Elemental and stable isotopic constraints on river influence and patterns of nitrogen cycling and biological productivity in Hudson Bay |
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Authors: | Zou Zou A. Kuzyk,Robie W. Macdonald,Jean-É ric Tremblay,Gary A. Stern |
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Affiliation: | 1. Centre for Earth Observation Science, Department of Environment and Geography, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2;2. Freshwater Institute, Fisheries & Oceans Canada, 501 University Crescent, Winnipeg, Manitoba, Canada R3T 2N6;3. INRS-Eau Terre Environnement, Université du Québec, 490 rue de la Couronne, Québec, Canada G1K 9A9;4. Institute of Ocean Sciences, Fisheries & Oceans Canada, 9860 West Saanich Road, P.O. Box 6000, Sidney, British Columbia, Canada V8L 4B2;5. Québec-Océan, Département de Biologie, Université Laval, Québec, Canada G1V 0A6 |
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Abstract: | Elemental (carbon and nitrogen) ratios and stable carbon and nitrogen isotope ratios (δ13C and δ15N) are examined in sediments and suspended particulate matter from Hudson Bay to study the influence of river inputs and autochthonous production on organic matter distribution. River-derived particulate organic matter (POM) is heterogeneous, nitrogen-poor and isotopically depleted, consistent with expectations for OM derived from terrestrial C3 vascular plant sources, and distinct from marine OM sources. Both δ13C and C/N source signatures seem to be transmitted to sediments with little or no modification, therefore making good tracers for terrigenous OM in Hudson Bay. They suggest progressively larger contributions from marine sources with distance from shore and secondarily from south to north, which broadly corresponds to the distribution of river inputs to Hudson Bay. Processes other than mixing of marine and terrigenous OM influence sedimentary δ15N values, including variability in the δ15N of phytoplankton in the Bay's surface waters due to differences in relative nitrate utilization, and post-production processes, which bring about an apparently constant 15N-enrichment between surface waters and underlying sediments. Variability in the δ15N of phytoplankton in the Bay's surface waters, in contrast, seems to be organized spatially with a pattern that suggests an inshore–offshore difference in surface water nitrogen conditions (open- vs. closed-system) and hence the δ15N value of phytoplankton. The δ15N patterns, supported by a simple nitrate box-model budget, suggest that in inshore regions of Hudson Bay, upwelling of deep, nutrient-rich waters replenishes surface nitrate, resulting in ‘open system’ conditions which tend to maintain nitrate δ15N at low and constant values, and these values are reflected in the sinking detritus. River inflow, which is constrained to inshore regions of Hudson Bay, appears to be a relatively minor source of nitrate compared to upwelling of deep waters. However, river inflow may contribute indirectly to enhanced inshore nutrient supply by supporting large-scale estuarine circulation and consequently entrainment and upwelling of deep water in this area. In contrast to previous proposals that Hudson Bay is oligotrophic because it receives too much fresh water (Dunbar, 1993), our results support most of the primary production being organized around the margin of the Bay, where river flow is constrained. |
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Keywords: | Hudson Bay Organic carbon Nitrogen Sediment Particulate organic matter Stable isotopes |
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