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  • Print publication year: 2012
  • Online publication date: June 2012

10 - The shelf edge system

Summary

At the outer edge of the shelf is a region where the gentle slopes of the shelf give way to the much steeper topography of the continental slope, and bottom depths rapidly increase down to the abyssal plains of the deep ocean. On average, the depth at which this slope transition occurs is about 130 metres, but this varies through the world's oceans. Off NW Europe the shelf edge is at a depth of 200 metres, while at high latitudes the shelf edge is deeper, typically 400–500 metres around Antarctica and off Greenland. Because the topography is steep, with slopes as large as 1:10, the transition between shelf and the deep ocean is usually limited in extent (~50 km). It is in this rather narrow region that the very different regimes of the shelf and the deep ocean adjust to each other. In this chapter, we shall consider how this adjustment occurs and how it controls the important exchanges between shelf and deep-ocean. We will look at wind-driven upwelling, the most studied process linking the physics and the ecology of the shelf edge which, in many parts of the world, supports important stocks of plantivorous fish. We shall also consider the upwelling (and downwelling) driven by the bottom Ekman layer of along-slope flows, and the consequences for nutrient supply to, and organic material export from, shelf seas. We will describe the density contrasts that develop in winter between temperate shelf seas and the adjacent ocean that can lead to downslope cascades of shelf seawater and its constituents. Finally, we consider the role of the internal tide, a prominent shelf edge process which strongly influences the biochemistry and is important in relation to commercial fisheries.