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 (Que´bec, Canada). Denitrification rates were measured on intact sediment cores collected at three stations down the slope of the Laurentian Trough at 103, 200 and 320m depth. For samples collected in June 1997, total denitrification rates ranged from 1.8 to 3.3 lmolNm 2 h 1 among the three stations. Only 3–5% of the NO3 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 O2 (102–187 lmolO2m 2 h 1) and NO 3 (13.9–25.5NO 3 m 2 h 1) increased significantly downslope, as did total denitrification rates. Denitrification made a negligible (<0.2%) contribution to carbon mineralization in our cores, but may have a greater impact on the nitrogen cycle. In addition, data suggest the presence of a major dissimilatory nitrate sink in these sediments that far exceeds the requirements of denitrification. The latter observation is considered in terms of proposed interactions between the nitrogen cycle and metal reduction/oxidation. We conclude by considering limitations to the extrapolation of these data to annual and regional scale nitrogen budgets for the deep waters of the St. Lawrence Estuary.