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Role of sediment denitrification in water column oxygen dynamics: comparison of the North American east and west coasts

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Author(s): L. Bianucci | K. Fennel | K. L. Denman

Journal: Biogeosciences Discussions
ISSN 1810-6277

Volume: 9;
Issue: 1;
Start page: 1;
Date: 2012;
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ABSTRACT
The occurrence of hypoxic conditions in the coastal ocean has been increasing in the last decade. Low oxygen concentrations, either natural or anthropogenically driven, can severely affect coastal marine ecosystems. A deeper understanding of the oxygen cycle is required in order to improve numerical models and to predict the timing and severity of hypoxia. In this study we investigate the effect of sediment denitrification on oxygen concentrations in bottom waters over the continental shelf. We used a coupled physical-biological model based on the Regional Ocean Modelling System (ROMS) to compare summer simulations with and without denitrification within the sediments for two North American shelves: the Middle Atlantic Bight (MAB) and the Vancouver Island Shelf (VIS). These regions belong to Western and Eastern Boundary Current Systems, respectively, and are characterized by different physical and biological dynamics. Both models assume coupled nitrification-denitrification within the sediments. Denitrification represents a loss of bioavailable nitrogen through the production of dinitrogen gas, with the potential to affect biogeochemical cycles. In our MAB model, denitrification within the sediments efficiently decreases the total pool of available nutrients, since recycled nitrogen supports most of the primary production in that region. The diminished primary production and consequent decrease of organic matter flux to the seafloor leads to less sediment oxygen consumption and higher oxygen concentrations in bottom waters. However, on the VIS changes in regenerated nitrogen barely affect primary production due to the efficient supply of new nutrients through wind-driven upwelling during summer and the nutrient-rich coastal current. We recommend that modelling experiments focusing on oxygen dynamics (as well as oxygen budget calculations) should include this process in coastal regions where regenerated primary production dominates productivity.
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