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Benthic nutrient fluxes along the Laurentian Channel: Impacts on the N budget of the St. Lawrence marine system
Journal
Estuarine, Coastal and Shelf Science
Volume
90
Number
4
Pages / Article-Number
195-205
Keywords
St. Lawrence, estuary, geochemistry, nitrogen cycle, denitrification
Abstract
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 and LSLE, respectively) to assess the nitrogen (N) budget in the St. Lawrence (SL) system, as well as to elucidate 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 the vicinity of the hypoxic bottom water of the LSLE. Given that DO concentrations in the water column have remained above 50 mmol L1, nitrate reduction in suboxic sediments, rather than in the water column, is most 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 the hypoxic western LSLE to 100 mmol m2 d1 in the Gulf. The average total benthic nitrate reduction rate for the Laurentian Channel (LC) is on the order of 690 mmol m2 d1, with coupled nitrification-nitrate reduction accounting for more than 70%. Using average nitrate reduction rates derived from the observed water column nitrate deficit, the annual fixed-N elimination within the three main channels of the Gulf of St. Lawrence and LSLE was estimated at 411 106 t N, yielding an almost balanced N budget for the SL marine system.
