Estimating the Efficiency of Cross-Shelf Transport of Terrestrially Derived Materials in River Plumes
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Rivers connect land and sea, delivering large amounts of terrestrially derived materials (such as nutrients, sediments, and pollutants) to the coastal ocean. Understanding the fate of this delivery is critical. Nutrients can accumulate on shelves, driving high levels of primary production, which can lead to hypoxia, or they can be exported rapidly across the shelf to the open ocean where their impact is minimized. With global ocean models unable to resolve the small-scale processes of riverine export to the open ocean, they are often parameterized with an “all or nothing” approach: either all of the riverine material enters the open ocean (ignoring shelf processes), or none of it does. Both approaches potentially misrepresent riverine export. Using an idealized river plume model, I assess the impact of latitude, river discharge, winds, and tides on the cross-shelf export of riverine material. From these numerical experiments, latitude is shown to be a strong controlling factor in determining the cross-shelf export within river plumes, which can only be slightly offset by external forcing from winds. It is possible to parameterize the resulting export using the Sp number—a dimensionless number relating the cross-shelf extent of a plume to the local shelf width. I use this simple relationship to estimate global export of riverine fresh water and nutrients to the open ocean. Globally, I estimate that just 15–53% of riverine fresh water reaches the open ocean through direct transport within river plumes, with greatly reduced nutrient export due to shelf processing.