| Despite contributing 8% of the global ocean area, the coastal ocean is more productive than the open ocean and a source of iron and organic matter to the open ocean. Currently, global biogeochemical models do not resolve the coastal ocean. A nonhydrostatic, three-dimensional model is implemented for an idealized shelf-slope geometry to simulate an eastern and western boundary coastal ocean. A biogeochemical model and tracers are designed to investigate mechanisms for nutrient supply, export, and ventilation.;The interaction between the winds and bottom boundary layer determines the fate of the nutrients on both eastern and western boundaries. For example, on western boundary margins, where exchange between the open and coastal ocean is regulated by dynamics of a shelf break front, nutrient-rich water from the bottom boundary layer is pumped up to the surface in response to the oscillation between the wind directions. Downwelling-favorable winds increase the supply of nutrients to the coastal ocean from the open ocean by creating convergence around the foot of the shelf break front. If this mechanism is extrapolated to the western margin of the US, then nutrients supplied to the coastal ocean from the open ocean is three times the nitrogen flux from estuaries.;Particulate organic matter on western boundaries behaves differently. A biogeochemical model to predict biological production, organic carbon flux to the sea floor, and f ratios was designed. Two choices were crucial to predict all three variables: the addition of a second particle size and the remineralization rate changing with depth. Organic carbon is always exported from the shelf.;On eastern boundary margins, upwelling supplies open ocean nutrients to the coastal ocean, and dissolved nutrients are exported offshore in the bottom boundary layer during downwelling-favorable winds. In the model, about 10% of the iron originating from shelf sediments is exported offshore in the bottom boundary layer. If this mechanism exists in all upwelling regimes, then iron exported to the open ocean is equivalent to the estimated dissolved flux from aerosols. The coastal ocean contributes globally significant exchange of biogeochemical material with the open ocean and needs to be included in global budgets. |
