Benthic nutrient fluxes along the Laurentian Channel: Impacts on the N budget of the St. Lawrence marine system

Water column concentrations and benthic fluxes of dissolved inorganic nitrogen (DIN) and oxygen (DO)were measured in the Gulf of St. Lawrence and the Upper and Lower St. Lawrence Estuary (USLE andLSLE, respectively) to assess the nitrogen (N) budget in the St. Lawrence (SL) system, as well as toelucidate the impact of bottom water hypoxia on fixed-N removal in the LSLE. A severe nitrate deficit,with respect to ambient phosphate concentrations (N*w10 mmol L1), was observed within and in thevicinity of the hypoxic bottom water of the LSLE. Given that DO concentrations in the water column haveremained above 50 mmol L1, nitrate reduction in suboxic sediments, rather than in the water column, ismost likely responsible for the removal of fixed N from the SL system. Net nitrate fluxes into the sediments,derived from pore water nitrate concentration gradients, ranged from 190 mmol m2 d1 in thehypoxic western LSLE to 100 mmol m2 d1 in the Gulf. The average total benthic nitrate reduction ratefor the Laurentian Channel (LC) is on the order of 690 mmol m2 d1, with coupled nitrification-nitratereduction accounting for more than 70%. Using average nitrate reduction rates derived from the observedwater column nitrate deficit, the annual fixed-N elimination within the three main channels of the Gulfof St. Lawrence and LSLE was estimated at 411 106 t N, yielding an almost balanced N budget for the SLmarine system.