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

J.-É. Tremblay,N. Garcia,M. Babin,P. Raimbault,B. Lansard,J. Gagnon

doi:10.5194/bg-11-4853-2014

Abstract

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.

Highlights

The Arctic Ocean is currently experiencing severe and rapid physical changes that are expected to alter primary productivity as well as the structure and function of marine food webs
Concentrations of dissolved organic nitrogen (DON) and Dissolved organic carbon (DOC) measured at river sites during May and June and on the shelf at stations with negligible salinity (< 0.23) during August were related exponentially to daily river discharge (Supplement Table S1 and Fig. S2)
Our results provide new information and insights into the interplay of riverine and oceanic processes on a narrow interior Arctic shelf, and into the biogeochemical consequence of these processes at local and larger scales

Summary

Introduction

The Arctic Ocean is currently experiencing severe and rapid physical changes that are expected to alter primary productivity as well as the structure and function of marine food webs. While any process conducive to the erosion of the halocline or upwelling will inject more nutrients into the upper euphotic zone, the unusual stoichiometry (or mixture) of nutrients imposes an additional constraint on productivity in the Pacific sector of the Arctic Ocean. In these waters, a chronic deficiency of N relative to phosphorus (P) is maintained by a host of local and remote biogeochemical processes. North American rivers tend to deliver Pdeficient waters (Le Fouest et al, 2013), but their impact on primary production and on the nutrient composition of waters exiting the high Arctic is not well understood

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