Incubations of intact sediment cores and sediment slurries reveal that anammox is an important sink for fixed nitrogen in sediments from the Lower St. Lawrence Estuary (LSLE), where it occurs at a rate of 5.5 ± 1.7 µmol N m⁻² h⁻¹. Canonical denitrification occurs at a rate of 11.3 ± 1.1 µmol N m⁻² h⁻¹, and anammox is thus responsible for up to 33% of the total N₂ production. Both anammox and denitrification are mostly (> 95%) fueled by nitrate and nitrite produced in situ through benthic nitrification. Nitrification accounts for > 15% of the benthic oxygen demand and may, therefore, contribute significantly to the development and maintenance of hypoxic conditions in the LSLE. The rate of dissimilatory nitrate reduction to ammonium is three orders of magnitude lower than denitrification and anammox, and it is insignificant to N-cycling. NH₄⁺ oxidation by sedimentary Fe(III) and Mn(III/IV) in slurry incubations with N isotope labels did not occur at measurable rates; moreover, we found no evidence for NH₄⁺ oxidation by added Mn(III)-pyrophosphate.