Carbon, oxygen, and nitrogen cycles on the Vancouver Island shelf




Bianucci, Laura

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A quasi-two dimensional model for the southern Vancouver Island shelf was developed with the Regional Ocean Modelling System (ROMS) to study coupling of the carbon, oxygen, and nitrogen cycles in a summer wind-driven upwelling region. The physical model is coupled to an ecosystem module that includes a simple representation of a sediment layer and considers non-fixed C:N ratios for detritus and dissolved organic matter (i.e., explicitly modelled pools of carbon and nitrogen for those variables). The model accounts for denitrification within the sediments as well as within the water column when oxygen concentrations are low (below 5 mmol-O2 m-3). The objective is to identify the dominant processes controlling the cycles, their coupling, and their sensitivity to changes in environmental forcing. Results demonstrate how low oxygen and low pH events are tightly coupled in the coastal study region, especially through local ecosystem processes. In particular, exchange with the sediments plays a dominant role in consuming oxygen from and releasing inorganic carbon to the bottom waters on the shelf. Two key features distinguish the southern Vancouver Island shelf from other coastal regions in the California Current System and protect inner shelf waters from severe hypoxia and corrosive (i.e., undersaturated in aragonite) conditions. First, the greater width of the shelf reduces the penetration of subsurface offshore high-carbon and low-oxygen waters into shallower waters; and second, the relatively fresh Vancouver Island Coastal Current (VICC) brings oxygen-rich and carbon-poor waters to the bottom layer over the inner shelf. Sensitivity experiments show that carbon and oxygen cycles on the southern Vancouver Island shelf may be significantly affected by an altered upwelling season, a shallower offshore Oxygen Minimum Zone, a warmer ocean, and a carbon-enriched environment. Combinations of these scenarios suggest a potential increasing risk for the development of coastal hypoxia and corrosive conditions in the future. Further sensitivity simulations indicate that sedimentary denitrification provides an additional coupling between the carbon, oxygen, and nitrogen cycles. Total alkalinity generated by sediment denitrification has the potential to buffer anthropogenic ocean acidification. However, this alkalinity effect over the Vancouver Island shelf in late spring and summer simulations is small compared with studies for other locations at annual scales. Longer time scales need to be examined in this region to confirm whether the role of alkalinity generation in the sediments is significant. In conclusion, this dissertation not only demonstrates the coupled nature of biogeochemical cycles in the coastal ocean, but also the importance of this coupling as we try to estimate how coastal ecosystems will respond to human modifications of shelf waters and the climate.



Biogeochemical cycles, Numerical modelling, Continental shelf, British Columbia