Abstract
Abstract. Low oxygen concentrations, either natural or anthropogenically driven, can severely affect coastal marine ecosystems. A deeper understanding of oxygen dynamics is required in order to improve numerical models, eventually 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 two coupled physical-biological models 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, this loss of regenerated nutrients through denitrification within the sediments significantly affects primary production, 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, changes in regenerated nitrogen on the VIS 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 sediment denitrification in coastal regions where regenerated primary production dominates productivity.
Highlights
Anaerobic remineralization processes become important where oxygen concentrations are low in the water column and in the sediment layer below the oxygenated upper few millimetres (Jørgensen, 1982)
We investigated the effect of this anaerobic process on the oxygen concentrations of bottom waters and found different behaviour in our two study regions
The loss of bioavailable nitrogen through sediment denitrification significantly decreased primary production in the Middle Atlantic Bight (MAB), such that less organic matter reached the seafloor in the DNF simulation compared with the NoDNF experiment
Summary
Anaerobic remineralization processes become important where oxygen concentrations are low in the water column and in the sediment layer below the oxygenated upper few millimetres (Jørgensen, 1982). In contrast to aerobic remineralization, end products of denitrification are dinitrogen gas or nitrous oxide and not readily available to most primary producers (Seitzinger, 1988) This anaerobic process represents a loss of bioavailable nitrogen from the ecosystem and affects water column processes such as primary production and nitrification, with repercussions for the carbon and oxygen cycles. Direct denitrification represents an extra sink of fixed nitrogen for the water column, with the potential to further decrease primary production (Process 1). These pathways connecting sediment denitrification and oxygen concentrations in the water column have not been explored in detail. Allochthonous sources of NH+4 and the input of NO−3 from below the nutricline that resulted from previous nitrification of NH+4 prevent the association of NO−3 as an unambiguous “new” nutrient (e.g., Walsh et al, 1981; Falkowski et al, 1983)
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