Articles | Volume 11, issue 17
Biogeosciences, 11, 4853–4868, 2014

Special issue: How changes in ice cover, permafrost and UV radiation impact...

Biogeosciences, 11, 4853–4868, 2014

Research article 11 Sep 2014

Research article | 11 Sep 2014

Impact of river discharge, upwelling and vertical mixing on the nutrient loading and productivity of the Canadian Beaufort Shelf

J.-É. Tremblay3,2,1, P. Raimbault4, N. Garcia4, B. Lansard5,6, M. Babin3,7,2,1, and J. Gagnon1 J.-É. Tremblay et al.
  • 1Québec-Océan & Départment de biologie, Université Laval, Québec, QC, Canada
  • 2Takuvik Joint International Laboratory, Département de Biologie, Université Laval, Québec, Qc, Canada
  • 3Takuvik Joint International Laboratory, CNRS, Québec, Qc, Canada
  • 4Aix-Marseille University, Mediterranean Institute of Oceanography (MIO), UMR7294, CNRS/INSU, UMR235, IRD, 13288, Marseille, Cedex 09; Université du Sud Toulon-Var (MIO), 83957, La Garde CEDEX, France
  • 5Department of Earth and Planetary Sciences-GEOTOP, McGill University, Montréal, Canada
  • 6LSCE, IPSL-CEA-CNRS-UVSQ, Gif-sur-Yvette, France
  • 7Laboratoire d'Océanographie de Villefranche, Université Pierre et Marie Curie, BP8, 06238 Villefranche-sur-Mer CEDEX, France

Abstract. The concentrations and elemental stoichiometry of particulate and dissolved pools of carbon (C), nitrogen (N), phosphorus (P) and silicon (Si) on the Canadian Beaufort Shelf during summer 2009 (MALINA program) were assessed and compared with those of surface waters provided by the Mackenzie river as well as by winter mixing and upwelling of upper halocline waters at the shelf break. Neritic surface waters showed a clear enrichment in dissolved and particulate organic carbon (DOC and POC, respectively), nitrate, total particulate nitrogen (TPN) and dissolved organic nitrogen (DON) originating from the river. Silicate as well as bulk DON and DOC declined in a near-conservative manner away from the delta's outlet, whereas nitrate dropped non-conservatively to very low background concentrations inside the brackish zone. By contrast, the excess of soluble reactive P (SRP) present in oceanic waters declined in a non-conservative manner toward the river outlet, where concentrations were very low and consistent with P shortage in the Mackenzie River. These opposite gradients imply that the admixture of Pacific-derived, SRP-rich water is necessary to allow phytoplankton to use river-derived nitrate and to a lesser extent DON. A coarse budget based on concurrent estimates of primary production shows that river N deliveries support a modest fraction of primary production when considering the entire shelf, due to the ability of phytoplankton to thrive in the subsurface chlorophyll maximum beneath the thin, nitrate-depleted river plume. Away from shallow coastal bays, local elevations in the concentration of primary production and dissolved organic constituents were consistent with upwelling at the shelf break. By contrast with shallow winter mixing, nutrient deliveries by North American rivers and upwelling relax surface communities from N limitation and permit a more extant utilization of the excess SRP entering through the Bering Strait. In this context, increased nitrogen supply by rivers and upwelling potentially alters the vertical distribution of the excess P exported into the North Atlantic.

Final-revised paper