua sorokinii/brodae cluster. However, we also found sequences related to Candidatus S. wagneri and Candidatus Brocadia fulgida, which suggests a high anammox bacterial diversity in this lake comparable with estuarine sediments.