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Nutrient transport pathways during winter in the Lower St. Lawrence Estuary
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  • Cynthia Bluteau,
  • Peter Galbraith,
  • Daniel Bourgault,
  • Vincent Villeneuve,
  • Jean-Éric Tremblay
Cynthia Bluteau
Université du Québec à Rimouski

Corresponding Author:[email protected]

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Peter Galbraith
Fisheries and oceans Canada Maurice-Lamontage Institude
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Daniel Bourgault
University du Québec à Rimouski
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Vincent Villeneuve
Université Laval
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Jean-Éric Tremblay
Université Laval
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The St. Lawrence Estuary connects the Great Lakes with the Atlantic Ocean. During summer, the Estuary’s surface layer receives its nutrient supply from vertical mixing processes caused by the estuarine circulation and tidal-upwelling at the Head of the Laurentian Channel (HLC). There has been few oceanographic process studies during winter when ice forms and flows on the surface. Winter monitoring is typically confined to vertical profiles of salinity and temperature and near-surface water samples collected from a helicopter. In 2018, however, the Canadian Coast Guard approved a science team to sample in tandem with its icebreaking and ship escorting operations. This opportunistic sampling provided the first winter turbulence observations, which covered the largest spatial extent ever measured during any season within the St. Lawrence Estuary and Gulf. The nitrate enrichment from tidal mixing resulted in an upward nitrate flux of about 30 nmol m$^{-2}$s$^{-1}$, comparable to summer values obtained at the same tidal phase. Further downstream, deep nutrient-rich water from the Gulf was mixed into the subsurface nutrient-poor layer at a rate more than an order of magnitude smaller than at the HLC. These fluxes were compared to the nutrient load of the upstream St. Lawrence River. Contrary to previous assumptions, fluvial nitrate inputs are the most significant source of nitrate in the Estuary. Nitrate loads from vertical mixing processes would only exceed those from fluvial sources at the end of summer when fluvial inputs reach their annual minimum.