Preprints
https://doi.org/10.5194/os-2021-59
https://doi.org/10.5194/os-2021-59

  07 Jul 2021

07 Jul 2021

Review status: this preprint is currently under review for the journal OS.

Nutrient transport pathways in the Lower St. Lawrence Estuary: seasonal perspectives from winter observations

Cynthia Evelyn Bluteau1, Peter S. Galbraith2, Daniel Bourgault1, Vincent Villeneuve3, and Jean-Éric Tremblay3 Cynthia Evelyn Bluteau et al.
  • 1Institut des sciences de la mer, Université du Québec à Rimouski, Rimouski, Canada
  • 2Institut Maurice-Lamontagne, Fisheries and Oceans Canada, Mont-Joli, Canada
  • 3Département de biologie, Université Laval, Québec, Canada

Abstract. The St. Lawrence Estuary connects the Great Lakes with the Atlantic Ocean. The accepted view, based on summer conditions, is that the Estuary's surface layer receives its nutrient supply from vertical mixing processes. This mixing is caused by the estuarine circulation and tidal-upwelling at the Head of the Laurentian Channel (HLC). During winter when ice forms, historical process-based studies have been limited in scope. Winter monitoring has been typically confined to vertical profiles of salinity and temperature and near-surface water samples collected from a helicopter for nutrient analysis. 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.

Cynthia Evelyn Bluteau et al.

Status: open (until 01 Sep 2021)

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Cynthia Evelyn Bluteau et al.

Cynthia Evelyn Bluteau et al.

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Short summary
In 2018, the Canadian Coast Guard approved a science team to sample in tandem with its icebreaking and ship escorting operations. This collaboration provided the first mixing observations during winter, that covered the largest spatial extent of the St. Lawrence Estuary and Gulf ever measured in any season. Contrary to previous assumptions, we demonstrate that fluvial nitrate inputs from upstream (i.e., Great Lakes) are the most significant source of nitrate in the Estuary.