Denitrification in sediments of the Laurentian Trough, St. Lawrence Estuary, Québec, Canada

The isotope pairing technique was used to obtain the first sedimentary denitrification rates for the St. Lawrence Estuary/Gulf system (Québec, Canada). Denitrification rates were measured on intact sediment cores collected at three stations down the slope of the Laurentian Trough at 103, 200 and 320 m depth. For samples collected in June 1997, total denitrification rates ranged from 1.8 to 3.3 μmol N m −2 h −1 among the three stations. Only 3–5% of the NO 3 − consumed by the sediments was subsequently denitrified. Nitrification was the major source of NO 3 − for denitrification, representing 56–79% of total denitrification rates. Sediment consumption of water column O 2 (102–187 μmol O 2 m −2 h −1) and NO 3 − (13.9–25.5 NO 3 − m −2 h −1) increased significantly downslope, as did total denitrification rates. Denitrification made a negligible (