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

[摘要] The concentrations and elemental stoichiometry of particulate and dissolvedpools of carbon (C), nitrogen (N), phosphorus (P) and silicon (Si) on theCanadian Beaufort Shelf during summer 2009 (MALINA program) were assessedand compared with those of surface waters provided by the Mackenzie river aswell as by winter mixing and upwelling of upper halocline waters at theshelf break. Neritic surface waters showed a clear enrichment in dissolvedand particulate organic carbon (DOC and POC, respectively), nitrate, totalparticulate nitrogen (TPN) and dissolved organic nitrogen (DON) originatingfrom the river. Silicate as well as bulk DON and DOC declined in anear-conservative manner away from the delta's outlet, whereas nitratedropped non-conservatively to very low background concentrations inside thebrackish zone. By contrast, the excess of soluble reactive P (SRP) presentin oceanic waters declined in a non-conservative manner toward the riveroutlet, where concentrations were very low and consistent with P shortage inthe Mackenzie River. These opposite gradients imply that the admixture ofPacific-derived, SRP-rich water is necessary to allow phytoplankton to useriver-derived nitrate and to a lesser extent DON. A coarse budget based onconcurrent estimates of primary production shows that river N deliveriessupport a modest fraction of primary production when considering the entireshelf, due to the ability of phytoplankton to thrive in the subsurfacechlorophyll maximum beneath the thin, nitrate-depleted river plume. Awayfrom shallow coastal bays, local elevations in the concentration of primaryproduction and dissolved organic constituents were consistent with upwellingat the shelf break. By contrast with shallow winter mixing, nutrientdeliveries by North American rivers and upwelling relax surface communitiesfrom N limitation and permit a more extant utilization of the excess SRPentering through the Bering Strait. In this context, increased nitrogen supplyby rivers and upwelling potentially alters the vertical distribution of theexcess P exported into the North Atlantic.

[学科分类] 地球化学与岩石