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A carbon and nitrogen flux model in a coastal upwelling region Ianson, Debby C.


An improved understanding of oceanic biogeochemical cycles is critical for predicting global climate. In coastal upwelling regions fluxes of both carbon and nitrogen are disproportionately large relative to the global ocean. This is especially true of the downward flux of organic matter that transports carbon away from the ocean surface layer causing absorption of atmospheric carbon dioxide (i.e. the 'biological pump'). In addition to biologically limiting nutrients, upwelling brings inorganic carbon to the surface, potentially providing a source of carbon to the atmosphere. However, global models seldom resolve the complex, non-homogeneous coastal ocean. I have developed a carbon and nitrogen flux model for coastal upwelling regions that considers all important processes both within and below the euphotic zone over time scales of days to decades. Physical circulation is represented by six boxes, an upper and lower box for three horizontal regions: the continental shelf, slope and open ocean. Dissolved inorganic, dissolved organic and particulate organic forms of both carbon and nitrogen as well as salinity are modelled. The model is parameterized and physically forced to apply to the west coast of Vancouver Island, Canada. In addition, a field study during July 1998 was undertaken to complement and constrain the model. The model predicts annual primary production, net air-sea CO₂ gas exchange and net export fluxes between the open ocean and the model system under different physical forcing scenarios (including El Niño-Southern Oscillation Events). Model results suggest that coastal upwelling regions do not contribute to the oceanic sequestration of CO₂; rather, they provide a conduit for subsurface inorganic carbon to the surface ocean. There is net annual air-sea CO₂ gas influx over the shelf and slope, but it is small. In winter, subsurface waters enriched in inorganic carbon and nitrogen are mixed into the surface, causing gas evasion that almost balances the summer invasion. On the other hand, there is a large flux of inorganic carbon from the lower ocean to the surface ocean via the model system, probably leading to CO₂ outgassing offshore of the shelf. Meanwhile, fluxes of organic carbon from the model system to the open ocean are small (compared with the inorganic carbon flux), especially in the lower layer. Thus, temperate coastal upwelling regions do not operate as strong biological pumps.

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