Low oxygen and high inorganic carbon on the Vancouver Island Shelf

Recently, independent concerns about declining oxygen and pH conditions in the coastal ocean have emerged. In coastal upwelling regions, hypoxia can be driven by onshore advection of oxygen-depleted offshore waters as well as by local biological consumption triggered by high productivity. As both mechanisms can also decrease pH and carbonate saturation states, coupled studies of oxygen and carbon are imperative. A quasi two-dimensional model coupling carbon, oxygen, and nitrogen was developed for the summer wind-driven upwelling region off southern Vancouver Island, using the Regional Ocean Modeling System. The physical model is coupled to an ecosystem module that tracks 11 state variables and allows nonfixed C: N ratios for detritus and dissolved organic matter. Given uncertainties in sediment parameterizations in biophysical models, three sediment models are compared and discussed. Results demonstrate that sediment-associated processes play a dominant role in consuming oxygen from, and releasing inorganic carbon to, the bottom waters over the shelf. This study also examines the unique characteristics of the southern Vancouver Island shelf. Two key features distinguish this region from other shelves in the California Current System and protect inner shelf waters from severe hypoxia and corrosive (i.e., undersaturated in aragonite) conditions. First, the near-shore Vancouver Island Coastal Current provides a source of oxygen and nutrients and forms a barrier that prevents upwelled waters (depleted in oxygen and rich in carbon) from penetrating the inner shelf. Second, the greater width of the shelf dilutes these upwelled offshore waters and reduces their penetration onto the shallower shelf region.