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Tremblay J.E.,University of Quebec | Tremblay J.E.,University Laval | Tremblay J.E.,Takuvik Joint International Laboratory | Raimbault P.,Aix - Marseille University | And 10 more authors.

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 nonconservatively 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 nonconservative 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. © Author(s) 2014. Source

Matsuoka A.,Laval University | Matsuoka A.,Takuvik Joint International Laboratory | Ortega-Retuerta E.,CSIC - Institute of Marine Sciences | Ortega-Retuerta E.,CNRS Microbial Oceanography Laboratory | And 4 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography

Colored dissolved organic matter (CDOM), a significant fraction of dissolved organic carbon (DOC), plays various roles in physical and biogeochemical processes in natural waters. In the Arctic Ocean, CDOM is abundant because of major input by large rivers. To better understand the processes that drive variations in CDOM, light absorption coefficients of CDOM [aCDOM(λ), m-1] were extensively documented together with temperature, salinity, chlorophyll a, nitrate concentrations, and bacterial production (BP) and abundance (BA) in the Western Arctic Ocean (WAO) from early to late summer as part of the MALINA and the ICESCAPE expeditions. The data set covered contrasting situations, from bloom to post-bloom conditions and from river-influenced to oceanic water masses. While CDOM photobleaching occurred in the surface layer (<20m), we observed significantly lower spectral slopes for CDOM absorption spectra (SCDOM) in addition to higher aCDOM(440) in the layer below (intermediate layer: 30.7CDOM values were found in the Chukchi Sea and the western part of the Beaufort Sea, which coincided with high BP and BA values. Considering the high primary production observed in these areas during our cruises (Arrigo et al., 2012. Science. 10.1126/science.1215065), we hypothesize that SCDOM variations reflect the degradation of phytoplankton that is associated with heterotrophic bacterial activity. In our datasets, a simple regression analysis showed that SCDOM was significantly correlated with BP and BA. A principal component analysis further supported this conclusion. From our field observations, it was shown that variations in aCDOM(440) and SCDOM result to a large extent from bacterial activity, at least in the WAO. © 2015 Elsevier Ltd. Source

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